Can occur any time, by being aroused to certain stimuli
At puberty, the testes begin to secrete androgens (male sex hormones) which produces secondary sex characteristics like growth of bodily hair and change in tone of voice
People who have literally changed their sex, surgically and hormonally altered so that they appear to be a sex different from the one they were born in
Gender roles vary over time, in 1965 the Harvard Business School had never granted a degree to a woman, but at the turn of the twenty-first century, 30 percent of its graduates were women