Are Twins More Likely to Be Left-Handed?
Understanding left-handedness in twins and triplets.
Posted Sep 20, 2019
About 3.3 percent of Americans are twins, with identical twins being much rarer (only about 0.3 percent of the population) than non-identical twins. Twins have much that's special about them, but one somewhat surprising finding about twins is that twinning seems to affect left-handedness. Here's what you need to know.
Are twins more likely to be left-handed than singletons?
Yes! An analysis of 19 different studies on left-handedness in twins (McManus, 1980) had shown that 15.09 percent of identical twins and 12.08 percent of non-identical twins were left-handers. Large-scale meta-analyses in the general population have shown that here, the overall frequency of left-handedness is about 9 percent (Papadatou-Pastou et al., 2019). So it's true—twins indeed have a slightly higher chance of being left-handed than the rest of us.
Is there a difference in the frequency of left-handedness between identical and non-identical twins?
No! In 1999, Nancy L. Sicotte, Roger P. Woods and John C. Mazziotta from the UCLA School of Medicine conducted a so-called meta-analysis on left-handedness in identical and non-identical twins (Sicotte et al., 1999). A meta-analysis integrates the results of several empirical studies; the advantage is that the sample size is much larger, increasing statistical power and rendering the analysis less likely to be affected by sample characteristics of individual studies.
Overall, Sicotte and colleagues statistically integrated 28 studies that included 9,969 twin pairs overall. They statistically compared the overall frequency of left-handedness in identical and non-identical twins to see whether there was a difference between the two groups. Despite the somewhat higher percentage of left-handers in identical compared to non-identical twins in the earlier study by McManus (1980), the analysis by Sicotte and colleagues showed that there was no difference in the frequency of left-handedness between identical and non-identical twins pairs.
Do twins always have the same handedness?
So, for example, if one twin is left-handed, is the other one always left-handed, too? This idea is based on the fact that identical twins develop from the same egg cell initially and are thus genetically identical. If left-handedness were determined 100 percent by genes, it would, therefore, be very likely that identical twins both have the same handedness.
Interestingly, Sicotte et al. (1999) found that identical twins were more likely to have the same handedness than non-identical twins. However, even identical twins do not always have the same handedness. About 20 to 25 percent of identical twin pairs do not have the same handedness (Gurd et al., 2006). So no—twins do not always have the same handedness.
What about triplets?
If twins have a higher chance of being left-handed, what about triplets? Do they have an even higher chance of being left-handed? A Finnish study that analyzed two large triplet cohorts (one from Japan and the other one from the Netherlands) found that in the Japanese sample, the chance of a triplet being left-handed was 11.9 percent, while in the Dutch sample it was 15.5 percent (Heikkilä et al., 2015). This indicates that triplets have a higher chance of being left-handed than the general population, but a similar chance of being left-handed compared to twins.
Interestingly, this study also identified a factor that could contribute to left-handedness in triplets: low birth weight. On average, triplets weight 1.5 kg less than singletons. In both the Dutch and the Japanese samples, left-handed triplets had a lower birthweight than right-handed triplets. Thus, developmental effects associated with low birth weight could affect left-handedness.
Gurd JM, Schulz J, Cherkas L, Ebers GC. (2006). Hand preference and performance in 20 pairs of monozygotic twins with discordant handedness. Cortex, 42, 934-945.
Heikkilä K, Van Beijsterveldt CEM, Haukka J, Iivanainen M, Saari-Kemppainen A, Silventoinen K, Boomsma DI, Yokoyama Y, Vuoksimaa E. (2018). Triplets, birthweight, and handedness. Proc Natl Acad Sci USA, 115, 6076-6081.
McManus IC. (1980). Handedness in twins: a critical review. Neuropsychologia, 18, 347-355.
Papadatou-Pastou, M., Martin, M., Munafo, M. R., Ntolka, E., Ocklenburg, S., & Paracchini, S. (2019). Papadatou-Pastou, Martin, Munafò, Ntolka, Ocklenburg, & Paracchini. The prevalence of left-handedness: Five meta-analyses of 200 studies totaling 2,396,170 individuals. PsyArXiv, preprint.
Sicotte NL, Woods RP, Mazziotta JC. (1999). Handedness in twins: a meta-analysis. Laterality, 4, 265-286.