Handedness in twins: meta-analyses

Background
In the general population, 10.6 % of people favor their left hand over the right for motor tasks. Previous research suggests higher prevalence of atypical (left-, mixed-, or non-right-) handedness in (i) twins compared to singletons, and in (ii) monozygotic compared to dizygotic twins. Moreover, (iii) studies have shown a higher rate of handedness concordance in monozygotic compared to dizygotic twins, in line with genetic factors playing a role for handedness.

Methods
We identified 59 studies from previous literature and performed three sets of random effects meta-analyses on (i) twin-to-singleton Odds Ratios (21 studies, n = 189,422 individuals), (ii) monozygotic-to-dizygotic twin Odds Ratios (48 studies, n = 63,295 individuals), and (iii) concordance Odds Ratios (44 studies, n = 36,217 twin pairs). We also tested for potential effects of moderating variables such as sex, age, the method used to assess handedness and the twins’ zygosity.

Results
We found (i) evidence for higher prevalence of left- (Odds Ratio = 1.40) and non-right- (Odds Ratio = 1.36), but not mixed-handedness (Odds Ratio = 1.08) among twins compared to singletons. We further showed a reduction of difference over time, which suggests that higher levels of left-handedness observed in twins are linked to a higher frequency of birth complications which might have dropped nowadays in their overall prevalence because of better healthcare. While there was (ii) no difference between monozygotic and dizygotic twins regarding prevalence of left- (Odds Ratio = 0.98), mixed- (Odds Ratio = 0.96), or non-right-handedness (Odds Ratio = 1.01), we found that (iii) handedness concordance was elevated among monozygotic compared to dizygotic twin pairs (Odds Ratio = 1.11). Moderator analyses showed our results to be robust against potentially confounding variables.

Conclusion
We provide the largest and most comprehensive meta-analysis on handedness in twins. Results suggest that the previously reported effect that twins are more likely to be left-handed than singletons is robust. Furthermore, we also provide robust evidence that monozygotic twins are more likely to be concordant for handedness than dizygotic twins. In line with recent genome-wide association studies, this finding supports a partially genetic determination of handedness.