We live in a democracy, and people have different perspectives, interests, and values. This means we can honestly disagree on questions about how to run our society, and the political process is supposed to work out those differences through compromise and democratic processes. However, the political process should be based on objective facts as much as possible. Senator Daniel Moynihan is quoted as saying, “Everyone is entitled to their own opinions, but they are not entitled to their own facts,” or some variation of that basic idea. If we lose the objectivity of facts, if everything is opinion, then the democratic process breaks down. There is no longer any common ground for discussion. In fact, what seems to be happening is that not only has fact devolved into opinion, opinion has become an alternate reality constructed and maintained through conspiracy theories.

Assuming we can get back to a world in which facts exist, let alone matter to some degree, they can very usefully inform public debate on political topics. Facts alone don’t determine political outcomes, because values can still differ, but they help. Let’s take the association between undocumented immigrants and crime as an example.

One of the prominent arguments put forward for aggressive policing of our borders is to stop undocumented immigrants, because they bring “large-scale crime and disease” across the border. But do they? Citing dramatic anecdotes will never give us a meaningful picture of reality. We are a nation of over 300 million people – everything happens somewhere. You can find anecdotes to support any narrative you wish. What we really need is data.

One problem is that undocumented immigrants are – undocumented. This makes it hard to track them and to get data on them. But we do have several lines of evidence that give us at least a partial picture. For example, some studies do surveys of individuals, asking their immigrant status and their involvement with crime. One such study, published in 2017, found that immigrants were 2-3 times less likely to report criminal behavior than demographically matched US born individuals. The two main weaknesses of this approach is that, first, it mixes documented and undocumented immigrants into one category. Second, this tells us only that immigrants report less crime, which may not be the same as engaging in crime.

Experts point to the fact that you can make sense of this data either way. Immigrants are highly motivated to avoid criminal activity because the stakes for them are high. It can mean deportation, sometimes to a horrific situation that they fled in the first place. Immigrants also often live in cultural enclaves that provide a great deal of social support, which is a factor that can reduce crime by itself. At the same time, immigrants, especially those who are undocumented, may not trust that they can report their criminal activity for a survey and not receive repercussions.

However, this study does not exist in isolation. It builds on a large body of research all telling the same story. For example, a 2014 epidemiological survey on alcohol and related conditions found immigrants to engage in less anti-social behavior than native born Americans.

An entirely different line of evidence addressing this question is incarceration rates. This does not rely on self-reporting, and there are hard numbers on people in prison. Multiple studies show that incarceration rates for immigrants are much lower than native born, at about one fifth the rate per population. The more recent the immigrant, the less likely they are to be incarcerated.

There is yet another way to address this question – what is the effect of immigration, specifically undocumented immigrants, on crime rates in locations where they live? There are several studies looking into this question, the most recent of which was just published. This study specifically looked at undocumented immigrants, using two independent measures to estimate their numbers. They found that in the 140 metropolitical areas studied, the rate of violent crime was statistically not affected by the number of undocumented immigrants. Meanwhile, the rate of property crime dropped. Looking at all different types of crime there was either no effect or a negative correlation – so the influx of undocumented immigrants into a city tends to decrease crime.

These results agree with prior similar studies showing overall no correlation between undocumented immigrants and crime rates, and if anything for some types of crime there is a decrease. This makes sense if immigrants themselves are less likely to commit crimes. But researchers also point to the positive correlation between immigrants and economic vitality. A good economy seems to correlate with lower crime rates (although this is admittedly a complex area).

There is no single perfect study addressing this question (as is the case with all complex social questions), but when various lines of evidence are all showing effects in the same direction, that creates greater confidence in the results. It is also highly plausible that immigrants would avoid crime for the reasons stated. It is also worth pointing out that there is no evidence linking undocumented immigrants to more crime.

There is still a political discussion to be had about how to optimize our immigration policy, and what our goals and values should be. But at the very least discussion should be informed by the best facts we have available.