In this article, we use data from a new longitudinal survey—the Fragile Families and Child Wellbeing Study—to examine union formation among unmarried parents who have just had a child together. We used multinomial logistic regression to estimate the effects of economic, cultural/interpersonal, and other factors on whether (relative to having no romantic relationship) parents are romantically involved and living apart, cohabiting, or married to each other about one year after the child’s birth. Net of other factors (including baseline relationship status), women’s education and men’s earnings encourage marriage. Cultural and interpersonal factors also have strong effects: women’s trust of men, both parents’ positive attitudes toward marriage, and both parents’ assessment of the supportiveness in their relationship encourage marriage. Supportiveness also encourages cohabitation, while fathers having a problem with alcohol or drugs and reporting higher conflict in the relationship discourage cohabitation. Fathers’ physical violence deters couples’ remaining in romantic nonresident relationships.

We use data from the Fragile Families and Child Wellbeing Study to investigate the association between coparenting quality and nonresident fathers’ involvement with children over the first five years after a nonmarital birth. We find that about one year after a nonmarital birth, 48% of fathers are living away from their child, rising to 56% and then to 63% at three and five years, respectively. Using structural equation models to estimate cross-lagged effects, we find that positive coparenting is a strong predictor of nonresident fathers’ future involvement, whereas fathers’ involvement is only a weak (but significant) predictor of future coparenting quality. The positive effect of coparenting quality on fathers’ involvement is robust across several techniques designed to address unobserved heterogeneity and across different strategies for handling missing data. We conclude that parents’ ability to work together in rearing their common child across households helps keep nonresident fathers connected to their children and that programs aimed at improving parents’ ability to communicate may have benefits for children irrespective of whether the parents’ romantic relationship remains intact.