In addition, the research used a measure of offense skewness that allowed for meaningful comparisons of offense skewness both within and across data sources. Launched in the late 1950's, the CPP was one of the largest medical studies ever undertaken. The Philadelphia sample of the CPP comprised about 10,000 pregnancies delivered at Pennsylvania Hospital between 1959 and 1965. The research followed 987 subjects (487 males and 500 females) who were selected from the first four cohorts (1959-1962) of 2,958 black mothers who participated in the CPP. Criminal history data on these individuals were obtained from official police records. Analysis showed a small group of offenders was responsible for a large proportion of offenses. Relative to one-time and recidivist offenders (2-4 offenses), chronic offenders (5 or more offenses) were more likely to display an early onset of offending and to participate in violent offending. These findings did not vary across gender. When examining measures of offense skewness, results suggested that, while the Philadelphia CPP had slightly lower offense skewness than the 1958 Philadelphia birth cohort, males had higher offense skewness compared to females. Implications for future research are addressed. 67 references, 9 notes, 7 tables, and 2 figures