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It's hard to talk about psychology for very long without mentioning the DSM—. Also known as the Diagnostic and Statistical Manual of Mental Disorders.

It's sometimes called the ‘Psychiatry Bible,' and for good reason: it's how most mental health professionals figure out if you have something they should be treating. But mental health professionals are just people. Regular, fallible people who have made mistakes— especially in the early days of the field, when psychology was often based more on what sounded good than on empirical evidence.

And the origin story of the DSM reveals one of the biggest ones. For a while, psychologists had a pretty limited definition of mental illness— they thought mental disorders were mostly physiological problems you were born with. And it took a couple of world wars for them to realize just how wrong they were.

One of the first times anyone tried to identify people with mental illness, at least in the US, was in 1840. The census that year had a question about whether people had what it called "insane" or "idiotic" dependents. Yeah, language has … changed a bit since then.

The problem was, the question didn't include any definitions of mental illness. So census takers kind of had to wing it. And without specific sets of symptoms to use when they were diagnosing people, doctors also had to wing it!

It took until 1918 for the American Medico-Psychological Association to write "The Statistical Manual for the Use of Institutions for the Insane," which had some of the first standardized descriptions of what counted as a disorder. It included things like "manic-depressive psychosis" or "dementia praecox"— which today we would call bipolar disorder and schizophrenia, respectively. Since these disorders were really poorly understood at the time, and there weren't great treatments for them, they were common among people who'd been institutionalized.

But the manual's whole approach to mental illness was really narrow. Doctors thought these disorders came entirely from physical problems, the same way a hard knock to the head can cause the symptoms of a concussion. Today, we know bipolar disorder and schizophrenia are some of the most highly heritable mental disorders, so it makes sense that they'd think of disorders as being physical—maybe even hereditary.

And it's true that mental illness can come with changes in brain chemistry. But it is not at all like a concussion! Environmental factors are incredibly important when it comes to mental health, and not considering those factors meant that doctors missed a lot.

Still, the system worked okay for a while ... until there was a problem. A pretty big problem, in fact—kind of a worldwide problem: World War II. Suddenly, there were millions of soldiers returning from a war where they'd seen their friends die and witnessed some of the cruelest possible treatment of others.

Meanwhile, families were split, communities were torn apart, and there was a lot of anxiety about the future of civilization in general. Many of these problems had come up in the aftermath of World War I, too, but they were generally diagnosed as shell shock. Today, we call shell shock post-traumatic stress disorder, or PTSD, but back then, doctors didn't really think of it as a mental illness.

They thought it was caused by the physical effects of artillery shells on nerves—hence the name. So doctors treated it like a neurological problem, not a psychological one. It wasn't until World War II that psychologists began to realize what had once been known as shell shock might be a mental illness.

They started calling it combat stress reaction, and treating it as a psychological thing. The thing was, all these people had previously been pretty healthy, so the manuals weren't much help. All disorders were considered biological and lifelong; there were no categories describing problems that developed in response to some life event.

So the American Psychological Association, or APA, decided to write a new guide. But first they went looking for some help understanding what was happening to people. What they found was detailed in the inauspiciously titled "War Department Technical Bulletin, Medical 203," a document published in 1943 that tried to explain how disorders could emerge from stressful life events.

It was written by a committee chaired by a military psychiatrist, but it relied a whole lot on the theories of Sigmund Freud. And I don't know if you've heard of the guy, but there are some things he kinda didn't get right about how people work. Like … almost everything.

But he saw patients after World War I, and he had an explanation for why they experienced shell shock— and why not everyone did. The symptoms of shell shock were very similar to symptoms of what Freud called a neurosis, which he thought was caused by childhood trauma and the repressed emotions or complexes that came from it. The idea was that if something that was too stressful to think about, it would be pushed into your unconscious mind, and then show itself through other anxieties or troubling behaviors.

It was also hard to tell if trauma was the real reason for the symptoms of neurosis, since by definition the memories might be repressed. But in the case of shell shock, it was obvious what the trauma was. So this War Department memo used Freud's ideas to explain how life circumstance could lead to a combat stress reaction in someone who didn't have a history of mental illness.

There were so many patients who fit the description that when the APA started to write the first edition of the DSM, which was published in 1952, these ideas became a major theme. They copied several passages verbatim from the War Department memo, including some about how disorders could be reactions to social factors or the environment. Also like the memo, the first edition of the DSM divided disorders into neurotic, psychotic, and character disorders— in addition to disorders that had a known physiological cause.

The DSM has been through a lot since. Removing some of Freud's theories for how disorders developed was one of the first orders of business, and today's DSM 5 focuses much more on just describing and categorizing symptoms. But we've kept some of the key lessons from the DSM's origin story: that disorders can have environmental and social causes— not just biological ones—and that mental illnesses can be much broader than what you'd find in an early 20th century mental hospital.

Thanks for watching this episode of SciShow Psych! For more on the strange—and sometimes awful— history of psychology, along with all kinds of other videos about our weird human brains, you can go to youtube.com/scishowpsych to subscribe. [OUTRO ♪].