Increases in nonmarital fertility, divorce and remarriage have contributed to higher proportions of mothers and fathers who have had children with more than one partner.  While policies often presume that families include two parents and their children in common, over the course of their childhood many children come to share a household and parent with half-siblings, and/or to share a non-resident parent with other half siblings who live elsewhere.

Previous research has documented the incidence of complex families, or multiple partner fertility, for a variety of populations. In this paper we document changes in the incidence of family complexity using a consistent sampling strategy and data source. Following three cohort of first-born children (born in 1997, 2002 and 2007) whose mothers were not married at the time of their birth, we document changes in family structure across cohorts and overtime. We consider both full and half siblings who are co-residential or who live in another household. We show that even in the earliest cohort, by the time they are 10 years old, over sixty percent of first-born nonmarital children have half siblings. These half siblings have implications for the resources available to children, the responsibilities of resident and nonresident parents, and for the challenges addressed by social policies, including those related to child support and public income support.

We use detailed longitudinal administrative data from the State of Wisconsin that include over 90 percent of nonmarital births. We consider the timing of subsequent births to the mother and father, together or with new partners, and account for siblings and half-siblings, even if they are not co-resident.  We follow cohorts of children born in 1997, 2002, and 2007, for 15, 10 and 5 year, respectively. We decompose the growth in family complexity accounting for changes in the characteristics of mothers of first-born nonmarital children, and changes in the probability of family complexity given these characteristics. We also examine the interaction between subsequent births and child support paid and received by the parents and other partners, and how this interacts with participation in TANF, Food Stamps and Medicaid programs.  We discuss implications of the incidence and timing of multiple partner fertility for the child support enforcement system as well as for public welfare eligibility and use.

This paper provides a context for the other papers in the panel, which consider complex families from the perspective of mothers and fathers, and the implications for child wellbeing.