Nuclear family (55%). Named after the nucleus (the tightly connected core particles of an atom), the nuclear family consists of a man and a woman and their biological offspring under 18 years of age. About half of all school-age children live in nuclear families. About 10 percent of such families also include a grandparent, and often an aunt or uncle, living under the same roof. These are extended families.

Stepparent family (10%). Divorced fathers usually remarry; divorced mothers remarry about half the time. When children from a former relationship live with the new couple, that is a stepparent family. If the stepparent family includes children born to two or more couples (such as children from the spouses’ previous marriages and/or children of the new couple), that is called a blended family.

Adoptive family (2%). Although as many as one-third of infertile married couples adopt children, few adoptable children are available and so most adoptive couples have only one or two children. Thus, only 2 percent of children are adopted, although the overall percentage of adoptive families is higher than that.

Grandparents alone (1%). Grandparents take on parenting for some children when biological parents are absent (dead, imprisoned, sick, addicted, etc.).

Two same-sex parents (1%). Some two-parent families are headed by a same-sex couple, whose legal status (married, step-, adoptive) varies.

Single mother—never married (14%). In 2010, 41 percent of all U.S. births were to unmarried mothers; but when children are school age, many mothers who were unmarried have married or have entrusted their children to their parents’ care. So, only about 13 percent of 6- to 11-year-olds, at any given moment, are in single-mother, never married, homes.

Single mother—divorced, separated, or widowed (12%). Although many marriages end in divorce (almost half in the United States, fewer in other nations), many divorcing couples have no children. Others remarry. Thus, only 12 percent of school-age children currently live with single, formerly married mothers.

Single father (4%). About 1 father in 25 has physical custody of his children and raises them without their mother or a new wife. This category increased at the start of the twenty-first century but has decreased since 2005.

Grandparent alone (1%). Sometimes a single grandparent (usually the grandmother) becomes the sole caregiving adult for a child.

Extended family (10%). Some children live with grandparents or other relatives, as well as with one or both of their parents. This pattern is most common with infants (20 percent) but occurs in middle childhood as well.

Polygamous family (0%). In some nations (not the United States), men can legally have several wives. This family structure is more favored by adults than children. Everywhere, polyandry (one woman, several husbands) is rare.