A Reply to Halpern's Commentary: Theory-Driven Methods for Classifying Groups Can Reveal Individual Differences in Spatial Ability within Females

Abstract

In Halpern's commentary, she strongly supports our theoretical and methodological framework for addressing the ways in which biological and environmental factors interact to influence spatial ability in females (see my review in this issue). I have reanalyzed a data set using two classification methods, to address Halpern's specific criticism that another researcher (McKeever, 1991) has found a pattern of results different from ours. The right-handed females from a suburban high school sample were given the Vandenberg Mental Rotation Test, the Edinburgh Handedness Inventory, and a family handedness questionnaire. When these females were divided into familial handedness subgroups based on the criteria used by McKeever, no significant differences were obtained. However, using a theory-driven classification system based on Annett's genetic theory, I found significantly reduced spatial ability among the predicted subgroup of right-handed females. In this reply: (1) I also address Halpern's main criticism regarding the genetic basis of handedness (which is the underlying assumption of Annett's theory), and (2) I have described a mechanism which may account for the impact of both genetic and environmental factors on individual differences in handedness and pattern of fetal brain development.