<P> The nuclear family has been considered the "traditional" family since the communist scare in the cold war of the 1950s . The nuclear family consists of a mother, father, and the children . The two - parent nuclear family has become less prevalent, and pre-American and European family forms have become more common . </P> <P> These include same - sex relationships, single - parent households, adopting individuals, and extended family systems living together . The nuclear family is also choosing to have fewer children than in the past . The percentage of married - couple households with children under 18 has declined to 23.5% of all households in 2000 from 25.6% in 1990, and from 45% in 1960 . </P> <P> A single parent (also termed lone parent or sole parent) is a parent who cares for one or more children without the assistance of the other biological parent . Historically, single - parent families often resulted from death of a spouse, for instance in childbirth . Single - parent homes are increasing as married couples divorce, or as unmarried couples have children . Although widely believed to be detrimental to the mental and physical well being of a child, this type of household is tolerated . </P> <P> The percentage of single - parent households has doubled in the last three decades, but that percentage tripled between 1900 and 1950 . The sense of marriage as a "permanent" institution has been weakened, allowing individuals to consider leaving marriages more readily than they may have in the past . Increasingly, single parent families are due to out of wedlock births, especially those due to unintended pregnancy . </P>

A person is usually born or adopted during childhood into this type of family