--- - |2 Abstract Turning points, between‐person differences, and within‐person changes have all been linked to desistance from crime. Nevertheless, the means through which between‐person differences are frequently captured in life‐course criminology makes them intertwined with, and perhaps confounded by, turning points in life. We propose that a new way of capturing the between‐person effect—the baseline between‐person difference—could benefit theoretically informed tests of developmental and life‐course issues in criminology. Because they occur at one time point immediately preceding a turning point in life, we demonstrate that baseline between‐person differences establish meaningful theoretical connections to behavior and the way people change over time. By using panel data from the Serious and Violent Offender Reentry Initiative, we estimate models capturing within‐person change and baseline between‐person differences in social bonds (family support) and differential association (peer criminality) at the time of release from prison. The results demonstrate that baseline levels of family support protect people from postrelease substance use but not from crime. Baseline between‐person differences and within‐person changes in peer criminality, however, are robustly related to crime and substance use. Collectively, baseline between‐person differences seem critical for behavior and within‐person change over time, and the results carry implications for reentry‐based policy as well as for theory testing in developmental criminology more broadly. - Criminology, Volume 56, Issue 1, Page 191-224, February 2018.