Today's young adults experience diverse union trajectories. Childhood family instability---defined as parents' transitions into and out of romantic, coresidential unions---offers one explanation for why some people are more likely than others to enter and exit unions during young adulthood. We evaluate whether the family instability hypothesis can explain union formation and dissolution among Black and White young adults using rigorous causal inference methods to account for time-varying selection. Preliminary results, employing Panel Study of Income Dynamics data on birth cohorts 1989-1999, question the generalizability of the family instability hypothesis. Despite experiencing more childhood family instability, Black young adults form fewer unions than Whites. Childhood family instability's marginal effects are weaker for Black than White young adults' union outcomes. Most racial differences in these outcomes are causally unassociated with childhood family instability (even accounting for racial differences in instability's prevalence, not only its marginal effects, via novel decompositions).