Multiple partner fertility (MPF) is defined as having children with more than one partner. Until the mid-20th century, MPF was uncommon and typically occurred in cases of spousal death or desertion. Since the 1960s, increasing rates of divorce, remarriage, and nonmarital births have resulted in opportunities for parents to have children with [Page 1465]new partners. MPF has become more prevalent over time, and unmarried parents are particularly likely to have children with multiple partners. MPF results in complex family structures, which often lead to complicated family relationships and poor outcomes for parents and children.