Quantitative research was completed on 9,945 juvenile male offenders between the ages of 10 and 18 in Philadelphia, Pennsylvania in the 1970s. The longitudinal birth cohort was used to examine a trend among a small percentage of career criminals who accounted for the largest percentage of crime activity. The trend exhibited a new phenomenon among habitual offenders. The phenomenon indicated that only 6% of the youth qualified under their definition of a habitual offender (known today as life-course persistent offenders, or career criminals) and yet were responsible for 52% of the delinquency within the entire study. The same 6% of chronic offenders accounted for 71% of the murders and 69% of the aggravated assaults. This phenomenon was later researched among an adult population in 1977 and resulted in similar findings. S. A. Mednick did a birth cohort of 30,000 males and found that 1% of the males were responsible for more than half of the criminal activity. The habitual crime behavior found among juveniles is similar to that of adults. As stated before most life-course persistent offenders begin exhibiting antisocial, violent, and/or delinquent behavior, prior to adolescence. Therefore, while there is a high rate of juvenile delinquency, it is the small percentage of life-course persistent, career criminals that are responsible for most of the violent crimes.