AGE, CRIMINAL CAREERS, AND POPULATION HETEROGENEITY - SPECIFICATION AND ESTIMATION OF A NONPARAMETRIC, MIXED POISSON MODEL

This article addresses three issues that are central to the criminal c areer debate. First, is the life course of individual offending patter ns marked by distinctive periods of quiescence? Second, at the level o f the individual do offending rates vary systematically with age? In p articular, is the age-crime curve single peaked or flat? Third, are ch ronic offenders different from less active offenders? Do offenders the mselves differ in systematic ways? Using a new approach to the analysi s of individual criminal careers-based on nested, mixed Poisson models in which the mixing distribution is estimated nonparametrically-we an alyze a panel data set that tracks a sample of males for more than 20 years. Our results provide empirical evidence in support of some featu res of criminal propensity theory and some in support of conventional criminal careers theory. In support of latent-trait criminal propensit y theory, the individual-level average offense rate (per unit of time) varies as a function of observable individual-level characteristics a nd unobservable heterogeneity among individuals, and the age trajector y of the offense rate is generally single peaked rather than flat. On the other hand, in support of conventional criminal careers theory, mo dels that incorporate a parameter that perm its periods of active as w ell as inactive offending across age have greater explanatory power th an those that do not. In addition, the nonparametric, discrete approxi mation to the population distribution of unobservable heterogeneity in the individual-level mean offense rate facilitates identification of four classes of offenders-nonoffenders as well as individual-level cha racteristics that are unique to each group. Problems of theoretical ex planation and empirical generalizability of these results are describe d.