## Left-Handedness Research Paper

## Introduction

Students' achievement in mathematics is a matter of high practical relevance. Mathematical skill is necessary to major in Science, Technology, Engineering, and Mathematics (STEM) subjects, and therefore to attain STEM jobs. The job market requires worldwide more graduates in STEM subjects than in other disciplines (e.g., humanities, social sciences) and has also become increasingly more competitive (Halpern et al., 2007). For this reason, the cognitive and biological correlates of mathematical ability have been the object of extensive debate (e.g., Deary et al., 2007; Rohde and Thompson, 2007; Wai et al., 2009; Lubinski, 2010; Peng et al., 2016). One of these correlates is handedness.

Handedness is a manifestation of the lateralization of human brain function, and consequently, hand preference is believed to affect human overall cognitive skills (McManus, 2002). However, in spite of much research carried out on this topic, the shape of this relationship is still highly controversial.

The effect of handedness on mathematical ability have been a matter of interest too (e.g., Annett and Kilshaw, 1982; Benbow, 1986, 1987; Annett and Manning, 1990; Crow et al., 1998; Cheyne et al., 2010). However, no distinct pattern of results has emerged from the research addressing this topic. For example, while some studies considered left-handedness as a sign of giftedness in mathematics (e.g., Benbow, 1986), others found that left-handers performed slightly worse than right-handers on measures of mathematical ability (e.g., Johnston et al., 2013). The unclear relationship between handedness and mathematics reflects the discrepancies between the models relating handedness to cognitive abilities. In fact, according to Nicholls et al. (2010), four main models linking handedness to human cognition have been proposed. Each of these models makes different predictions about how handedness affects mathematical ability.

One of the most influential models linking handedness, cognition, and mathematical ability is Annett's (1985, 2002)*right shift theory*. According to this theory, most people inherit the so-called “right-shift factor,” which is a dominant allele (RS+) that predisposes them to be both right-handed and left-hemisphere dominant for language. Whoever inherits this allele has a good probability of being right-handed (see Corballis, 1997, for a review). While people with a heterozygous genotype (RS±) are mostly moderately right-handed (i.e., they do not show an exclusive preference for the right hand in experimental or daily life tasks), and benefit from a balanced cognitive profile, those who inherit a homozygous genotype for the RS+ are mostly strongly right-handed and may suffer from a deficit in spatial ability, because of the costs the RS+ allele to the right-hemisphere. Finally, in people who do not inherit the right-shift factor (i.e., homozygous for the RS– allele), handedness is determined by random factors (active during fetal development) and by environmental pressure on hand use. In this situation, a person ends up being randomly either right- or left-handed, and may suffer from a deficit specific to language ability.

Annett's right-shift theory thus predicts a general cognitive advantage for moderate right-handers. In line with this hypothesis, Annett (1992) found an advantage in spatial ability for moderate right-handers in a sample of 14–15-year-olds. Further support for this hypothesis was also provided by Casey's (1995, 1996a) studies, which found an advantage for moderate right-handers in general intelligence in a sample of primary school children, and mental rotation ability in a sample of female college students, respectively. However, some studies failed to replicate these outcomes (e.g., McManus et al., 1993; Cerone and McKeever, 1999). Finally, a more recent study (Nicholls et al., 2010) showed that moderate right-handers performed slightly better than the rest of the sample in a test measuring several cognitive skills such as attention, executive functions, language ability, and memory. Interestingly, in that study, both strong right- and left-handers achieved the worst results, thus showing a quadratic relationship between hand skill and test scores.

Concerning mathematical ability, Annett's right shift theory predicts a disadvantage for strongly right-handed individuals (Peters, 1991) and an inverse linear relationship between dextrality. Since mathematical ability relies to a large extent on spatial ability (Wai et al., 2009; Lubinski, 2010), those who inherit the homozygous dominant genotype are more likely to perform poorly in mathematics (Annett and Manning, 1990).

Following another line of research, Benbow (1986) claimed that left-handedness is a predictor of mathematical precociousness in young students. In that study, it was found that the rates of left-handers among students talented in mathematics were much greater than among the general population. Benbow (1986) found that the frequency of left-handedness among gifted students was significantly higher than in the general population. Moreover, this alleged superiority of left-handers seems to occur mostly in males and when mathematical ability is assessed with tasks involving reasoning (e.g., mathematical problems; Benbow, 1988).

Benbow's hypothesis—i.e., left-handers tend to be overrepresented among gifted students—is based on the fact that left-handers are more likely to have a more developed right-hemisphere (Geschwind and Behan, 1984; Geschwind and Galaburda, 1987; O'Boyle and Benbow, 1990), which is involved in processes related to mathematical ability—such as spatial reasoning (Ganley and Vasilyeva, 2011) and mental rotation ability (O'Boyle et al., 2005; Hoppe et al., 2012), and a larger corpus callosum (Witelson, 1985; Beaton, 1997). This may foster interhemispheric connectivity and bi-hemispheric representation of cognitive functions (Benbow, 1986), with positive effects on left-handed individuals' intellectual skills, such as verbal reasoning (Halpern et al., 1998) and verbal fluency (Hines et al., 1992), episodic memory (Christman and Propper, 2001), intelligence among gifted children (Hicks and Dusek, 1980), and spatial abilities (Casey et al., 1992; Reio et al., 2004). In addition, the relationship between left-handedness and giftedness seems to occur in several domains. For example, left-handedness appears to be more common among gifted musicians (Kopiez et al., 2006), chess players (Gobet and Campitelli, 2007; Oremosu et al., 2011), artists (Preti and Vellante, 2007), and mathematicians (Annett and Kilshaw, 1982).

Recently, however, the idea that left-handedness is a predictor of superior intellectual ability has been challenged. Several authors have claimed that left-handedness is not related to any advantage in cognitive skills, and may even exert detrimental effects on general cognitive abilities and hence academic achievement. Left-handedness may be caused by left-hemisphere damage occurring pre- or peri-natally (Satz et al., 1985), and consequently, a portion of left-handed individuals may suffer from an overall cognitive deficit. In line with this hypothesis, Johnston et al. (2009) found that left-handed children slightly underperformed in a series of developmental measures, compared to right-handers. Also, two recent meta-analyses (Papadatou-Pastou and Tomprou, 2015; Somers et al., 2015) reported that left-handers were over-represented among intellectually challenged individuals and did slightly worse in spatial ability tasks, respectively. Consistent with these results, Johnston et al. (2013) found that left-handers underperformed in mathematical ability in a sample of children aged 5–14.

The last theory drawing a causal link from handedness to mathematical ability through cognition is the *hemispheric indecision* hypothesis (Crow et al., 1998). This theory focuses on the importance of handedness as a continuous variable, in opposition to the dichotomy left/right. The lateralization of brain function seems to be an advantage from an evolutionary perspective, because it obviates functional redundancy, and therefore makes neural processing run more efficiently (Gutwinski et al., 2011). Thus, the most decisive factor is *how much* a person is right or left-handed because a weak lateralization may be associated with a delay in development (Orton, 1937; Zangwill, 1960; Bishop, 1990).

This hypothesis has recently received empirical support. Crow et al. (1998) found that the tendency to show an equal skill for right and left hand in a square-checking task predicted deficits in verbal, non-verbal, and mathematical abilities in a sample of 11-year-old children. Peters et al. (2006) reported that individuals with no preferred hand in writing had the lowest performance in mental rotation ability. Corballis et al. (2008) observed that children who had no hand preference for writing performed significantly worse than right- and left-handers in several tasks including arithmetic, memory, and reasoning. Finally, Cheyne et al. (2010) found similar results in a large sample of 11-year-old children.

## Materials and Methods

### The Present Study

The research that has been carried out on the effects of handedness on mathematical ability depicts an intricate tapestry. The outcomes of the studies are, at least partially, contradictory, and it is hard to infer a definitive conclusion from them. However, research also suggests that one reason for the discrepant findings may be methodological inconsistencies. Studies differed with regard to (a) how participants were categorized according to handedness (e.g., right-/left-handers, right-/left-/mixed-handers, non-right-/right-handers), which causes difficulties for comparing the outcomes between studies (Casey, 1996b; Cerone and McKeever, 1999; Li et al., 2003; Nicholls et al., 2010); (b) ages and educational levels (e.g., primary school children, middle- and high-school students, adults) of the participants; and (c) the specific mathematical abilities assessed (e.g., simple arithmetic or problem-solving).

The aim of this study was to reconcile the discrepancies observed in previous research in the field. First, we used a continuous measured of handedness without using any categorization. Second, we systematically manipulated the age of the participants and the mathematical tasks to evaluate the effect of these moderating variables on the relationship between handedness and performance on tests of mathematics.

### A Theoretical Challenge: The Use of Quartic Functions

Along with the abovementioned methodological issues, another aspect of the research in the field may be a critical limitation. While the four theories we have reviewed differ in important ways, a common characteristic is that they consider the link between handedness and mathematical ability as dichotomous (e.g., left-handers vs. than right-handers), or, when a continuous measure of handedness is used, linear or quadratic. However, no study, to the best of our knowledge, has investigated the possibility that the relationship between handedness and mathematical ability is more complex and requires a polynomial function with a cubic and quartic term to be described properly.

Quartic functions can have up to five maxima and minima (three relative, and two at the extremes of the domain), be both monotonic and non-monotonic, and reduce into smaller-degree functions if necessary (by attributing the value 0 to one or more coefficients). For these reasons, quartic functions may be able to detect patterns of the relationship between handedness and mathematical ability impossible to identify with categorical measures or quadratic functions. Thus, our hypothesis is that including cubic and quartic terms in polynomial functions substantially contributes to the amount of variance in mathematical ability accounted for by handedness.

### Procedure

We ran five experiments differing from each other regarding the age of the participants and the type of mathematical skills assessed (e.g., arithmetic, reasoning). The aims of these experiments were (a) to evaluate which of the models introduced above best describes the relationship between handedness and mathematical ability; (b) to investigate the mediating effect of age, gender, and type of task on the link between handedness and mathematical ability; and (c) to quantitatively assess the effect of handedness on mathematical ability by calculating the percentage of variance (*R*^{2}) of the participants' scores in mathematics explained by handedness, using 4th-degree polynomials.

The participants, aged 6–17, were recruited in Italian schools, between December 2013 and June 2015. Most of the participants (~80–85%) were from Italian middle-class families, while the rest were from foreign families. All the participants spoke fluent Italian and were not diagnosed with any learning disability. Parental consent was obtained for all the participants. The participants were administered (a) a set of different tests assessing mathematical ability (one for each experiment), and (b) the 10-item version of the Edinburgh Handedness Inventory (EHI; Oldfield, 1971). EHI is a multiple-item questionnaire, and thus is more sensitive and reliable than categorical measures of hand preference (Johnston et al., 2009). EHI provides a continuous measure of handedness (*H*), which was calculated using the formula:

where *R* and *L* indicate the number of preferences for the right and left hand, respectively. The range of values is between −1, for extreme left-handedness, and +1, for extreme right-handedness. Importantly, the participants were not divided into groups according to their hand preference. In fact, categories (e.g., right-handers, left-handers, and mixed-handers) are always arbitrary to some extent and hence may cause difficulties for comparing outcomes.

### Data Analysis

Since our data were nested (i.e., most of the participants were recruited from different schools), a multi-level linear modeling (Goldstein, 2011) approach was applied to control for possible confounding effects (e.g., Type I error) due to the school of provenance of the participants. As noticed by several authors (e.g., Cheyne et al., 2010; Nicholls et al., 2010), the relationship between handedness and academic skills is not necessarily linear. Therefore, preliminarily to building the models, a series of linear regression analyses (method backward) was performed with *H, H* quadratic (*H*^{2}), cubic (*H*^{3}), and quartic (*H*^{4}) functions as possible predictors, to look for potential non-linear (i.e., polynomial) relationships between *H* and scores in mathematics. Then, the functions of *H* [i.e., *F(H)*] calculated by the linear regression analysis were inserted—along with the participants' age and gender—into the models as independent variables. For each experiment, three multi-level linear models were run and compared to each other: the intercept model, the model including all the independent variables except the functions of handedness, and the model including all the independent variables. Finally, two additional regression analyses were performed (with predictors *H* and its functions) for males and females separately, in order to investigate possible gender differences in the relationship between handedness and mathematical ability.

## Experiment 1

In this experiment, we investigated the relationship between handedness and mathematical precocity in a sample of children aged 7–9. To evaluate whether Benbow's (1986, 1988) hypothesis—i.e., left-handers tend to be more precocious in mathematics—generalizes to the general population of third and fourth graders, we used items designed to assess the mathematical ability of 15-year-old students.

### Method

#### Participants

A total of 413 third and fourth graders (187 males, 226 females) with a mean age of 8.32 (*SD* = 0.62) years took part in this experiment only. The participants were recruited from five different schools in Italy.

#### Procedure

Along with the EHI, the participants were administered a set of seven OECD-Pisa items (OECD, 2012) assessing mathematical skill (score range 0–7). These items require the student to infer the correct solution from a given set of data and hence involve mathematical reasoning ability. In all the seven items, the participants were asked to choose one of the five possible answers.

The OECD-Pisa items are designed for students aged 15. Therefore, we expected the children to perform relatively poorly. Nonetheless, the contents of the selected items were manageable for children of third and fourth grades (e.g., problems involving only whole numbers, operations of addition and subtraction, and simple geometry), and the instructions were easily understandable.

### Results

The mean score for *H* was 0.584 (*SD* = 0.470), while the mean score for mathematics was 1.55 (*SD* = 1.16). The linear regression analysis showed that only the quartic function of *H* (i.e., *H*^{4}) was significantly correlated to the scores in mathematics (*b*_{4} = −0.312, *t* = −2.073, *r* = 0.102, *R*^{2} = 0.010, *p* = 0.039; *intercept* = 1.684, *p* < 0.001). The quartic function can be appreciated in Figure 1A.

**Figure 1. (A)** The function [*F(H)*] of handedness (*H*) correlating with the score of mathematics in the whole sample. The blue circles represent *F(H)* values for the *H* values for which there were human observations. The black dots represent the trend line. The values on the y-axis were normalized by dividing *F(H)* by the possible maximum score (i.e., 7) on the test of mathematical ability. **(B)** The function [*F(H)*] of handedness (*H*) correlating with the score of mathematics in the female sample. The blue circles represent *F(H)* values for the *H* values for which there were human observations. The black dots represent the trend line. The values on the y-axis were normalized by dividing *F(H)* by the possible maximum score (i.e., 7) on the test of mathematical ability.

The multi-level linear models showed a significant effect of age, no effect of gender, and only a marginally significant (*p* < 0.10) effect of *H*^{4} (Table 1). The effect of the school of provenance (random factor) was not significant.

**Table 1.** Parameters, coefficients, and standard errors in the multilevel models of Experiment 1.

#### Gender Analysis

Two linear regression—one for males and one for females—were performed. The analysis showed no predictor in males, whereas it did in females (*b*_{1} = −1.275, *b*_{3} = 1.242, *b*_{4} = −0.885, *t* = 2.547, *r* = 0.284, *R*^{2} = 0.081, *p* < 0.001; *intercept* = 2.102, *p* < 0.001). The following function

depicting the relationship between *H* and mathematics scores in females can be seen in Figure 1B.

### Discussion

The results of this experiment suggest a quartic relationship between handedness and mathematical ability, especially for females. Benbow's (1986) hypothesis that left-handedness is a predictor of precocity in mathematical reasoning ability is not supported. In fact, the results appear to suit more—marginally in the whole sample, and significantly in females—Annett's (2002) conception of the disadvantage of the extremes.

## Experiment 2

Experiment 1 showed a quartic relationship between *H* and scores in mathematics, especially in females. However, the low scores achieved by the participants, due to the difficulty of the mathematical tasks, may have hidden other potential relationships between the two variables (e.g., the same relationship in males as well).

In this experiment, we replaced the OECD-Pisa items with six items designed for assessing mathematical literacy in fourth graders, and hence more suitable for the participants. Thus, we investigated the relationship between handedness and mathematical reasoning ability in primary school children again, but not focusing on mathematical precocity.

### Method

#### Participants

A total of 300 (151 males, 149 females) third and fourth graders took part in this experiment only. The participants' mean age was 8.46 (*SD* = 0.67) years. The children were recruited from nine schools in northern Italy.

#### Procedure

The participants were administered the EHI and a test consisting of six items of IEA-TIMSS (Mullis and Martin, 2013) international survey assessing mathematical literacy in fourth graders. Similar to OECD-PISA, the items of the IEA-TIMSS survey require solving a mathematical problem from a given set of data. The participants have to select an option among four possible answers.

### Results

The mean score for *H* was 0.614 (*SD* = 0*.5*29), while the mean score for mathematics was 2.57 (*SD* = 1.33). The regression analysis showed that only the quartic function of *H* (i.e., *H*^{4}) was significantly correlated to the scores in mathematics (*b*_{4} = −0.438, *t* = −2.244, *r* = 0.130, *R*^{2} = 0.017, *p* = 0.026; *intercept* = 2.801, *p* < 0.001; Figure 2A).

**Figure 2. (A)** The function [*F(H)*] of handedness (*H*) correlating with the score of mathematics in the whole sample. The blue circles represent *F(H)* values for the *H* values for which there were human observations. The black dots represent the trend line. The values on the y-axis were normalized by dividing *F(H)* by the possible maximum score (i.e., 6) on the test of mathematical ability. **(B)** The function [*F(H)*] of handedness (*H*) correlating with the score of mathematics in the sample of males. The blue circles represent *F(H)* values for the *H* values for which there were human observations. The black dots represent the trend line. The values on the y-axis were normalized by dividing *F(H)* by the possible maximum score (i.e., 6) on the test of mathematical ability.

The multi-level linear models showed a significant effect of gender, *H*^{4}, and age (Table 2), while the effect of the school of provenance (random factor) was not significant.

**Table 2.** Parameters, coefficients, and standard errors in the multilevel models of Experiment 2.

#### Gender Analysis

Regression analysis showed that *H*^{4} was still a predictor of the dependent variable (mathematics scores) for males (*b*_{4} = −0.779, *t* = −2.649, *r* = 0.212, *R*^{2} = 0.045, *p* = 0.009; *intercept* = 3.239, *p* < 0.001; Figure 2B), but not for females.

### Discussion

The results showed that the children at the two extremes of the distribution tended to achieve the poorest performance, but among males only. This outcome again supported Annett's (2002) conception of the disadvantage of the extremes. In this experiment too, gender moderated the effect that handedness exerted on the scores in mathematics. We will take up this issue in the General Discussion.

## Experiment 3

While the previous two experiments examined the effect of handedness on children's ability to solve mathematical tasks involving reasoning, this experiment evaluated the role of handedness on children's arithmetical ability. The used arithmetical tasks demanded only the knowledge and the application of simple algorithms (e.g., adding in column). Moreover, those who took part in this and the following two experiments were administered a mental rotation ability (MRA) task. Since MRA has been proposed as one possible link connecting handedness to mathematical ability (Annett and Manning, 1990; Casey et al., 1992), we tested whether the effect of handedness on arithmetical ability would remain significant even when MRA was controlled for.

### Method

#### Participants

One-hundred and sixty-two (78 males, 84 females) children took part in this experiment only. The participants were first, second, and third graders, and their mean age was 7.79 (*SD* = 0.89) years. The participants were recruited from one school in northern Italy.

#### Procedure

The participants were administered (a) the EHI, (b) a test of arithmetic, designed by the experimenters (score range 0–27), and (c) a 2-D mental rotation ability task suitable for children (score range 0–16; for details, see Cheng and Mix, 2014). In the test of arithmetic, the participants solved simple mathematical equations (e.g., 3 + 4 = ?) and missing-term problems (e.g., 3 +? = 7).

### Results

The mean score was 0.604 (*SD* = 0.473) for *H*, 17.43 (*SD* = 8.28) for the scores in arithmetic, and 13.65 (*SD* = 2.51) for mental rotation ability. The regression analysis showed that only the cubic function of *H* (i.e., *H*^{3}) was significantly correlated to the scores in mathematics (*b*_{3} = −3.192, *t* = −2.192, *r* = 0.171, *R*^{2} = 0.029, *p* = 0.030; *intercept* = 18.759, *p* < 0.001), which can be appreciated in Figure 3.

**Figure 3.** The function [*F(H)*] of handedness (*H*) correlating with the score of mathematics in the whole sample. The blue circles represent *F(H)* values for the *H* values for which there were human observations. The black dots represent the trend line. The values on the y-axis were normalized by dividing *F(H)* by the possible maximum score (i.e., 27) on the test of mathematical ability.

The multi-level linear models showed a significant effect of *F*(*H*), mental rotation skills, and age, but no effect of gender (Table 3).

**Table 3.** Parameters, coefficients, and standard errors in the multilevel models of Experiment 3.

Moreover, the analysis showed no significant correlation between *F*(*H*) and MRA scores (*r* = −0.053, *p* = 0.504).

#### Gender Analysis

The regression analysis did not find any significant predictors either in males or females.

### Discussion

The results showed that handedness exerted a significant effect on the scores in mathematics even when MRA was controlled for. Interestingly, MRA and the function of handedness correlated with the scores in mathematics were not significantly correlated. This outcome suggests that the effects of handedness and MRA did not overlap. With respect to the shape of the relationship between handedness and arithmetical ability, the function showed a monotonic trend in favor of left-handers.

However, the relatively small number of participants may have been insufficient to detect potential alternative patterns among left-handers. Since left-handers were underrepresented compared to right-handers (as in the general population), the function might have fit the dependent variable regardless of the few left-handers of the sample. Put simply, the advantage of left-handers may have been due to a statistical artifact.

## Experiment 4

This experiment aimed at improving the design of the previous one by recruiting a larger sample. We thus wanted to test the advantage of left-handers in arithmetical tasks found in the previous experiment.

### Method

#### Participants

Seven-hundred and ninety-eight (417 males, 381 females) children took part in this experiment only. The participants were first, second, and third graders, and their mean age was 7.22 (*SD* = 0.91) years. The participants were recruited from six schools in northern Italy.

#### Procedure

Along with the EHI and the MRA task, the participants were administered a test of arithmetical abilities (AC-MT 6-11; Cornoldi et al., 2012). This test consisted of 26 items (score range 0–26) of basic arithmetic (e.g., addition, subtraction, multiplication, and identifying the greatest or the smallest number in a series).

### Results

The mean scores were 0.626 (*SD* = 0.522) for *H*, 21.81 (*SD* = 4.77) for arithmetical ability, and 13.04 (*SD* = 3.04) for mental rotation ability. The linear regression analysis showed that the quadratic (*H*^{2}) and the quartic (*H*^{4}) functions of handedness were correlated to the scores in the arithmetic test (*b*_{2} = 7.763, *b*_{4} = −9.083, *t* = 5.375, *r* = 0.260, *R*^{2} = 0.068, *p* < 0.001; *intercept* = 21.693, *p* < 0.001). We thus built the following function:

which is shown in Figure 4A.

**Figure 4. (A)** The function [*F(H)*] of handedness (*H*) correlating with the score of mathematics in the whole sample. The blue circles represent *F(H)* values for the *H* values for which there were human observations. The black dots represent the trend line. The values on the y-axis were normalized by dividing *F(H)* by the possible maximum score (i.e., 26) on the test of mathematical ability. **(B)** The function [*F(H)*] of handedness (*H*) correlating with the score of mathematics in males. The blue circles represent *F(H)* values for the *H* values for which there were human observations. The black dots represent the trend line. The values on the y-axis were normalized by dividing *F(H)* by the possible maximum score (i.e., 26) on the test of mathematical ability. **(C)** The function [*F(H)*] of handedness (*H*) correlating with the score of mathematics in females. The blue circles represent *F(H)* values for the *H* values for which there were human observations. The black dots represent the trend line. The values on the y-axis were normalized by dividing *F(H)* by the possible maximum score (i.e., 26) on the test of mathematical ability.

The multi-level linear model showed a significant effect of *F*(*H*), MRA, and age, whereas no significant effect of gender was found (Table 4). The effect of the school of provenance (random factor) was not significant.

**Table 4.** Parameters, coefficients, and standard errors in the multilevel models of Experiment 4.

The correlation analysis showed that *F*(*H*) and MRA were correlated (*r* = 0.150, *p* < 0.001). We thus calculated the partial correlation—with the effect of the scores in MRA being partialled out—between *F*(*H*) and the scores in mathematics, and still found a significant correlation (*r* = 0.221, *R*^{2} = 0.049, *p* < 0.001).

#### Gender Analysis

The regression analysis showed that *H*^{4} was a predictor (*b*_{4} = −2.346, *t* = −4.220, *r* = 0.203, *R*^{2} = 0.041, *p* < 0.001; *intercept* = 23.337, *p* < 0.001) of the dependent variable (mathematics scores) in males, while *H*^{2} and *H*^{4} were predictors (*b*_{2} = 11.788, *b*_{4} = −12.739, *t* = 4.542, *r* = 0.314, *R*^{2} = 0.098, *p* < 0.001; *intercept* = 20.824, *p* < 0.001) in females. The two functions are shown in Figures 4B,C.

No correlation was found between MRA scores and the quartic function in males. By contrast, a significant correlation was found between MRA scores and *F(H)* in females (*r* = 0.271, *p* < 0.001). We thus calculated the correlation between the females' scores in mathematics and *F(H)* with the scores in MRA being partialled out. This partial correlation was still significant (*r* = 0.234, *p* < 0.001).

### Discussion

The results of this experiment revealed once again that the participants occupying the two extremes of the handedness distribution achieved the worst scores in mathematics, and that the pattern obtained in the previous experiment (i.e., an advantage for left-handers over right-handers) was probably a statistical artifact. Moreover, the mixed-handed children—the ones in the center of the distribution—also obtained a relatively poor performance, especially among females. The latter outcome lends some support to the idea that mixed-handers are disadvantaged in mathematical abilities due to their hemispherical indecision (Crow et al., 1998; Cheyne et al., 2010). Interestingly, the results show an M-shaped pattern—indicating the inferior performance of the strong right- and left-handers, and of the mixed-handers—similar to the one found for mental rotation ability in Peters et al. (2006).

The shape of the relationship between handedness and scores in arithmetical ability does not differ substantially between genders. In fact, strong right- and left-handers achieved the worst results both in males and females, and the fact that mixed-handers do not seem to underperform among males might only be due to lack of statistical power. However, handedness appears to exert a greater influence on females' than does on males' arithmetical ability (*R*^{2} = 0.098 and *R*^{2} = 0.041, respectively).

## Experiment 5

The previous experiments tested the role of handedness in affecting children's mathematical ability. This experiment dealt with the relationship between handedness and high-school students' mathematical ability. The aim of this experiment was to examine whether handedness maintains a significant effect also on adolescents' performance in mathematics.

### Method

#### Participants

A total of 641 (211 males, 430 females) youngsters (aged 14–17) took part in this experiment only. The participants were ninth and tenth graders, and their mean age was 14.71 (*SD* = 0.76) years. The participants were recruited from three high schools in northern Italy.

#### Procedure

The participants were administered (a) the EHI, (b) a set of 10 OECD-Pisa items (OECD, 2012; score range 0–10) to assess mathematical ability, and (c) a revised version of Vandenberg and Kuse's (1978) 3-D mental rotation task assessing MRA (Version A; Peters et al., 1995; score range 0–24).

### Results

The mean scores were 0.655 (*SD* = 0.432) for *H*, 5.46 (*SD* = 2.03) for mathematical ability, and 9.16 (*SD* = 5.03) for MRA. The linear regression analysis showed that the quadratic (*H*^{2}) and the cubic (*H*^{3}) functions of handedness were correlated to the participants' scores in mathematics (*b*_{2} = 0.934, *b*_{3} = −1.024, *t* = 2.524, *r* = 0.140, *R*^{2} = 0.020, *p* = 0.002; *intercept* = 5.369, *p* < 0.001). We thus built the following function:

which can be appreciated in Figure 5A.

**Figure 5. (A)** The function [*F(H)*] of handedness (*H*) correlating with the score of mathematics in the whole sample. The blue circles represent *F(H)* values for the *H* values for which there were human observations. The black dots represent the trend line. The values on the y-axis were normalized by dividing *F(H)* by the possible maximum score (i.e., 10) on the test of mathematical ability. **(B)** The function [*F(H)*] of handedness (*H*) correlating with the score of mathematics in the sample of males. The blue circles represent *F(H)* values for the *H* values for which there were human observations. The black dots represent the trend line. The values on the y-axis were normalized by dividing *F(H)* by the possible maximum score (i.e., 10) on the test of mathematical ability.

The model showed a significant effect of *F*(*H*), scores in MRA, and age, whereas no significant effect of gender was found (Table 5). The effect of the school of provenance (random factor) was not significant.

**Table 5.** Parameters, coefficients, and standard errors in the multilevel models of Experiment 5.

The correlation analysis showed no significant correlation between *F*(*H*) and MRA scores (*r* = −0.050, *p* = 0.207).

#### Gender Analysis

The linear regression analysis showed that *H*^{2} and *H*^{3} were significant predictors (*b*_{2} = 1.968, *b*_{3} = −1.436, *t* = 2.625, *r* = 0.249, *R*^{2} = 0.062, *p* = 0.001;

## Current Left-handed research

This section gives information on research that has been in the news recently or is particularly relevant to current events.

## General information on left-handed research

If you are currently involved in research or studies involving left-handedness and would like more detailed information than we have available, you may find the following links helpful. You can view articles and often search for any papers or articles relating to your particular area of interest.

Research websites:

http://bmj.com

http://www.nature.com/nature

http://www.genetics.org

Research books:

Right-Hand, Left-Hand by Prof. Chris McManus

Available from our online shop

The Left-Handers Handbook by Diane Paul

Available from our online shop

If you discover any interesting research relating to left-handedness that we have not covered, please let us know about it, and we will add a link to this page.

## Are left handed people more inhibited?

This article appeared in the New Scientist – click here for original article

Lefties face a daily battle with a world designed for right-handers. Now it seems that left-handed people face a similar struggle in the mental sphere: behavioural research suggests they are prone to inhibition and anxiety.

When about to do something, left-handers tend to dither, says Lynn Wright, a behavioural psychologist at the University of Abertay Dundee, UK, who led the study. Right-handers tend to jump in a bit.â€

On tests of behavioural inhibition, 46 left-handed men and women scored higher than 66 right-handers. Women, too, tended to rack up higher scores on the tests of reticence.

Wright and her colleagues uncovered these predilections by giving subjects a behavioural test that gauges both personal restraint and impulsiveness, qualities which seem to emanate from opposite hemispheres of our brains.

Compared to right-handers, lefties and women were likelier to agree with statements such as, I worry about making mistakesâ€ and Criticism or scolding hurts me quite a bitâ€. All groups responded similarly to statements such as I often act on the spur of the momentâ€ and I crave excitement and new sensationsâ€, Wright’s team found.

The results could be due to wiring differences in the brains of left-handers and right-handers, she says. Research suggests that handedness is a matter of degree (see “Edinburgh handedness inventory”). But in left-handers the right half of the brain is dominant, and it is this side that seems to control negative aspects of emotion. In right-handers the left brain dominates.

It’s all relative, you see,â€ says Philip Corr, a behavioural neuroscientist at Swansea University, UK, noting that the differences in the brains of left and right-handers are usually slight.

However, he says handedness is not so much a predictor of personality as a great way to understand how emotions are handled in our brains. Although we may have a predisposition to an inhibition, that may encourage us during adulthood or childhood to develop coping strategies,â€ he says. It could act as a blessing.â€

Wright, a lefty, agrees. They [left-handers] like to colour-code things, they like to write lists, it’s almost a way to alleviate their stress,â€ she says, adding that she is the classic example of the things that she finds in her work – “which is frighteningâ€.

An international group of scientists, led by a team from the Wellcome Trust Centre for Human Genetics at

## Researchers at Oxford University find gene for left-handedness

An international group of scientists, led by a team from the Wellcome Trust Centre for Human Genetics at the University of Oxford, have discovered a gene that increases the chance of being left handed. The study is published on-line today by the journal Molecular Psychiatry.

The research, which involved over 40 scientists from 20 research centres around the world, revealed a gene called LRRTM1; the first to be discovered which has an effect on handedness. Although little is known about LRRTM1, the Oxford team suspects that it modifies the development of asymmetry in the human brain. Asymmetry is an important feature of the human brain, with the left side usually controlling speech and language, and the right side controlling emotion. In left-handers this pattern is often reversed. There is also evidence that asymmetry of the brain was an important feature during human evolution; the brains of our closest relatives, the apes, are more symmetrical than humans' and they do not show a strong handedness.

The Left-Handers Club welcome these new findings, as a genetic link has long been considered the most likely cause of left-handedness yet a specific gene has until now remained elusive. This is the first potential genetic influence on human handedness to be identified, and the first putative genetic effect on variability in human brain asymmetry. LRRTM1 is a candidate gene for involvement in several common neurodevelopmental disorders, and may have played a role in human cognitive and behavioral evolution.

The researchers also discovered that LRRTM1 might slightly increase the risk of developing schizophrenia. People with schizophrenia often have unusual patterns of brain asymmetry and handedness, so the researchers were not surprised when LRRTM1 also showed a possible effect on the risk of developing schizophrenia. Schizophrenia is a disorder of the brain which results in impaired perception and thought. It affects roughly one percent of adults worldwide.

There has not, however, been any assumption that left-handedness and schzophrenia are linked. The study leader, Dr Clyde Francks, said: “People really should not be concerned by this result. There are many factors which make individuals more likely to develop schizophrenia and the vast majority of left-handers will never develop a problem. We don't yet know the precise role of this gene.”

Some of the researchers involved in this discovery are now planning further study on the roles of LRRTM1 in the developing brain, and to find other genes with which LRRTM1 interacts. Dr Francks said: “We hope this study's findings will help us to understand the development of asymmetry in the brain. Asymmetry is a fundamental feature of the human brain that is disrupted in many psychiatric conditions.”

For more information contact:

Dr Clyde Francks (Study Leader)

GlaxoSmithKline

Email: clyde.2.francks (at) gsk.com

Phone: +39 045 821 8059

Prof Anthony Monaco (Laboratory Head)

Wellcome Trust Centre for Human Genetics University of Oxford

Email: anthony.monaco (at) well.ox.ac.uk

Phone: +1 302 945 5349

07795 690173

Higher breast cancer risk among left-handed women (Sep 2005)

Source **: BMJ, doi: 10.1136/bmj.38572.440359.AE (Pub. 26 Sept 2005) **

New research suggests that left-handed women may be more at risk from breast cancer. The study, published online by the British Medical Journal, found left-handed women were more than twice as likely to develop premenopausal breast cancer as non-left handed women.

The researchers, from the University Medical Center Utrecht, looked at the relationship between handedness and cases of breast cancer in more than 12,000 middle aged women born between 1932 and 1941. As part of their examination, t he researchers also took body measurements and assessed risk factors such as economic status, smoking habits, family history of breast cancer and reproductive background. |

Even when taking into account all risk factors, the study found that the overall association was hardly affected

The team of Dutch researchers believe the common link may be exposure to high levels of sex hormones testosterone in the womb. Previous research has suggested that exposure to high levels of sex hormones before birth may induce left-handedness. This exposure can also trigger changes in the breast tissue that make tumour growth more likely in later life. The researchers concluded: “Although the underlying mechanisms remain elusive, our results support the hypothesis that left-handedness is related to increased risk of breast cancer.”

Breast cancer is the most common cancer for women in the UK, with more than 41,000 new cases diagnosed each year. It accounts for one in three of all cancer cases in women, while the lifetime risk for women is one in nine.

Emma Taggart, director of policy and campaigns at charity Breakthrough Breast Cancer, said: “Women who are left-handed should not worry about these findings. Although this is an intriguing study, it doesn't give us enough evidence to link left handedness with breast cancer.

“Breast cancer is an extremely complex disease and very little is known about the causes.”The answer is likely to be a complex interplay of factors.”

Ms Taggart said women could minimise their risk by eating a balanced diet, drinking less alcohol and exercising regularly.

“It's also vital for women to be breast aware and visit their GP if they are concerned about changes in their breasts.”

Lesley Walker, of Cancer Research UK, said the study was based on a relatively small sample.

“A much bigger study is required to determine the actual risks of left handedness.

“If the results of this study are borne out, it could highlight a group of pre-menopausal women to whom early screening could be targeted. She continued “We know that the strongest risk factor for breast cancer is age. Eighty per cent of breast cancers occur in women over the age of fifty.”

** Left-Handers Club comment: **

The results of this study are based on an extremely small sample, as out of the 12,000 women initially included in the study, only 165 women studied in the final sample were left-handed. The causal link between exposure to high levels of testosterone in the womb and subsequent left-handedness has not been conclusively proven, and indeed more recent research strongly suggests a genetic link to left-handedness being far more likely.

However, if we accept the hypothesis the research was based on, these initial findings would certainly indicate a need for further and more detailed studies to establish a possible link. If these results are conclusive, left-handedness would be a valuable signal to encourage early screening.

## The loneliness of the left handed surgeon (Jan 2005)

Source: BMJ 2005;330:10 (1 January), doi:10.1136/bmj.330.7481.10-f

Left handed surgeons lack access to left handed instruments while training, receive little mentoring about their left handedness, and are more prone to needle stick injuries than their right handed colleagues. They also have considerable difficulty handling some instruments.
One in 10 left handed surgeons was also uncomfortable with the idea of being operated on by a left handed surgeon, says a report in |

The perils and pitfalls of being left handed emerge from a survey of surgeons in New York city, Manhattan, and Brooklyn; there were 68 responses from clinicians aged 27 to 60.

The authors, from New York’s Memorial Sloan-Kettering Cancer Center and other institutions, say that left handed surgeons feel that they are the last unorganised minority. The survey found that only 13% of left-handed surgeons were provided with left handed instruments while training.

“Having basic sets of left-handed instruments (scissors, clamps, and needle holders) available in the teaching hospitals for medical students and surgical residents may minimise the inconveniences associated with learning,” the authors wrote.

The report says there is a lack of laterality related mentoring for left handed surgical residents: “There are no studies or teaching material available to teach left-handed surgical residents. Laterality-related guidance was reported to be minimal in medical school.

“Nearly half of the left-handed surgeons surveyed were anxious about their laterality related difficulties and sought advice during surgical residency, but only one in 10 programs mentored for laterality predominance. Provision of a left-handed mentor and other environmental modifications could be used to minimise the recurring difficulties for left-handed learners.’’

The report says that left handed surgeons preferred an approach that might be different from a right handed surgeon in an invasive procedure: “Mentoring by a right handed surgeon will only leave the left-handed residents to teach themselves a procedure.”

It says that left handed surgeons felt that several surgical procedures were difficult to learn standing on the right side of the operating table, including open cholecystectomy and pelvic surgical procedures: “Left-handed surgical residents should be given a chance to stand on the left side of the operating table.”

Another finding was that laparoscopy and laparoscopic instruments have not eliminated the problems of instrument handling: “The popular belief that laparoscopy and minimally invasive surgical instruments have completely eliminated difficulties for the left-handed surgeons does not hold true for the respondents in this study. Laparoscopic surgery involves more static posture of the neck and trunk with more frequent awkward movements of the upper extremities than open surgery.”

Another unexpected finding was that some respondents were uncomfortable with the idea of being operated on by a left handed surgeon: “We were surprised to learn that one in 10 left-handed surgeons have perceivable difficulty in being treated by another left handed surgeon. Unfortunately, our survey did not have the provision to inquire into the reasons for this perception.”

## Left-Handers – Nature's Fighters?

**If anyone picks a fight with you, tell them you're left handed and they may well think again!**

**Scientists have found we lefties often have the upper hand in combat.**

The endurance of left-handedness has puzzled researchers, considering the links to disadvantages including an increased risk of some diseases. But researchers at the University of Montpellier in France believe left-handers continue to thrive because they do well in combat.

*Proceedings of the Royal Society B*, saw that left-handers had the advantage in sports such as fencing, tennis and baseball. They said that Western interactive sports such as these can be classed as “special cases of fights – with strict rules, including the prohibition of killing and intentionally wounding the opponent”.

This led them to speculate the same advantage may persist in more aggressive contexts, such as war, so societies which are more violent would have a higher frequency of left-handers.

The suggestion that left-handers have an advantage in combat is not new. It has long been thought that, in the days when arguments were resolved by hand-to-hand combat, being left-handed gave people the benefit of surprise against a right-handed opponent. This advantage, however, would only have persisted if left-handers remained in the minority. Otherwise, right handers would soon get accustomed to fighting with left-handed opponents. |

For this latest study, the researchers analysed data for eight traditional societies; the Kreyol people of Dominica, the Ntimu of Cameroon, the Dioula-speaking people of Burkina Faso, the Baka of Gabon, Inuit people and the Eipo people of Irian Jaya, New Guinea. Charlotte Faurie and Michel Raymond compared homicide rates (which includes murders and executions) and the frequency of left-handedness, and found they appeared to be linked.

The Dioula were found to have a homicide rate equivalent of one hundredth of a death per 1,000 people per year, and a left-handedness rate of just 3%.

But the Eipo had around three homicides per 1,000 people and a left-handedness rate of 20%.

Dr Faurie said “We have found a direct correlation between the level of violence in a given society and the proportion of left-handers. This indicates that fighting can be an important selection pressure in the evolution of left-handedness.”

The researchers admitted that a homicide rate that includes executions and gang murders is probably not an accurate measure of one-to-one fights in society, but it was the best measure available. “This result strongly supports the fighting hypotheses. More generally, it points to the importance of violence in understanding the evolution of handedness in humans.” she said.

Chris McManus, a professor of psychology at University College London who has made a study of the pros and cons of left-handedness, detailed in his book **“Right Hand Left Hand”**, said it was true that left-handers did have an advantage in a fight – “It's the same advantage as you see with tennis players, baseball players and cricketers”.

But he added: “The question is whether that advantage in fights then goes on and dominates the rate of left-andedness in societies, and I think the answer is ‘no it doesn't'. The explanation must be much more complex than that.”

There must have been an advantage for a minority of people to be born left-handed, but trying to find out what this advantage is remains unclear, he said. “The theory I've put forward is that despite the drawbacks of being left-handed, there are advantages in terms of creativity and other positive aspects,” said the professor, “and society needs a subgroup who are different.”

He added that the French study had also examined too few people, raising concerns over its conclusions. “The sample sizes were small and the methods they used were not as reliable as they could have been. I'm far from convinced” he said.

**Left-Handers Club Comment:** The suggestions that left-handers have good combative skills is not unreasonable, and has been proven many times by the high number of successful left-handers in combative sports such as fencing, tennis, and boxing.

What is interesting in this study is the suggestion that the instance of left-handers increases in a more violent society. There is no suggestion that the left-handers are the perpetrators of the violence, only that they are good fighters. Perhaps this is the key to their success, since they will have a kudos and elevated position in society, as well as longevity, enabling them to breed more successfully and pass on the left-handed genetic trait to more offspring.

Even if this were the case, however, the random nature by which left-handedness passes through generations (as detailed by Prof. McManus) would inhibit left-handers becoming the majority of the population, and thus losing their combative advantage.

Whilst this is an interesting hypothesis, the size and nature of the study do, as pointed out by Dr Chris McManus, undermine its credibility and a more controlled study on a far wider range of societies would be most welcome, to provide more reliable results.

Links :

BBC News – http://news.bbc.co.uk/1/hi/health/4073775.stm

New Scientist – http://www.newscientist.com/news/print.jsp?id=ns99996773**Chris McManus Book “Right Hand Left Hand” **buy this book now in our online shop

## Handedness develops in the womb (July 2004 )

Source: New Scientist Print Edition, 22 July 2004, Laura Spinney, Lisbon

The hand you favour as a 10-week-old foetus is the hand you will favour for the rest of your life, suggests a new study.

The finding comes as a surprise because it had been thought that lifelong hand preferences did not develop until a child was three or four years old.

A team led by Peter Hepper of the Fetal Behaviour Research Centre at Queen's University, Belfast in the UK reached this conclusion after studying ultrasound scans of 1000 fetuses.

In one study, nine out of 10 fetuses at 15 weeks' gestation preferred to suck their right thumbs. Hepper's team followed 75 of those fetuses after birth, and found that at 10 to 12 years old all 60 of the right thumb-suckers were right-handed, while 10 of the 15 left thumb-suckers were left-handed and the rest right-handed.

At 10 weeks old, even before they suck their thumbs, fetuses wave their arms about. A second study found that most prefer to wave their right arm, a preference that persisted until 24 weeks, after which the fetus is too cramped to move. Hepper reported the findings at the Forum of European Neuroscience in Lisbon, Portugal, earlier in July.

##### Reflex arc

Hepper is quick to point out that these observations do not show that the fetus can control its movements at such a young age. Nervous connections to the body from the brain are not thought to start developing until around 20 weeks' gestation.

In addition, at the same stages of development fetuses that lack a brain cortex, a condition called anencephaly, move their limbs in a similar way, also favouring their right arm over the left.

“There is no evidence that the brain has any control over these movements at this stage,” says Hepper. “It's most likely to be a local reflex arc involving the spinal cord.” He speculates that the fetus may have a preference for one side of its body simply because that side develops slightly faster.

The findings challenge the favourite theory of how handedness in humans develops. According to that theory, it is a side effect of brain lateralisation, in which one side of the brain predominantly handles certain functions, such as language. As the fetal scans show that handedness appears long before the brain has any control over limb movement, that theory cannot be correct.

##### Sensory connections

Instead, Hepper speculates that the reverse may be true: the fetus's body movements

may somehow lead to the development of an asymmetrical brain. He points out that the sensory connections from the body to the brain develop before the connections that allow the brain to control the body's movement.

But Stephen Wilson, a developmental biologist at University College London, is sceptical. “The movements you see in a fetus don't have to be influencing brain asymmetries.”

It is more likely, he says, that in the early fetus there is already a difference in gene activation between the right and left sides of the brain and that this leads to lateralisation.

## Hair Growth Clue to Handedness

Researchers have discovered that you can tell if someone is right or left-handed literally off the top of your head – by checking which way their hair grow out of their scalp. Right-handed people tend to have hair that swirls clockwise, from the whorl or crown (the place at the back of the head where hair appears to grow in a spiral). People who are left-handed or ambidextrous, however have no such pattern – the hair can coil in either direction. Amar Klar of the National Cancer Institute in Frederick, Maryland, surreptitiously inspected people's pates by spying on them in airports and shopping malls – ignoring the long-haired and the bald. More than 95% of right-handers' hair whorls clockwise on the scalp, he found. The locks of lefties and the ambidextrous are equally likely to coil either way. |

A single gene with either ‘right' or ‘random' forms might underlie the trend, says Klar. People with one or two copies of the right version would be right-handed, with clockwise hair; those with two random versions would split 50/50 for handedness and hair whorls. He is now seeking such a gene.

“It's one of the most exciting things [I've seen] in a while,” says geneticist Ralph Greenspan of the Neurosciences Institute in San Diego, California. A gene causing asymmetric cell division in the young embryo might set up asymmetry throughout the body, he suggests.

But many genes might influence handedness, counters Clyde Francks of the University of Oxford, UK, who is hunting for such genes. Only finding these molecules will reveal the answer, he argues.

**Right, left**

Around 90% of people favour their right hand for writing and throwing. Researchers argue about whether genes or learning create this preference. Most people assume that there is no single ‘handedness' gene because it is not simply inherited. Two left-handed parents, for example, will often have right-handed children. Klar believes his hypothesis accounts for these puzzles. If children of left-handers inherit the ‘random' gene, they could be left- or right-handed. This would also explain why identical twins can be right- and left-handed.

The genes underlying handedness might also explain why our brains are asymmetrical. And left-handed or ambidextrous people are more likely to store language in the right side of the brain, are more prone to schizophrenia and, anecdotally, are more often creative or even geniuses.

**References**

1. Klar, A.J.S. Human handedness and scalp hair whorl direction develop from a common genetic mechanism, *Genetics*, in the press, (2003).

2. Nature Science Update – Handedness equals hairstyle

## BBC Test the Nation – Lefties have upper hand in IQ test

Left-handers across the UK edged ahead in the intelligence stakes this week, when the nation took part in the biggest IQ test ever undertaken. The British were all tuned to their TV sets on Saturday 11th May, as more than 90,000 people took part in Test the Nation hosted by quiz show presenter Anne Robinson on BBC1. Everyone had to complete 70 questions to assess their language, perception, memory, maths and logic in what the BBC called a ‘light-hearted way of gauging Britain's brain power'

An IQ score of 90 to 110 indicates average intelligence, while anything over 110 points is in the highest 25 per cent. Anyone with 130 or above can consider themselves in the top 2 per cent of the population. **The results revealed that left-handed people on average scored 109, slightly higher than the right-handers at 108**. Smokers, scoring 108 were shown to be brainier than non-smokers with 107. ‘Silver surfers' topped the hair colour stakes with 113 over brown hair at 108, blonde at 107, red 106 and black 106. Blue eyes beat brown with 109 over 108.

However, the overall highest scorer, Keith Jowett from Surrey did not fit into the stereotypes, being a right-handed non-smoker living nowhere near the cleverest city dwellers of Leicester.

Any budding Einsteins who would like to increase that left-handers lead can still take part, as the BBC's Test the Nation website will run the test for the next 2 weeks and results are constantly being updated with new figures. Visit www.bbc.co.uk/testthenation to find out more.

Americans may also soon be brushing up their brainpower, as it is rumoured that presenter Anne Robinson, well known in the States for her hostile hosting of ‘The Weakest Link' has held talks with a US television company about taking the idea to the USA.

## Ultrasound scans and left-handedness

Controversial new evidence suggests that ultrasound scans on unborn babies can not only make them more likely to be left-handed, but may cause mild brain damage, particularly in boys.

In the most comprehensive study yet on the effect of ultrasound scanning, doctors have found that men born to mothers who underwent scanning were more likely to show signs of subtle brain damage. The implications of the study are to be raised at an international meeting of scientists being held in Edinburgh.

The work of the Swedish scientists, which has been published in the journal *Epidemiology,* backs up earlier research which was conducted in the 1990's suggesting ultrasound scanning affected unborn babies. Previous research had suggested subtle brain damage could increase the risk of conditions ranging from learning difficulties to epilepsy, as well as causing people who ought genetically to be right-handed to become left-handed. This conclusion is based on a theory that if the right-handed brain is in any way hindered from developing to become the dominant half of the brain, the left-hand brain will take over to compensate – in itself a theory still under investigation.

The Swedish team from the Karolinska Institute in Stockholm compared almost 7,000 men whose mothers underwent scanning in the 1970's with 172,000 men whose mothers did not, looking for differences in the rates of left and right-handedness. The team found men whose mothers underwent scanning were significantly more likely to be left-handed than normal.

The most significant difference was found among those born after 1975 when doctors introduced a second scan later in pregnancy. These men were 32% more likely to be left-handed. There are strong indications that, normally, left-handedness is genetic. The likelihood of two left-handed parents having a left-handed child has been put as high as 35%, while for two right-handed parents it is thought to be only 9%.

Reporting their findings, the researcher warned that scans in late pregnancy were now routine in many countries. “The present results suggest a 30% increase in left-handedness among boys pre-naturally exposed to ultrasound” they wrote. “If this association reflects brain injury, this means as many as one in 50 male foetuses pre-natally exposed to ultrasound are affected.” It is important to stress the “if” in this statement, as much research still needs to be undertaken to clarify whether any hindrance in right-brain development does lead to the left-brain taking dominance to compensate.

According to the Swedish team, the human brain undergoes critical development until relatively late in pregnancy, making it vulnerable to damage. The male brain is especially at risk, as it continues to develop later than the female one.

Professor Juni Pamgren, a member of the team, said “I would urge people not to refuse to have ultrasound scanning, as the risk of brain damage is only a possibility – but this is an interesting finding and needs to be taken seriously.”

Dr Francis Duck of the British Medical Ultrasound Society will chair a discussion of the results at the International Meeting of Ultrasound Experts being held this week in Edinburgh. “When the first study suggesting a link came out, it was possible to ignore it, but now this is the third”, he said. “What it demonstrates is the need to investigate the link further, and to look at possible mechanisms.” Dr Duck cautioned, however, that ultrasound scanning has saved the lives of countless babies: “This research must be seen in context, and it should not deter anyone from having an antenatal scan.

British obstetrics professionals have also tried to quash potential fears, saying the findings should be met with “extreme caution”. One critic of the research, Gordon Stirrat, Emeritus Professor of Obstetrics at Bristol University, notes the study was done in the 1970's when powerful, unsophisticated ultrasound was used. “Today's ultrasound is much more sophisticated,” he said. “There's no comparison.”

And even more significantly, in the 1970's ultrasound was reserved for pregnancies already suspect. “The males in the study might already have had abnormalities or a tendency to left-handedness before they had the ultrasound,” said Professor Stirrat. “I think we can be confident today's ultrasound is very safe.”

We will keep you informed of any further developments, but it must be stressed that this study focuses on only one of many situations that can result in a person being left-handed and is still in the very first stages, and needs further work to reach definite conclusions.

## Left-Handers Remember Events Better Than Facts

Members of a family that is dominated by left-handers tend to be better at remembering events than facts, according to research published yesterday, in the journal “Neuropsychology”.

Dr Stephen Christman and Dr Ruth Propper, of the University of Toledo in Ohio also think they may have a clue as to why few of us can remember things that happened in our lives before we were four years old. Dr Christman and Dr Propper studied two types of memory -episodic (the recall and recognition of events) and non-episodic (factual memory and implicit memory, which concerns things people “just know”). They concluded that the two halves of the brain work together in episodic memory to help to remember events because left-handers and those with left-handedness in their families recall events better than facts.

They also point out that the onset of episodic memory at around four years of age roughly coincides with the maturation of a structure called the “corpus callosum” that connects the two halves of the brain. More research is being conducted by the team to ascertain why episodic memory involves both halves of the brain whereas implicit memory appears to only be processed in one half.

## Inflammatory Bowel Disease

Recent research has shown that left-handers are twice as likely to suffer from Inflammatory Bowel Disease (also known as Crohns disease or ulcerative colitis) as right-handers. This was how the story was reported in the Cardiff Western Mail in July 2001

See our archive pages of left-handed research

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