In this paper we answer three questions about demographic change and violent crime in New Zealand: How has the age-sex profile of violent offending been changing? How have changes in age-sex structure affected overall offending rates for violent crime? How will likely changes in age-sex structure and offending rates affect future justice sector expenditure on violent crime? We do not claim that population ageing is the main determinant of trends in violent crime rates and expenditure, or even one of the most important. However, population ageing is one of the few determinants that can be readily quantified, and doing so provides a useful context for studies of more central causes.
Our principal data source is recorded offences and apprehensions for the eight classes of violent crime between 1994 and 2008, by sex and single year of age, provided by the New Zealand Police. We combine these data with Statistics NZ population estimates and projections between 1994 and 2018, and published estimates of government expenditure per violence offence (Roper & Thompson, 2006). The data on recorded offences miss many actual offences. However, we show that our methods are robust to substantial under-reporting. Our main method of analysis is demographic decomposition, which we use to assess the effect of demographic change on crime rates.
We find that the age-sex profile of violent offending has been changing, with the largest proportional increases occurring at age 30 years and over, and among women. We find that changes in the age-sex structure have helped to moderate increases in violent offence rates since 1994, and that prospective changes in age-sex structure should help moderate future growth in offence rates and justice sector expenditure.
We document trends in age-sex-specific offence and apprehension rates for violent crime in New Zealand, and estimate the impact of population change on offence rates and justice sector expenditure for violent crime.
The main data source is New Zealand Police data on recorded offences and apprehensions for violent crime between 1994 and 2008. These data are combined with Statistics New Zealand population data, and published estimates of government expenditure per offence. The impact of population change is assessed using demographic decompositions. Complex visualisations of the data allow trends and impact to be examined.
The age-sex profile of violent offending in New Zealand has been changing, with the largest growth rates occurring at age 30 years and over, and among women. Changes in population age-sex structure have moderated increases in violent offence rates since 1994. Prospective changes in age-sex structure should moderate future growth in offence rates and justice sector expenditure.