Using the Wisconsin Court Record Data, I show that among nonmarital cases joint legal custody increased from 2% in 1988–93 to 20% in the late 90s, jumping further in 2000 and staying relatively high at around 70% in the 2000s. I hypothesize that an increasing preference for joint legal custody, a policy change that made joint legal custody presumptive, a change in the demographic composition of never-married parents, or a combination of these influences explains this trend. Logit models and Blinder–Oaxaca decomposition analyses both suggest that the difference in joint legal custody is mostly explained by the process (the coefficients) rather than the changes in parental characteristics (the independent variables). The patterns of the data suggest that an increasing parental or societal preference for joint legal custody, encouraged by the policy change, is the primary drive for the recent rise in joint legal custody among nonmarital cases. •The prevalence of joint legal custody increased abruptly around 2000 among nonmarital cases in Wisconsin.•Joint legal custody was made presumptive by law in Wisconsin in 1999.•The welfare reforms and the expansion of child support enforcement may also change unmarried parents who went to court.•I examine the driving forces of this trend using court data with logit models and the Blinder-Oaxaca decomposition.•This trend is mostly explained by a social change and a policy change rather than changes in parental characteristics.