Gender is probably the most important individual difference in our society. Although the range of opportunities available to men and women continue to converge, the difference between social roles and styles remain striking. How do these differences develop? To what extent are they due to social norms and expectations or to biological differences? Definitive answers to these questions are still a long way off, but research on both the biological and social aspects of the development of gender has led to important insights. While thinking about differences between men and women, it is important to keep the distinction between sex and gender in mind. In psychology, the former is used to refer to the clear biological differences between men and women (e.g. reproductive organs, secondary sexual characteristics) and the latter is used inclusively to refer to the all of the differences between men and women, including the vast amount of differences that are due to social influences.

It is clear that there are almost always striking differences between the bodies of people who have two XX chromosomes and those who have an X and a Y (although there are some notable exceptions, such as when a genetic male is born with a genetic mutation that causes insensitivity to testosterone). These purely physical differences may themselves be responsible for some of behavioral differences that are observed. For instance, the fact that women have the capacity to carry a child influences some of the activities in which they may choose to involve themselves. More interesting are the differences in behavior that may be directly caused by sex differences in hormonal or neural systems. For instance, testosterone has been associated with aggression in males. The fact that females both have less testosterone than males and tend to be less physically aggressive suggests that hormonal differences are important determinants of behavior.

Eleanor Maccoby's research has shown that male and female children segregate themselves by sex very early in childhood. In their separate groups, they choose different kinds of games and establish different kinds of social hierarchies. Males may create larger sub-groups and engage in active and direct competition with each other. Female children, on the other hand, have been characterized as forming smaller, more intimate groups in which competition is more likely to take place on a social plane than on a physical one. While these differences may be instigated by biological differences--such as increased levels of aggression in males--they are continued and exaggerated by the social segregation of the two groups. Young girls and boys learn gender expectations and gender-appropriate behaviors by observing their parents, older children, and media representations of behavior.

The biology vs. society debate is never a very productive one, but it continually recurs in discussions of gender differences. A useful model to follow in understanding the interaction between the two is that slight gender differences--such as boys' slight preference for active competition--may create a basis for social influences that greatly amplify those original differences. Of course, biology does not drop out of the picture entirely; it continues to influence, and be influenced by, experience.