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The federal government partial shutdown drags on as politicians can’t agree on what to do with illegal aliens. Trump wants a wall to keep out “illegal immigrants … bringing with them crime, tremendous amounts of crime,” but Democrats say that a wall is immoral and unnecessary.

The media usually describe illegal aliens in glowing terms. Many are undoubtedly good people, but new data I have obtained show that illegal aliens as a group are not very law-abiding.

The existing research is fatally flawed. Most studies just examine all immigrants as a monolithic group, rather than separately studying legal and illegal immigrants. There is a good reason to believe that people who come to America legally tend to be more law-abiding than those who do so illegally.

Other studies depend on individuals to self-report their criminal histories and even whether they are here illegally. Illegal aliens may not want to tell a stranger that they have been in prison, fearing that their criminal record and illegal status will make them prime candidates for deportation.

Fortunately, new research from the Crime Prevention Research Center uses newly obtained data on everyone who entered the Arizona prison system from January 1985 through June 2017. This data includes a lot of demographic information including citizenship and documentation status. The data paint a very disturbing picture of illegal aliens, particularly younger ones.

Over the entire period, illegal aliens were more than twice as likely to be convicted of crimes than other Arizonans. These convictions are not for illegally being here or for working. They also served 10% longer prison sentences than did U.S. citizens.

The relative rate that illegal aliens are committing crime depends on not only their share of prisoners, but also their share of the population. I used Department of Homeland Security/Immigration and Naturalization Service for data up to 2012 and PEW Research Center data after that, Arizona averaged 4.8 percent of their population being illegal. They would have to make up almost 12% of the population to have the same conviction rate as U.S. citizens.

Take murder. During the period we studied, there were 7,948 convictions out of about 11,900 cases through June last year. We know the citizenship of 7,905 of those convicted. Illegal aliens killed 1,037; U.S. citizens 6,812; and legal residents 56. Illegal aliens make up over 13.1% of the murderers, but just 4.8% of the population.

Compared to American citizens, illegal aliens are more than twice as likely to be convicted for armed robbery, child molestation, and for sexual assault.

They are nearly three times as likely to be convicted of murder and manslaughter.

They are more than four times as likely to be convicted of a drive-by shooting, and for extortion.

They are more than five times as likely to be convicted of kidnapping.

For smuggling, they are 16 times as likely to be convicted – no surprise, since it involves crossing the border.

If illegal aliens committed crime nationally as they do in Arizona, in 2016 they would have been responsible for over 1,000 more murders, 5,200 rapes, 8,900 robberies, 25,300 aggravated assaults, and 26,900 burglaries.

All these numbers, detailed in a report on the Crime Prevention Research Center website, support Trump’s concerns.

Trump has differentiated legal versus illegal aliens, and it turns out for good reason. Legal permanent resident immigrants are extremely law-abiding. They made up only 1.5% of the prison population in 2014, while their share of the state’s population is about 3.9%. Other studies that lump legal and illegal aliens together hide the high crime rate for illegals.

Hispanics who are legally in the US are also relatively law-abiding. Unfortunately, Hispanics get a bad reputation because they are combined with Hispanics who are illegal aliens.

Young illegal aliens who are 15 to 35 years of age may reportedly make up only about two percent of Arizona’s population, but they account for almost 8% of the state’s prison population. After adjusting for the fact that young people commit most crime, they still commit crimes at about twice the rate on average as other young people.

As bad as these numbers are, there are two strong reasons to believe that they underestimate the problem. Most violent crimes are never solved. Even a third of murders in Arizona didn’t result in conviction and these were more likely to be committed by illegal immigrants. The most likely victims of illegal aliens are other illegal aliens, and there is evidence that these victims are particularly reticent to report crimes to the police. Illegal aliens are also 45% more likely to be gang members according to the new data, and crimes committed by gangs are less likely to be solved.

Among U.S. citizens who are criminals, a small subset keeps going in and out of prison. That isn’t true for illegal aliens, who are deported after prison and who may not return to the United State. 24.8% of U.S. citizen convicted criminals were admitted five or more times to the Arizona Department of Corrections, but that is only true of 2.95% of illegal aliens convicted criminals.

Given that illegal aliens show such a low recidivism rate, it is remarkable that they make up such a large share of the Arizona prison population. The only explanation for this is that a much larger share of illegal aliens are committing crimes compared to U.S. citizens.

Trump’s insistence on a wall is the right choice for public safety.