Gender differences in cognitive, social, and personal characteristics have been investigated since the early 1900s. Research has identified differences in several specific cognitive skills as well as in a range of social and personal characteristics. Some differences are apparent from infancy; others do not emerge until late childhood or adolescence. Interestingly, in several skills the differences between boys and girls have shrunk over the last two to three decades. This indicates that socialization and differential experiences play roles in gender differences. Even when gender differences are significant and consistent over time, we still do not fully understand why they exist. Different experiences and socialization are almost certainly involved, but biological factors may also have important effects.

However, the most striking finding in the study of gender is that in most areas the similarities between girls and boys far outweigh the differences. One comprehensive review found that of the 124 meta-analyses included (which represented over 7,000 individual research reports investigating a wide range of cognitive, social, and personality variables), 78% showed small or close-to-zero effect sizes—this indicates few statistical differences between males and females in these studies (Hyde, 2005, 2006). For some variables, context affects whether gender differences were found. For example, when participants were told that gender differences had been found on previous administrations of a math test, males taking the test performed better than females. In contrast, when the participants were told the test was gender-fair, no gender differences were found (Spencer, Steele, & Quinn, 1999). Similar context effects have been found for both aggressive and helping behavior (Hyde, 2005). While it is important to understand how, when, and why gender differences exist, it is equally important to know when they do not exist so that neither girls nor boys are kept from developing their individual potentials.

Cognitive Skills

In cognitive skills, the largest and most consistent gender differences are found in verbal, language, and certain spatial skills. For example, girls tend to produce words at an earlier age, have a larger vocabulary, and show a higher level of language complexity beginning in early childhood (Feingold, 1993; Halpern, 2000; Hyde & Linn, 1988). The biggest differences in verbal skills during school-age years—all favoring girls—are in spelling, overall language measures, and writing. Some of these gender differences seem to get smaller during adolescence, whereas differences in other areas (e.g., writing) remain (Halpern, 2000). These differences have remained relatively stable over 30 or more years of research. Differences in other specific skills tend to be small, and some have decreased in recent decades (Campbell, Hombo, & Mazzeo, 2000). Clear and consistent gender differences favoring males exist for some spatial skills such as mental rotation (the ability to visualize how an object would look if you viewed it from a different angle). Differences in these areas emerge at around 9 to 13 years and widen throughout adolescence. Like the verbal skills we discussed above, gender differences in mental rotation have remained stable over the last few decades (Masters & Sanders, 1993; Voyer, Voyer, & Bryden, 1995).

Is it true that boys are better than girls in mathematics, as so many people seem to believe? It depends on students' ages and skill levels, as well as on the particular area of mathematics being assessed. The only consistent differences found in elementary school favor girls, both for computation and for grades in math (Halpern, 2000). Girls continue to earn higher grades in math throughout the school years, but their superior performance in computation disappears after about age 15. In studies of very talented populations, boys perform better on several mathematics skills. Gender differences favoring boys appear at adolescence and increase during the high school years, but only in areas involving mathematics problem solving. Since the late 1970s boys have consistently scored about 10% higher than girls on the math portion of the SAT (a standardized test required by many colleges for admission). On national assessments of 4th, 8th, and 12th graders, however, math gender differences have decreased since the early 1980s. The most recent national assessment shows small differences for all grade levels (2 point differences at each grade level, out of 500 points for 4th and 8th graders and out of 300 points for 12th graders) (Byrnes, 2001; Grigg, Donahue, & Dion, 2007; Perie, Grigg, & Dion, 2005). Some studies find that girls hold less positive attitudes toward math, show less interest in math, and receive less encouragement for engaging in math-related activities; but the gender difference in taking higher-level math courses such as calculus has declined (Eccles, Wigfield, & Schiefele, 1998; Maccoby, 1998; Perie, Moran, & Lutkus, 2005).

Social Behavior and Personality Traits

Prevailing stereotypes in Western cultures portray boys as more active and aggressive and girls as more emotional and helpful. The research evidence supports some of these images, but not all. On average, boys do show higher activity levels than girls from infancy onward. They are more likely to engage in outdoor play, rough play, and activities that cover large areas of physical space (Eaton & Enns, 1986; Lindsey, Mize, & Pettit, 1997; Maccoby, 1998). Girls, on the other hand, perform better on tasks involving flexibility and fine-motor coordination. These differences increase with age. Both girl and boy infants explore new objects, but they tend to use different strategies for doing so. Boys are more likely to handle a new object physically; girls are more likely to use visual exploration, looking carefully at a novel object without actually touching it. Interestingly, male and female infants show different reactions when left alone to explore. Boys are more likely to explore objects and become more independent, while girls show less exploration and greater attempts to establish or maintain contact with their caregiver (e.g., not letting go of their parents, reaching for the door their parent left through, sitting at the door crying) (Mayes, Carter, & Stubbe, 1993).

What about aggression and assertiveness? These two categories of behavior differ by whether they can hurt others; aggression is directed against someone or something, whereas assertiveness may be defined as speaking up for oneself, being self-confident. Beginning at an early age, boys show more physical aggression, such as hitting or kicking, than girls; this difference continues throughout childhood and into adulthood (Coie & Dodge, 1998; Hyde, 2005). Boys also show higher levels of assertiveness than girls, though the difference is not as great as for physical aggression (Feingold, 1994). Gender differences like these have been found in several studies of girls and boys in numerous countries (Bettencourt & Miller, 1996; Coie & Dodge, 1998; Maccoby & Jacklin, 1974). But the gender gap may be closing for physical aggression, at least among 10- to 17-year-olds. In 1993, adolescent boys reported seven times more violent behavior than girls. By 1998, the ratio had gone down to 3.5 to 1. Arrest rates showed the same trend—in 2003, females accounted for 18% of all arrests of juveniles for violent crimes, up from 10% in 1980 (Snyder & Sickmund, 2006; U.S. Department of Health & Human Services, 2001). It's also important to remember that aggression can take different forms; it does not have to be physical. Relational aggression seeks to hurt others through social means such as name-calling or exclusion. Girls are significantly more likely than boys to show relational aggression (Crick & Grotpeter, 1995).

Researchers have not found consistent gender differences in prosocial behavior or emotions. Girls often receive ratings from others, and evaluate themselves, as more helpful, cooperative, and sympathetic than boys, but their actual behavior is not consistently different from that of boys. However, girls are more likely to seek and to receive help than are boys, and some studies indicate that girls are more easily influenced than boys (Eisenberg & Fabes, 1998; Ruble & Martin, 1998). When attempting to influence others, boys are more likely to use threats and physical force. Girls tend to use verbal persuasion or, if that does not work, simply to stop their efforts to influence the other person (Serbin, Moller, Gulko, Powlishta, & Colburne, 1994).

Some researchers suggest that male infants are more emotionally reactive than female babies, but that culture socializes boys to express less emotion as they get older (with the possible exception of anger). As a result, boys become less skilled at understanding both their own and others' emotions. As this view predicts, research shows that by adolescence there are clear gender differences in the expression of emotions, particularly of negative ones. For example, girls are more likely to show symptoms of depression or anxiety and to attempt suicide; boys are significantly less likely to report that they experience sadness, shame, or guilt. However, boys are significantly more likely to actually commit suicide. It seems that adolescent boys learn to bear their negative feelings alone and in silence, with potentially deadly results (Eisenberg, Martin, & Fabes, 1996; Kindlon & Thompson, 2000).