Handedness and birth order: a critical note on a critical note.

Abstract

Hicks, et al. ( 6 ) have combined five disparate studies of the relationship between birth order and handedness and, on the basis of an over-all test of significance, they argue that there is no relationship. Their pooling of data might be legitimate if it could be assumed that the five samples were drawn from the same population. In my (7, 10) artempt to account for two failures to confirm my reported ( 1 ) relationship between birth order and handedness, I have shown that differences between the samples studied are likely in saio-economic level, infant mortality, and rate of pregnancy and birth complications. These variables are crucial in determining whether or not a relationship exists between birth order and handedness. To quote from my 1977 paper ( 3 ) : The factor of bitth order .is not by itself important except for its correlation with pregnancy and birth complications. But the birth order/pregnancy complication relationship varies according to variables such as socio-economic status, infant mortality, nutritional factors, medical care, etc. If these variables are not equated in the samples combined by Hicks, et al., then their pooling procedure leads to an inference about a population which cannot be specified. More telling with respect to the etiology of deviations from right-handedness is the more direct relationship found berween pregnancy and birth complications (and the related hypoxia) and both left-handedness and ambidexterity (2 , 4 ) . Even more direct is the evidence from a prospective study by Barnes ( 5 ) showing that hypoxia at birth is a goad predictor of deviations from right.handedness at age three. Finally it should be noted that Hicks, et d. ( 6 ) have failed to cite or include in their analysis two reports in support of a relationship between birth order and handedness (8, 9) .