Homicide rates in Philadelphia: Contribution of age distribution to rate differences

In an effort to put the recent increase of homicides in context, we compared Philadelphia trends over a 25 year period to U.S. and several large cities, including Boston, Chicago, NYC and D.C. In comparison, Philadelphia rates are consistently higher than the U.S., Boston, and NYC but lower than D.C. Homicide rates peaked in 1985 to 1996, and the 2000s saw stable rates for most of the cities. However, in contrast to the general pattern, Philadelphia rates started to increase in the 2000s. Criminal justice literature on youth violence suggested several themes, including cohort size that can contribute to rate variability. To measure the contribution of age distribution to the recent increase of homicides in Philadelphia, we performed decomposition analysis on Black Male crude homicide rates between 2000 and 2006. Of the sharp increase in rates between 2005 and 2006, decomposition indicates that 80% of the increase was due to differences in the rates' schedules. Age distribution differences only accounted for 20% of the increase. Between 2000 and 2005, age distribution differences accounted for 70% of the increase. Each successive year between 2000 and 2006, the cohorts for the 15-19, 20-24, 25-29 age-groups grew. The relative size of these cohorts played a larger role in the increase in rates in the first half of the 21st century but 2006 is a different story. In considering homicide trends and patterns, there is a need to examine the contribution of demographic factors such as relative cohort size.