Gender is a category system made up of many levels, many interwoven strands. Physiology defines the most fundamental level, designating people as male or female on the basis of their observable sexual anatomy, but every known society surrounds the basic facts of sexual form and function with a system of social rules and customs concerning what males and females are supposed to be and do. As children master and internalize this system, they learn to discriminate and label themselves and others on the basis of sex, to recognize attributes, attitudes, and behaviors that are typical of or considered appropriate for each sex, and to learn how to do what is seen as appropriate and to avoid what is not. What’s more, the gender category system is infused with affect to an extent few other knowledge bases can match, making it what is perhaps the most salient parameter of social categorization for the young child.