Does Left-Handedness Run in Families?

Is it true that left-handed parents are more likely to have left-handed kids?

Posted Jun 07, 2019

It is often said that left-handedness runs in families, and most lefties know one or more family members that are also left-handed. So is there any scientific data supporting these anecdotal reports?

Turns out, there is.

In a German 1999 study (Reiss and Reiss, 1999), the authors investigated the distribution of handedness in 292 families and found a relation between left-handedness in parents and children. They observed that overall, children had a chance of 9.7 percent of being left-handed. However, parental handedness strongly influenced the chance of a child being left-handed. If both mother and father were right-handed, the chance of their offspring being left-handed was only 7 percent. However, if one parent was left-handed, the chance of the offspring being left-handed, too, was 21.4 percent, more than three times as much as for two right-handed parents.

A statistical integration of 25 family datasets that included handedness data from more than 72,600 individuals also showed a clear family effect (McManus and Bryden, 1992). Here it was shown that two right-handed parents had a chance 9 percent to raise a left-handed child. This chance increased to 19 percent if at least one parent was left-handed. If both parents were left-handed, the chance of their offspring also being left-handed was highest: 26 percent.

This indicates that children of two left-handed parents have a higher chance of being left-handed, but also that three-quarters of them are still right-handed. So how do parents influence their children’s’ handedness? There are two possibilities: On the one hand, this could be a genetic effect, i.e., the child inherits a specific genetic configuration from their parents that increases their chance of being left-handed. On the other hand, it could be a learning effect, i.e., the child observes how the parents use their hands and imitates them. Of course, a combination of both factors is also possible.

One way to differentiate between these possibilities is through adoption studies. If the effect of parental left-handedness on offspring left-handedness is mediated mainly by non-genetic factors like imitation learning, biological and adoptive children should be affected by parental handedness in the same way. In contrast, if the effect of parental left-handedness on offspring left-handedness is mainly mediated by a specific genetic setup, only biological children—but not adoptive—should be affected by parental handedness.

This was exactly what Louise Carter-Saltzman from the Department of Psychology at the University of Washington investigated in a famous 1980 study (Carter-Saltzman, 1980). She found a clear effect: Left-handedness in children was significantly related to left-handedness in biological parents, but not in adoptive parents. For biological parents, she found that if both parents were right-handed, their offspring had an 11 percent chance of being left-handed. If one parent was left-handed, this chance increased to 25.5 percent, on average. In adoptive children, two right-handed parents had 14 percent chance of raising a left-handed child. If one parent was left-handed, this chance increased only slightly: 15 percent, on average.

Statistical analysis revealed that only for biological parents, but not for adoptive parents, it was possible to predict offspring handedness based on parental handedness. This shows that the increase in left-handedness among children that have at least one left-handed parent is likely due to genetic factors, rather than learning mechanisms.

References

Carter-Saltzman L. (1980). Biological and sociocultural effects on handedness: comparison between biological and adoptive families. Science, 209, 1263-1265.

McManus IC, Bryden MP. (1992).  The  genetics  of  handedness, cerebral  dominance,  and  lateralization. In:  Boller  F,  Grafman  J, editors. Handbook of neuropsychology, 6, 115-143.

Reiss M, Reiss G. (1999). Earedness and handedness: distribution in a German sample with some family data. Cortex, 35, 403-412.