Critics of illegal immigration argue that the crime rates of illegal aliens are higher than those of the American population generally, or at least of legal immigrants. The New York Times has denied that illegals commit more crime than other groups, but the paper bases its claim on a Cato Institute study that relies on questionable data. In fact, nobody can calculate with accuracy the crime rates of illegal immigrants or any other social group unless they have reliable data on the size of the group, and we simply don’t know how many illegal aliens there are in the United States.

Nationwide data on crime by illegal aliens is unavailable mainly because most states don’t keep such records. For instance, California, with Hispanics making up more than 43 percent of its incarcerated population, provides no information on the alienage of its inmates. Texas does, though, and its Department of Public Safety reports that illegal aliens were arrested and charged with more than 298,000 crimes, an average of over 39,000 per year, from June 1, 2011 to the end of 2018. Though some of these arrests were for nonviolent crimes, such as theft, burglary, or drug offenses, they also include many violent crimes: 624 homicides, 1,911 robberies, and 3,955 sexual assaults (which, under Texas law, include rapes).

While these figures sound disturbing, we can’t say with certainty if they are high relative to the size of the illegal immigrant population because, as noted above, we really don’t know how many there are. A 2014 estimate by the Pew Research Center pegged the Texas figure at 1,650,000, or 6.3 percent of the state’s entire population. Homeland Security offered a higher estimate for 2015: 1,940,000, which accounted for 7.3 percent of the state’s population.

Among all arrests for selected offenses over the period 2012 to 2017, illegal aliens were taken into custody for homicide (which includes murder and manslaughter) in numbers greater than their population size would predict. They accounted for nearly 10 percent of all apprehended killers, whereas, using the high-end DHS estimate, they make up 7.3 percent of the Texas population. For all other crimes, however, including burglary, drugs, theft, robbery, and weapons offenses, their apprehension percentages ranged from 2.5 to 6.7 percent—in other words, below their putative population size.

The crime of homicide provides the most accurate measure, though, because a much higher proportion of murders are solved by police—around 70 percent—than for any other crime; by contrast, fewer than 15 percent of property offenses lead to an arrest. As a result, we have much more accurate demographics for murderers than for, say, burglars. The indication that illegal aliens commit disproportionate numbers of murders is corroborated by crime rates, shaky though they may be, for 2014 and 2015—the two years for which we have population estimates from Pew and DHS. In 2014, Texas illegal-alien murder-arrest rates were 4.99 per 100,000—56 percent higher than the rates for all other apprehended murderers (3.2 per 100,000). In 2015, the rates were 35 percent higher for illegal aliens (4.2 per 100,000, versus 3.1 per 100,000).

Granted, neither the rates nor the percentages of illegal aliens arrested are overwhelmingly high. And the rates and percentages for other crimes that they commit are below those of the arrested citizen and legal-alien populations. Still, illegal aliens account for nearly 10 percent of the apprehended murderers in Texas, and over 39,000 of the annual arrests for crime overall. These figures are significant, reflecting crime in a single state with an outsize number of illegal aliens—a small part of the nationwide picture.

No amount of crime by those who enter this country unlawfully should be acceptable, because it is “extra” crime that wouldn’t occur if our border security were effective. Crime by illegal aliens is costly. The real issue underlying the current public debate is whether the crimes of illegal immigrants are so numerous that they provide a compelling reason, or at least a powerful supporting argument, for urgent spending to secure our southern border. Judging by Texas the answer, though not incontestable, seems to be “yes.”

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