Why some generations are more violent than others: assessment of age, period, and cohort effects.

Empirical longitudinal studies assessing why community-level violence rates change over time are lacking. Despite a wide-ranging literature, questions remain as to whether changes over time are due to factors occurring in specific periods (period effects) or individuals in successive cohorts (cohort effect). The objective was to assess the relative contribution of age, period, and cohort effects on violence trends. The authors assessed differences in self-reported violence between two cohorts of males (n = 1,009) from the Pittsburgh Youth Study, which tracked delinquency and risk factors from 1987 to 2000. The youngest cohort were aged 7-19 years, and the oldest cohort were aged 13-25 years. Yearly measures of violence were examined through generalized estimating equations. The oldest cohort reported higher levels of violence even after adjustment for age and major individual-level risk factors (odds ratio (OR) = 1.45, 95% confidence interval (CI): 1.17, 1.81) such as gang participation and drug dealing, as well as community-level factors (OR = 2.16, 95% CI: 1.65, 2.82). However, when period effects were included, cohort differences were rendered insignificant (OR = 1.23, 95% CI: 0.78, 1.94). The authors conclude that differences in the rates of violence over time may be attributed to changing social factors (period effects) and not to differences between the individuals (cohort effect) of cohorts.