tion: Different demographic groups, particularly different age and ethnic groups, display very different rates of involvement in crime.

Some of these factors could be addressed in the context of generating policies intended to reduce crime. For example, to the extent that unemployment among teenagers and young adults is a major contributing factor to the crimes they commit, then efforts at providing job assistance, job training, or extending unemployment support for those groups could well be stimulated by their anticipated crime trends.

We begin by examining trends in violent crimes, which are the most serious crimes and attract the greatest public concern. We focus on robbery and murder, the two violent crimes that are best measured. We devote less attention to the other two violent crimes, forcible rape and aggravated assault, both of which exhibit important measurement problems. Aggravated assault is troubled by the room for discretion in classifying an assault as either “aggravated” or “simple”; only if it is aggravated is it recorded as a Part I crime in the Federal Bureau of Investigation’s Uniform Crime Reports (UCR). Moreover, comparisons with the assault trends measured in the National Crime Victimization Survey (NCVS) suggest that the police have “upgraded” the recording and classification of assaults over time and classify many as aggravated that they would have treated as lesser offenses in the past (Rosenfeld, 2007a). The measurement of forcible rape is subject to important variations in whether the incident is reported to the police and counted as a Part I crime.

In Figure 2-1 we compare the rates of homicide and robbery from 1972 to 2006. To provide a comparison of the two trends, we have divided the robbery rate by 25 to put robbery and murder on a comparable scale.

The first observation from comparing the murder and robbery trends is their striking similarity. Both reach their peaks and their troughs within a year of each other. This may suggest that similar factors are affecting both trends, but not necessarily. It also is possible that one is driving the other. Explaining the correspondence between trends in different types of crime is an important issue for future research (see LaFree, 1998).