Handedness and birth order.

Abstract

IN spite of much speculation on the matter, the aetiology of left-handedness remains elusive. Investigators have considered heredity, neurological pathology, imitation and negative personality, but the evidence in support of any particular hypothesis is inconclusive. There are, however, some suggestive facts. The frequency of left-handedness is greater in males and in twin births, both of which are also associated with greater birth and infant mortality and, in the case of males, a higher rate of spontaneous abortion. The pre-natal and peri-natal periods seem to be more stressful for these groups. Left-handedness is also associated with language disorders such as stuttering, dyslexia, and mental retardation, conditions where central nervous system pathology may be implicated1. This suggests that the incidence of left-handedness might be correlated with stressful pre-natal and birth conditions. Such difficulties are most characteristic of primiparous births (longer labour and more use of instruments) and births to older mothers. Thus high risk birth orders would be the first born and the late-born (defined here as fourth or later birth).