Group-Based Trajectory Modeling (Nearly) Two Decades Later

Nearly two decades have passed since the publication of ‘‘Age, Criminal Careers, and Population Heterogeneity: Specification and Estimation of a Nonparametric Mixed Poisson Model’’ by Nagin and Land (1993). In that article Nagin and Land laid out a statistical method that has come to be called group-based trajectory modeling. The principle objective of the paper was to address issues related to the ‘‘hot topic’’ of the time—the criminal career debate—not to lay out a new statistical methodology. As described in the paper’s abstract, these issues were: ‘‘First, is the life course of individual offending patterns marked by distinctive periods of quiescence? Second, at the level of the individual, do offending rates vary systematically with age? In particular, is the age-crime curve single peaked or flat? Third, are chronic offenders different from less active offenders? Do offenders themselves differ in systematic ways?’’ Figure 1 reports Nagin’s (2005) updated version of the trajectories reported in Nagin and Land (1993). The analysis is based on the classic dataset assembled by Farrington and West (1990), which includes data on convictions from age 10 to 32 in a sample of over 400 males from a poor neighborhood in London, England. A four group model, analyzed using the zero-inflated Poisson modeling option, was found to best fit the data. The largest trajectory group accounted for 69.5% of the population, and was composed of individuals who generally had no convictions. The three offending trajectories included an adolescentlimited group (12.4% of the population), which peaked sharply in late adolescence, and then declined to a near zero rate of offending by age 20, a high chronic trajectory (5.9% of the population) with a high-humped shaped trajectory and a low rate chronic trajectory that accounted for the remaining 12.2% of the population. Also, shown in the figure are 95% confidence intervals around each trajectory. The dominant legacy of Nagin and Land (1993), however, was not its answers to the specific questions listed in the abstract but the methodology itself. A review of applications