Joint custody reforms are among the most consequential family-law changes for children, yet little is known about their long-term effects. Exploiting staggered adoption across US states and 13 million ACS observations, we show that childhood exposure reduces adult fertility by 7 percent, symmetrically for women and men and operates primarily through lower parenthood and couple formation. Effects are sub-additive within couples, while exposed individuals assortatively match. Our findings point to the formation of family preferences during childhood and align with a US fertility decline increasingly driven by rising childlessness and declining couple formation rather than smaller families among parents.

That is from a recent research paper by Daniel Fernández-Kranz and Sébastien Fontenay.  Via the excellent Samir Varma.