Since mid-century, dramatic changes in family demographics have characterized patterns of parenthood and sexual partnerships in America. As age at marriage has increased, the age at initiation of sexual inter course has decreased so that adolescents and young adults are spending several years sexually experienced but not married. Cohabitation is becoming a common experience during this stage of their lives. The age at which people start having children has not changed as much as has age at marriage so that an increasing proportion of children are born to single parents. At the same time, marriages have become much less stable so that adults are spending more time single after marriage, and children are more likely to live at least part of their lives with a single parent.