the United States between 1975 and 1988, there are some interesting differences among the crime-type components of the total rate. Rates for violent crimes increased more than property crimes in the FBI's crime index. The modest 7.2 percent increase in the total index crime rate in the United States from 1975 to 1988 resulted from a 4.7 percent increase in the property crime rate and a much larger 32.3 percent increase in the violent crime rate.10 If robbery (which shares features of both violent and property crimes) is excluded, the rate per total population for the remaining violent offenses increased 58.0 percent between 1975 and 1988, reflecting increases in the rates of rapes and aggravated assaults that are reported by the police;11 murder rates actually declined by 12.5 percent between 1975 and 1988.

Changes in population crime rates may be affected by changes in the composition of the population toward increased (or decreased) representation of population subgroups that are characterized by higher (or lower) crime rates. Age, for example, is a potentially important factor distinguishing population crime rates,12 as illustrated by age-specific arrest rates in 1988 that peaked in the late teens or early twenties and then dropped slowly through the adult years (Federal Bureau of Investigation, 1990b). The period 1965 to 1988 was characterized by a general aging of the U.S. population, with adults (age 18 and over) rising from 64 percent of the population in 1965, to 69 percent in 1975, and then to 74 percent by 1988. This population shift, combined with age-varying arrest rates, would contribute to increases in general population crime rates.

The increasing representation of adults in the total population is a particularly important factor in the differences observed between violent and property crime rates for the total population. Violent crimes are especially characteristic of adults. While the peak arrest rate for murder was at age 18 in 1988, murder rates declined slowly with age and did not reach a rate equal to one-half the peak rate until ages in the early thirties (Federal Bureau of Investigation, 1990b). Rape and aggravated assault rates peaked at ages 23 and 21, respectively, and remained above the half-peak rate into the late thirties. This contrasts with arrests for property crimes, which peaked at age 16 and reached the half-peak rate by age 23. Robbery rates fell between violent and property crimes, peaking at age 18 and reaching the half-peak rate in the late twenties.13

After partially controlling for age and using rates of the estimated number of crimes committed by adults per adult population,14



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