Family is a social institution found in all societies that unites people through cooperative living and caring for one another, including children. There are several types of families including the extended family, which includes multiple adult relatives living together, and the nuclear family consisting of two parents and their children living as a unit. Marriage is a legal relationship that often involves economic cooperation, sexual activity, and childbearing, and it can take various forms such as monogamy, polygamy, or exogamy in which mates are chosen from outside a specified group.