Sex Differences in Left-Handedness Are Also Evident in Scandinavia and in Twins: Comment on Papadatou-Pastou, Martin, Munafo, and Jones (2008)

Acknowledgement: This research was supported by the Centre of Excellence in Complex Disease Genetics, Academy of Finland, Helsinki, Finland. Eero Vuoksimaa was supported by the Finnish Cultural Foundation (Helvi and Arvo Lehtovaara Foundation).

In their extensive and well-conducted meta-analysis of sex differences in left-handedness Papadatou-Pastou, Martin, Munafò, and Jones (2008) stated that the sex difference was moderated by testing location and year of study publication. With respect to geographic location, these authors concluded that sex differences in left-handedness do not exist in Finland and other Scandinavian countries. A key speculation in this regard is that this lack of sex difference could be a result of extremely low levels of cultural masculinity in Scandinavian societies.

We note, preliminarily, that the Scandinavian studies included in the Papadatou-Pastou et al. (2008) meta-analysis (i.e., Beckman & Elston, 1962; Götestam, 1990; Kauranen & Vanharanta, 1996; Levander & Schalling, 1988; Ofte, 2002) comprise small, non–population-based samples (each of which has an n < 1,000). Additionally, some key details should be taken into consideration when making interpretations from these studies. One Norwegian study had investigated handedness among students where 63% of the participants were students of architecture or music (Götestam, 1990). The prevalence of left-handedness was higher in students of architecture or music compared with the general student group, and most of the students of architecture (61.7%) were female, whereas in the general student group, half of the participants (50.6%) were female. Thus, these data might be biased and inappropriate for studying the sex difference in the prevalence of left-handedness in the general population. Furthermore, Papadatou-Pastou et al. included one Finnish study (Kauranen & Vanharanta, 1996) and reported a male-to-female odds ratio of 0.18 for left-handedness for this investigation. However, the study of Kauranen and Vanharanta (1996) included 100 male and 100 female participants; of these two groups, 7 male and 7 female participants were left-handed. These numbers should equal an odds ratio of 1.00. Finally, one Swedish study (Beckman & Elston, 1962) did not report how handedness was determined, which makes it difficult to draw conclusions from this investigation.

The overall purpose of the present comment is to report on sex difference in left-handedness in four population-based Scandinavian samples not included in the meta-analysis by Papadatou-Pastou et al. (2008). As shown below, we do find, in analyzing data from these samples, that males exhibit a greater tendency toward left-handedness than females.

We have recently studied handedness in a large population-based Finnish sample of singletons, twins, and triplets (Vuoksimaa, Koskenvuo, Rose, & Kaprio, 2009). Our study showed that in 4,068 singletons there exists significant sex difference in the prevalence of left-handedness: 5.3% in males as compared with 3.7% in females (p < .001). The male-to-female odds ratio, with 95% confidence intervals (CIs), was 1.46 [1.08, 1.97], which is greater than the figure of 1.23 reported in the Papadatou-Pastou et al. (2008) meta-analysis. We further investigated the prevalence of left-handedness in males and females separately in different age groups. Self-reported childhood left-handedness was more common in males in all age groups; similarly, self-reports of left-handed writing in adulthood were more common in male than female individuals in all age groups, with the exception of ages 60–69 years (see Table 1, which presents previously unpublished data on the prevalence of left-handedness in male and female individuals by age groups).

Our study also included information about forced right-handedness. When we excluded those participants who were forced to use their right hand and also those participants who reported inconsistent handedness from childhood to adulthood, the sex difference in left-handedness remained (4.4% in males vs. 2.4% in females), χ2(1) = 10.63, p < .01. We also found that there was no significant difference, χ2(1) = .24, p = .62, in forced right-handedness between male (49%) and female (52.3%) participants, which could be interpreted as an equal social pressure for the use of the right hand in males and females (Vuoksimaa et al., 2009). The proportion of forced right-handedness within left-handers for different age groups is also displayed in Table 1. These data show that the sex difference in left-handedness in Finland is quite consistent across cohorts and that it remains after controlling for forced right-handedness and inconsistent handedness.

In addition, one previous study including 70% of persons born in 1966 in the northern Finland (provinces of Oulu and Lapland) reported that 7.9% of males and 6.1% of females were left-handed (Pekkarinen, Salminen, & Järvelin, 2003), yielding a significant male-to-female odds ratio of 1.33 (95% CI = 1.12, 1.57).

Furthermore, Papadatou-Pastou et al. (2008) did not include studies with participants under age 16 years, leading to the exclusion of two additional population-based studies of the prevalence of left-handedness in both Norwegian (Salvesen, Vatten, Eik-Nes, Hugdahl, & Bakketeig, 1993) and Swedish (Kieler, Axelsson, Haglund, Nilsson, & Salvesen, 1998) samples of children ages 8–9 years. Both studies have investigated whether ultrasound exposure during pregnancy increases the probability of non–right-handedness. A prior meta-analysis of these two studies indicated that ultrasound exposure may increase the prevalence of non–right-handedness in boys (Salvesen & Eik-Nes, 1999). Because ultrasound exposure may affect handedness, we looked at the prevalence of non–right-handedness by sex within the participants from control groups of non–ultrasound-exposed individuals.

The Norwegian study included 802 participants who were not exposed to ultrasound prenatally. Here, 17.3% of boys and 12.5% of girls were non–right-handed, yielding a male-to-female odds ratio of 1.46 (95% CI = 0.98, 2.16), which fell just short of traditional levels of statistical significance. The Swedish study included 1,508 participants who were not exposed to ultrasound prenatally. In this instance, 17.1% of boys and 14.6% of girls were non–right-handed, yielding a male-to-female odds ratio of 1.21 (95% CI = 0.92, 1.60), also nonsignificant but in the expected direction. According to a literature search, these studies were the only population-based samples to have studied the sex difference in left-handedness, as measured with preference, in Norway and Sweden, although we note that both studies did not include high-risk pregnancies. Yet, this is not considered to be a problem, according to a study of 25 potential birth- and pregnancy-related risk factors, where maternal age had a weak association to handedness, accounting for only a tiny percentage of the variance in children's handedness (Bailey & McKeever, 2004).

Hand preference is known to be fairly undifferentiated at birth (Ottaviano, Guidetti, Allemand, Spinetoli, & Seri, 1989) but to develop during childhood to its ultimate adult form by approximately 6 years of age (Bryden, Pryde, & Roy, 2000; De Agostini, Paré, Goudot, & Dellatolas, 1992). The exact age at which adult handedness is established is not clear and depends on the measure in which handedness is studied (see Carlier, Doyen, & Lamard, 2006; De Agostini et al., 1992). Papadatou-Pastou et al. (2008) did not include participants under 16 years of age in order to avoid possible developmental effects, yet they also stated that “handedness is usually considered to be established around 3–7 years of age” (p. 680). Our own data of 532 Finnish individuals reveal that writing handedness at 14 years of age is a good predictor of handedness, as measured by the Edinburgh Handedness Inventory, at ages 21–24 years, with a receiver operating characteristic curve value of 0.944 (Vuoksimaa et al., 2009). These data therefore indicate that handedness is a stable phenotype from adolescence onwards. In short, the prevalence data from Norwegian and Swedish studies are likely to be reliable and therefore usable for studying the sex difference in non–right-handedness in Scandinavian samples.

On the basis of these four Scandinavian samples that were not included in the meta-analysis of Papadatou-Pastou et al. (2008), we calculated the Mantel-Haenszel combined odds ratio (Mantel & Haenszel, 1959), which yielded a statistically significant, χ2(1) = 21.61, p < .001, male-to-female odds ratio of 1.34 (95% CI = 1.18, 1.51). In short, the expected sex difference in left-handedness is evident in Scandinavian countries.

As Papadatou-Pastou et al. pointed out, the way in which handedness has been assessed can have an effect in the size of the sex difference. Therefore, we tested whether there is heterogeneity in the four new Scandinavian samples we reported. Although the Norwegian and Swedish studies categorized child participants with regard to right- versus non–right-handedness with questionnaires, and the two Finnish studies categorized adult participants with regard to left- versus right-handedness, the Mantel-Haenszel test of homogeneity indicated no heterogeneity across these four Scandinavian studies, χ2(3) = 1.04, p = .79. Furthermore, in our data, equal proportions of left-handed males and females were forced to use their right hand for writing, and equal proportions of these males and females actually reported using their right hand for writing: 73.2% of males as compared with 76.1% of females, χ2(1) = 0.12, p = .73. In short, the sex difference in left-handedness remained after controlling for forced right-handedness.

Papadatou-Pastou et al. (2008) found that the magnitude of the sex difference in left-handedness increased as a function of national masculinity and concluded that the lack of sex difference in Scandinavian countries could result from low levels of cultural masculinity in these countries. Whereas such cultural effects on handedness are unquestionable (see, e.g., Medland, Perelle, De Monte, & Ehrman, 2004), our finding of sex differences in left-handedness in Scandinavian countries challenges the view that low cultural masculinity could lead the sex difference in left-handedness to disappear in Scandinavia.

Although the association between left-handedness and low levels of cultural masculinity is questioned in Scandinavian countries, a large-scale Internet study of handedness showed that the sex difference in left-handedness can vary by country within samples from the same ethnicity group (Peters, Reimers, & Manning, 2006). There was no difference in the prevalence of left-handedness between Chinese males (8.5%) and females (8.4%) from Singapore, whereas the greatest sex difference for Chinese males (10.3%) and females (4.4%) was evident in participants who responded from China. These data appear to support the cultural masculinity hypothesis of Papadatou-Pastou et al. (2008). Indeed, the prevalence of left-handedness in male and female Chinese participants, respectively, from other countries reported in Peters et al. (2006) were as follows: 9.1% vs. 5.7% for Canada, 8.7% vs. 6.1% for the United Kingdom, and 7.6% vs. 5.3% for the United States. Interestingly, across these countries, the lowest cultural masculinity score of 48 (as rated by Hofstede) was obtained in Singapore, whereas the highest score of 66 was found in China and the United Kingdom. Canada and the United States have masculinity scores of 52 and 62, respectively (masculinity scores obtained from http://www.geert-hofstede.com/hofstede_dimensions.php). In all, we note that Peters et al. (2006) investigated handedness in 255,100 subjects from seven ethnic groups from more than 50 countries, which would make this a useful data set on which to test the association between the magnitude of sex difference in left-handedness and Hofstede's cultural masculinity score.

It is important to note that many of the studies included in the Papadatou-Pastou et al. (2008) meta-analysis included fewer than 1,000 participants. Furthermore, one individual study (Gilbert & Wysocki, 1992) included 1,177,507 participants, accounting for 66% of all participants in the entire meta-analysis. The male-to-female odds ratio in this study of participants from the United States was 1.31. Prior to Papadatou-Pastou et al. (2008)'s study, one meta-analysis of sex difference in left-handedness had been published. Sommer, Aleman, Somers, Boks, and Kahn (2008) included 43 studies and reported an odds ratio of 1.25, which is in line with the Papadatou-Pastou et al. meta-analysis.

Finally, Papadatou-Pastou et al. (2008) did not include studies with twins, but the sex difference in the prevalence of left-handedness appears to exist in twins as well. Although in our study twins were more likely than singletons to be left-handed, a parallel sex difference was also evident in 25,799 twins (8.9% in males vs. 7.3% in females; Vuoksimaa et al., 2009). Other Scandinavian twin studies have indicated that the sex difference in left-handedness also exists in Denmark (9.2% in males vs. 6.5% in females, n = 2,509; Basso et al., 2000), and in Norway (7.2% in males vs. 5.2% in females, n = 976; Tambs, Magnus, & Berg, 1987). Moreover, a recent study of more than 54,000 twins and their siblings, which did not find difference in left-handedness between singletons and twins, reported similar a sex difference in Australian (14.6% in males vs. 12.5% in females) and Dutch (17.6% in males vs. 15.2% in females) samples (Medland et al., 2009).

The fact that a sex difference in left-handedness also exists in twins suggests that the results obtained from twin samples can be generalized to nontwin populations. Thus, twin studies should be included when conducting handedness studies, because they often comprise population-based samples with large numbers of participants. Furthermore, such investigations enable such methods as classical monozygotic–dizygotic comparisons in concordance levels, case-control studies of handedness in discordant twin pairs, and genotyping for candidate genes (see Boomsma, Busjahn, & Peltonen, 2002).

Overall, we conclude that the sex difference in left-handedness reported in Papadatou-Pastou et al. (2008) also exists in Scandinavian countries when analyzed in population-based samples and, additionally, occurs in twins. Although cultural masculinity may be associated with the magnitude of sex difference in left-handedness, the data from Scandinavian countries suggest that male individuals are more likely to be left-handed even in countries that exhibit extremely low levels of cultural masculinity. Our results support the view that the causes that lead to sex difference in left-handedness are most likely of biological origin (e.g., genetic or hormonal), although cultural effects cannot be ignored. Further studies are needed to investigate the causes for sex differences in the prevalence of left-handedness.

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Submitted: October 28, 2009 Revised: December 10, 2009 Accepted: December 21, 2009