The most common alternative to a genetic explanation of left-handedness is that sinistrality arises because of birth stress factors. In a sample of 1398 subjects, the association between birth stress and left-handedness was confirmed. More importantly, it was found the left-handed mothers are more likely to have birth-stressed offspring and that the presence of any left-handed sibling increases the likelihood of a history of birth stress in the proband. This was interpreted as suggesting that a plausible alternative to the genetic explanation for the usual pattern of association observed in family studies of handedness (where a left-handed mother increases the probability of left-handed offspring) is that the mother-offspring association may actually be mediated by birth stress rather than representing heritable aspects of handedness.