From an early age, boys and girls can be easily distinguished according to sex. Although this is partly due to appearance and the way in which their parents dress them and cut their hair, it is also because of the things that children do. Boys and girls like to play with different toys, prefer different games, and engage in different activities. Long before they reach school age, it is possible to tell with a reasonable degree of accuracy whether a child is a boy or a girl simply on the basis of his or her behavior. This does not mean that all boys engage in male -typical activities, or that all girls engage in female -typical activities, all of the time. There is a great deal of overlap between the sexes, with some girls seeming quite " boyish " and some boys seeming quite " girlish. " Although there is considerable variation in the behavior of children within each sex, it is generally more acceptable for girls to behave like boys than it is for boys to behave like girls. This may explain why the term tomboy used to describe masculine girls is often used endearingly, whereas sissy is a much more derogatory term when applied to feminine boys.
What exactly are the differences in behavior shown by boys and girls? And how do these differences develop? These are the questions that will be addressed in this chapter. First, sex differences in social behavior will be described from the preschool to the elementary school years. This will be followed by a consideration of the various theories that have been put forward to explain this phenomenon. The sections on biological theories draw from Collaer and Hines (1995) and Hines (2004Hines ( , 2009) ) , whereas those on psychological theories draw from Golombok and Fivush (1994) . As will be seen, some theories have greater empirical support than others. Following Maccoby (1988) , the terms sex and gender will be used interchangeably throughout the chapter without any assumption that sex implies biological causes or that gender results from socialization.