The familiar nursery rhyme telling us what little boys and girls are made of suggests that each sex is made of fundamentally different things. When we say “boys will be boys,” we generally mean there are certain ways we expect boys to behave simply because they are male, for example, to be high-spirited, aggressive, and so on. On what do we base such expectations and beliefs? Are there sex-specific ways of behaving that are acquired by being born male or female or through growing up as a boy or girl? If there are differences, are they biologically determined or socially constructed?