There is a growing awareness among researchers that the magnitude of cognitive sex differences is affected by a number of subject variables. To examine spatial and verbal cognitive sex differences as a function of personal and family handedness, the 478 offspring who participated in the Minnesota family study and 454 offspring who participated in the Texas family study were tested. Results from these studies are contrasted with those presented by Hyde (1981) in her reanalysis of studies reviewed by Maccoby and Jacklin (1974). In aggregate, the results from the studies reviewed, which range in number of subjects studied from 44 to 45,222, show that the approximate magnitude of sex difference in spatial abilities is a .50 standard deviation (SD); the approximate magnitude of sex difference in verbal abilities is .25 SD. Cognitive sex differences explain only a small proportion of the total variation among individuals; however, small mean sex differences are shown to generate large differences in the proportion of males to females at the tails of the distributions for spatial and verbal cognitive abilities. These results have practical implications for career counseling and the study of developmental reading disabilities. (Author/JAC)