Defining Sex Differences
Because grades and overall test scores depend on many factors, psychologists have turned to assessing better-defined cognitive skills to understand these sex differences. Preschool children seem to start out more or less even because girls and boys, on average, perform equally well in early cognitive skills that relate to quantitative thinking and to knowledge of objects in their surrounding environment.
Around the time school begins, however, the sexes start to diverge. By the end of grade school and beyond, females perform better on most assessments of verbal abilities. In a 1995 review of the vast literature on writing skills, University of Chicago researchers Larry Hedges (now at Northwestern University) and Amy Nowell put it this way: “The large sex differences in writing … are alarming. The data imply that males are, on average, at a rather profound disadvantage in the performance of this basic skill.” There is also a female advantage in memory of faces and in episodic memory—memory for events that are personally experienced and are recalled along with information about each event's time and place.
There is another type of ability, however, in which boys have the upper hand, a skill set referred to as visuospatial: an ability to mentally navigate and model movement of objects in three dimensions. Between the ages of four and five, boys are measurably better at solving mazes on standardized tests. Another manifestation of visuospatial skill in which boys excel involves “mental rotation,” holding a three-dimensional object in memory while simultaneously transforming it [see illustration above]. As might be expected, these capabilities also give boys an edge in solving math problems that rely on creating a mental image.
Indeed, of all the sex differences in cognitive abilities, variation in quantitative aptitude has received the most media attention. This popular fascination is, in part, because mastery of these skills is a prerequisite for mathematically intensive disciplines such as physics and engineering. And, as Summers suggested, if women were disadvantaged in these skills, it would go a long way to explaining why women are typically underrepresented in these fields. But the data are much less clear-cut.
As we said before, females get higher grades in math classes at all grade levels and also do slightly better on international assessments in algebra, perhaps because of its languagelike structure. But boys shine on the math part of the SAT—resulting in a difference of about 40 points that has been maintained for over 35 years. When all the data on quantitative ability are assessed together, however, the difference in average quantitative ability between girls and boys is actually quite small. What sets boys apart is that many more of them are mathematically gifted.
At first, this statement seems almost paradoxical. If boys and girls are, on average, equally skilled at math, how could there be greater numbers of gifted boys? For reasons that are not yet fully understood, it turns out that males are much more variable in their mathematical ability, meaning that females of any age are more clustered toward the center of the distribution of skills and males are spread out toward the ends. As a result, men outnumber women at the very high—and very low—ends of the distribution. Data from the Study of Mathematically Precocious Youth exemplify this phenomenon. In the 1980s one of us (Benbow), along with the late psychologist Julian C. Stanley, who founded this study at the Johns Hopkins University Center for Talented Youth, observed sex differences in mathematical reasoning ability among tens of thousands of intellectually talented 12- to 14-year-olds who had taken the SAT several years before the typical age.
Among this elite group, no significant differences were found on the verbal part of the SAT, but the math part revealed sex differences favoring boys. There were twice as many boys as girls who achieved math scores of 500 or higher (out of a possible score of 800), four times as many boys who received scores of at least 600, and 13 times as many boys who had scores of at least 700 (putting these test takers in the top 0.01 percent of 12- to 14-year-olds nationwide).
Although it has drawn little media coverage, dramatic changes have been occurring among these junior math wizards: the relative number of girls among them has been soaring. The ratio of boys to girls, first observed at 13 to 1 in the 1980s, has been dropping steadily and is now only about 3 to 1. During the same period the number of women in a few other scientific fields has surged. In the U.S., women now make up half of new medical school graduates and 75 percent of recent veterinary school graduates. We cannot identify any single cause for the increase in the number of women entering these formerly male-dominated fields, because multiple changes have occurred in society over the past several decades.
This period coincides with a trend of special programs and mentoring to encourage girls to take higher-level math and science courses. And direct evidence exists that specifically targeted training could boost female performance even further. A special course created by engineering professor Sheryl A. Sorby and mathematics education specialist Beverly J. Baartmans at Michigan Technological University, for example, targeted improvement in visuospatial skills. All first-year engineering students with low scores on a test of this ability were encouraged to enroll in the course. This enrollment resulted in improved performance in subsequent graphics courses by these students and better retention in engineering programs, which suggests that the effects persisted over time and were of at least some practical significance for both women and men.