That viral chart showing a link between mental hospitals and prisons? It’s kind of misleading.

You may have seen this chart on social media recently — after Sean McElwee’s tweet about it went viral:

Charts like this have been fairly common in criminal justice writing over the past few years, with media outlets like the Economist, Mother Jones and, yes, Vox referencing something like it in previous articles.

But as criminal justice expert John Pfaff points out, depending on how you read this chart, it can be very misleading:

In short, the chart isn’t totally wrong, but it oversimplifies a rather complicated story: While deinstitutionalization did contribute to mass incarceration, it wasn’t the whole cause or even a big one.

Beyond what Pfaff noted, there are a couple of other problems with the chart.

For one, it suggests that people with mental illnesses are violent and have to be locked up somewhere. That’s not true: People with mental illness are more likely to be victims — not perpetrators — of violence. And only about 3 to 5 percent of violent acts in the US are carried out by people with serious mental illnesses, while about 4.2 percent of adults in the US experience a serious mental illness in a given year that substantially interferes with or limits their major life activities.

The chart can also be read to suggest that the deinstitutionalization of mental health care was a bad thing. While state-run mental hospitals are still underfunded, the asylums of old were, experts now widely agree, horrifically abusive. There’s a very good reason why popular depictions of these old hospitals tend to be all about abuse (like in One Flew Over the Cuckoo's Nest) or literal horror movies and shows (like in American Horror Story). The move to shut down these places don was a good thing — even if the better forms of treatment developed since then, such as community clinics, are frequently underfunded and unaffordable.

Still, it’s also true that the criminal justice system often mistreats people with mental illnesses. A 2015 report by the Treatment Advocacy Center found that someone with an untreated mental illness is 16 times more likely to be killed by police than other civilians approached or stopped by law enforcement. And 2 million people with a mental illness cycle through US jails each year, while at least 83 percent of jail inmates with a mental illness do not have access to adequate treatment, according to the National Alliance on Mental Illness.

But those problems can be signaled and studied without simplified graphs. Like many things in the policy world, the connection between mental illness and prison time is a bit more complicated than one chart can suggest. Getting that nuance right is crucial to understanding and fixing the problems surrounding mental health and the criminal justice system today.

For more on this topic, check out Vox’s in-depth explainer.

Watch: The psychology of police sketches

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