Children exposed to joint custody reforms during childhood become measurably less likely to form couples and become parents as adults, according to a new research paper that examines the long-term consequences of one of the most consequential family-law changes affecting children in the United States.

The study, by Daniel Fernández-Kranz and Sébastien Fontenay, is circulated as IZA Discussion Paper No. 18831 by the Bonn-based Institute of Labor Economics. Like other discussion papers, it circulates research early for discussion and comment, with many such papers later revised for journal publication. While joint custody reforms rank among the most consequential family-law changes for children, the authors note that little has been known about their long-term effects.

Exploiting the staggered adoption of joint custody reforms across U.S. states and drawing on 13 million observations from the American Community Survey (ACS) — the U.S. Census Bureau's ongoing survey that collects detailed demographic data from millions of American households each year — the researchers show that childhood exposure reduces adult fertility by 7 percent. The effect is symmetric for women and men and operates primarily through lower parenthood and lower couple formation.

Within couples, the effects are sub-additive, meaning that when both partners were exposed to the reforms as children, the combined reduction in fertility is smaller than the sum of their individual effects. Exposed individuals, the study finds, also assortatively match — that is, they tend to partner with others who were similarly exposed.

The authors read the results as evidence that family preferences are formed during childhood. The findings align with the broader U.S. fertility decline, which has been increasingly driven by rising childlessness and declining couple formation rather than by parents having fewer children.

The reforms at the center of the study reshaped custody practice across much of the country. Family law in the United States is set largely at the state level, and beginning in the late 1970s, a growing number of states adopted joint custody statutes and presumptions. These measures encouraged or required courts to consider or favor shared custody arrangements after divorce, in place of the earlier prevailing arrangement of sole custody, which courts most often awarded to mothers. Because states adopted the reforms at different times over subsequent decades, the resulting staggered pattern allows researchers to compare outcomes across states and cohorts. Shared-parenting legislation has remained an active issue in state legislatures in recent years, with several states debating or enacting custody-presumption bills, so the question of the reforms' long-run consequences for children continues to bear on current policy debates.

The findings come amid a long-running decline in U.S. fertility. The national total fertility rate has generally remained below the replacement level of roughly 2.1 births per woman since the early 1970s, and annual births fell from a peak of about 4.3 million in 2007 to around 3.6 million in recent years.

The paper was highlighted by Marginal Revolution, the economics blog published by George Mason University economists Tyler Cowen and Alex Tabarrok, in the post "Childhood Exposure to Joint Custody Reforms and Adult Family Formation", which credited the find to Samir Varma.