Relative Size versus Controlling for Size

Abstract

The corpus callosum is the main tract of fibers providing communication between the left and right cerebral hemispheres. The question of a sex difference between human males and females in the size of the corpus callosum relative to the size of the brain has been actively debated for almost 20 years. One group of investigators notes that when a ratio is calculated with corpus callosum size in the numerator and brain size in the denominator, the value is, on average, larger in females. The other group argues that ratios are misleading on this question and that better statistical methods, such as analysis of covariance, do not indicate a sex difference. I show that this debate has been unnecessary. The issue is resolved by recognizing the difference between statistical control, in which focus is on a single trait for which a second trait is defined as a confounding effect, and proportionality, which focuses on the relationship between two traits-the relationship that statistical control serves to remove. The calculation of ratios is not effective for statistical control but is essential for the evaluation of proportionality. Interpretation of the ratio is affected in important ways by dimensionality-the units of the numerator and denominator (lengths, areas, or volumes )-and this too has been mostly overlooked. A meta-analysis of published data indicates that the corpus callosum is relatively larger in females. However, the mean difference between sexes is small, males and females mostly overlap in relative size, and we do not know what the biological implications might be. [ABSTRACT FROM AUTHOR] Copyright of Current Anthropology is the property of Wenner-Gren Foundation and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)