The first problem with understanding crime is that measuring it is harder than it sounds. The Department of Justice approaches the problem in two ways. The F.B.I.’s Uniform Crime Reporting Program, or U.C.R., solicits data from about twenty thousand law-enforcement agencies around the country. Simultaneously, the Bureau of Justice Statistics’ National Crime Victimization Survey, or N.C.V.S., interviews about a hundred and fifty thousand nationally representative citizens, asking them whether they have been victims of a crime.

Both datasets have problems. An obvious one is that there’s no consensus about what counts as criminal activity. In some jurisdictions, only offenses worthy of incarceration are considered crimes. In others, fined infractions also count. (Is speeding a crime? What about manspreading, for which one can be fined seventy-five dollars in Los Angeles?) Because the U.C.R. draws its data from investigators, and the N.C.V.S. relies on victims, they can present starkly different pictures of crime. According to the U.C.R., the incidence of rape nearly doubled from 1973 to 1990. The N.C.V.S., by contrast, shows that it declined by around forty per cent during the same period. Researchers at Vanderbilt University looked into the discrepancy; they found that the upward trend in the U.C.R. data correlated with upticks in the number of female police officers, and with the advent of rape crisis centers and reformed investigative styles. It could be, in short, that a modernized approach to the policing of rape drastically increased the frequency with which it was reported while reducing its incidence. But coherent stories like these only sometimes emerge from the conflicting data.