The next phase in the testing movement occurred between the 1930s and 1940s, when there was a shift in focus to a method known as factor analysis in intelligence testing. L. L. Thurstone, for instance, developed the Primary Mental Abilities Test (PMA), which is based on factor analysis applied to seven areas of variation in ability, including mathematical ability, spatial ability, and verbal ability. Researchers began to use this style of analysis and to apply it to research on gender differences in these areas. By the 1930s there were numerous textbooks in differential psychology that reviewed a fairly large body of data on presumed group differences in abilities by gender, race, class, and age. Anastasi and Tyler drew similar conclusions: that women were superior in verbal abilities across the lifespan, and that men excelled in spatial relations tests, but that this difference emerged later than differences in verbal skills.
71 Another set of differences believed to emerge later in men were mathematical skills requiring reason rather than computation. These perspectives have dominated the field of psychology of gender differences for many years. In their important and comprehensive review of studies of gender differences, the psychologists Eleanor Maccoby and Carol Jacklin reviewed thousands of studies on gender differences in cognition, personality and behavior. 72 They concluded that much research was methodologically flawed and they on this basis dismissed conclusions about differences in the areas of achievement motivation, self-esteem, and higher-level cognitive tasks. They agreed that gender differences were well-supported by research in four areas: girl's greater verbal ability, boys greater mathematical and visual-spatial abilities and greater aggression. Gender differences in these areas have been reified and widely taught to many generations of undergraduates in introductory psychology classes.
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