Middle Childhood Gender Identity
The development of gender identity starts as early as age 2 ½ right after toddlers have enough language to be able to label themselves as boys and girls. Next, by age 3 or children realize that their sex does not change overtime, which is important for the final realization at age 6 or 7 that gender does not change at all; once a boy, always a boy and the same for girls. There is so much research on diverse aspects of human behaviors in relation to gender but perhaps the most solid and useful information in trying to understand gender identity development is the observation that boys like to spend more times with boys and girls with girls, which might strengthen their sense of their gender.
Sex differences seem to be the result of complex interaction of biological factors, cognitive gender schemas, parenting styles and peer relationships.