Psychiatric conditions like schizophrenia, bipolar disorder, autism, and ADHD can be debilitating for those who have them. And yet, they’re partially heritable, presenting a paradox. Why would evolution continue to select for a condition that can make life so difficult and even jeopardize one’s ability to find a mate and have children?

Now a huge new study in Nature Communications has found that some of these conditions are associated with specific forms of intelligence at the genetic level in European populations, such as creativity, accumulated knowledge, or reaction time. Previous cognitive research into psychiatric conditions had treated intelligence as a monolith, which had obscured differences in individual cognitive traits. The findings present a possible solution to the evolutionary puzzle presented by psychiatric illness.

I spoke with study author Diego Londoño-Correa, a graduate student in the department of psychology at the University of Texas at Austin, about what the findings tell us about the nature of intelligence, domestication syndrome in animals, and the dangers of designer baby genetics.

Scientists have historically lumped intelligence into a single category when studying its link to mental illness. Why?

People tend to be smart in different dimensions at the same time, but it’s not a perfect correlation. And with genome wide association studies (GWAS) like the one we did, it’s important to compare as many people as possible. If you want to have large numbers of people represented, it’s easier to give people a general test of intelligence and then find genetic associations with that than to focus on narrower traits. So that’s one thing.

Also, we’d already seen evidence that, for example, people with schizophrenia have some deficits in certain dimensions of intelligence at the trait level. These deficits can be evident even before the onset of schizophrenia, which tends to occur at around 25 years of age. So it was natural to ask, “Do we see the same thing happen at the genetic level?”

Now that we’ve collected more information on psychiatric disorders and intelligence and genetics at the population level, we have enough power to delve into the finer dimensions of intelligence and even educational attainment. Because there was a paradox in the data. While people with schizophrenia often have deficits in standard measures of intelligence, they also tend to be high in educational attainment. But educational attainment is correlated with intelligence, as well as curiosity and self-discipline. People started wondering if there were some nuances of intelligence that we were missing. One of those was creativity. The relatives of people with schizophrenia tend to be over-represented in artistic or creative jobs. But creativity is a dimension that isn’t captured by intelligence tests.

Then there was a paper published in March 2021, where they broke down educational attainment into two different factors, one was cognitive and the other was non-cognitive. And they found that the non-cognitive skills were positively correlated with schizophrenia and bipolar disorder. So that was the basis for our question.

Read more: “A Mental Disease by Any Other Name”

You wrote in your study that intelligence actually falls into three distinct categories with different developmental trajectories: reaction time, fluid reasoning, and crystallized knowledge. Is this the standard way most scientists understand intelligence today?

That is one way of separating them. It’s not the only way. We chose this one because they have different trajectories with aging. You see a very steep decline with age for reaction time, which is a basic measure of processing speed. When it comes to fluid reasoning, you also see a decline with age. But for crystallized knowledge, or knowledge accumulated over time, you see a more stable trend. People don’t lose crystallized intelligence as they age.

Also, when people have brain trauma, it has a bigger impact on reaction time or fluid reasoning than crystallized knowledge. But you can differentiate intelligence into different dimensions, according to different theories.

Why was it important that these dimensions of intelligence change over the course of a lifetime for the questions you were asking?

We’re finding genetic correlations with intelligence, right? And we can think that maybe intelligence is a very basic biological process. But when we see that crystallized knowledge is maintained, it suggests to us that maybe this dimension isn’t that biological. Maybe this is related to culture. Because your body loses some capacity as you age. But then you have to ask whether there are some genes associated with that form of cultural intelligence as well.

Observational studies had already linked certain psychiatric conditions to specific forms of intelligence, like problem solving and crystallized knowledge. But then you took it this step further and found the genetic links. What do the genetic links tell us that the observational studies did not?

The perception in the past was that psychiatric conditions are a form of impairment. But it was hard to explain why those variants persisted in the population. Because if they’re just bad for people, then it doesn’t make sense that evolution has continued to select for them over time. But if there are advantages as well in those genes, then it would make sense to say, “Okay, yeah, evolution has kept those variants because there are also some benefits.”

So some psychiatrists have already explored this hypothesis with autism, for example. That was my main motivation: trying to understand why those genetic variants are still with us and are very common. Everyone has some variants for schizophrenia, for autism, for ADHD. But the more variants you have, the higher your risk of actually having the condition. Depending on the quantity of variants, then you can also have different personality profiles or patterns, without getting the disorder itself.

That’s why it’s interesting to understand the genetics, because you don’t need to have the disorder to have the correlation. It’s not just the family members of people with schizophrenia and bipolar disorder who have more creative jobs. People with a higher risk for schizophrenia and bipolar disorder also tend to choose more artistic or creative jobs. Maybe that creativity can explain why schizophrenia persists in our genes.

But if these different forms of intelligence and disorders are all polygenic, associated with large numbers of genes, why wouldn’t evolution have figured out how to just keep the good stuff and get rid of the bad?

What happens is that this would require a very decoupled nervous system. There is a concept in genetics called pleiotropy. One gene can have very different effects. You have the protein keratin. That is in your hair, but it’s also in your nails. That’s pleiotropy. You have one gene that can have different phenotypes associated with it.

Everything in our genome has effects on many different traits at the same time, because evolution likes to reuse the same systems over and over again. It’s easier to build on a system that’s already there and create a new function than to create a new set of genes, a new set of tissues, to create a new function. Everything is related to everything else. It’s impossible to decouple.

Did any of the specific links between a particular disorder and form of intelligence surprise you?

Yes. There were no observational reports that ADHD had any positive association with any form of intelligence, but then I found a positive association with reaction time. That makes sense.

With autism, you usually find that there is a positive association between the genes associated with autism and intelligence. But we found that it was mainly crystallized knowledge. That also made sense, because autism is associated with having very specific and intense interests and wanting to learn a lot about those interests, being kind of obsessive about a topic and having to learn a lot about the topic. You also find that in relatives of people with autism. Some studies have found that they tend to be over-represented in science, technology, engineering, and math.

I thought that ADHD was actually linked to high general intelligence?

It depends. There should be more studies of people with ADHD. For example, scientists have found that when autism is diagnosed early in your life, versus later in your life, there are different genetic associations. ADHD could have a similar pattern. When people have a high risk for ADHD, they can also be very smart, which allows them to mask their symptoms early on. They may only get diagnosed later in life when they have kids, higher stress, or they’re doing a Ph.D., and so on.

That happened to me. I wasn’t diagnosed with ADHD as a kid, but then as I started my Ph.D., I began to have symptoms. I was like, “Okay, I cannot pay attention. I have too many ideas.” I was always in the future, thinking about possibilities. I could not focus on right now. I would have trouble focusing on, say, a lecture, but if I recorded it and brought it home, I could hyper-focus.

When I was doing the Ph.D., I simply couldn’t record everything. It would take too much time, so I went to a neuropsychologist, and she did some tests. She said, “You probably have higher cognitive ability than some other people so maybe you were able to mask the symptoms earlier in life.” That means that the genes we associate with ADHD might be biased toward early diagnosis. We weren’t actually testing a lot of the people who are diagnosed later in life, when they’re 30 or 40.

Read more: “Does Depression Have an Evolutionary Purpose?”

Do you have a hypothesis about why these genetic links between psychiatric conditions and certain kinds of intelligence might have been favored by evolution?

The problem is that we cannot do experiments to recreate human evolution, so it’s hard to confirm anything. We can only make hypotheses, and then we may find evidence that is consistent with the hypotheses. We believe that evolution works via polygenic selection—or genes that have many different effects—but it’s hard to find those patterns because individual genes have very small effect sizes. And it’s hard to figure out whether a particular combination of traits exist on a single gene or was inherited from different ancestors.

There is one clinical psychologist named Simon Baron-Cohen who wrote a book called The Pattern Seekers: How Autism Drives Human Invention. The book has a hypothesis about the evolution of autism, but it’s hard to prove it. He offers evidence that’s consistent with the hypothesis that the very logical way people with autism think could be a good driver of human invention. It could influence innovation. For schizophrenia, scientists have different hypotheses. One of them is the shaman hypothesis. People with schizophrenia could have been good shamans, because they tend to be better at divergent thinking and free association of concepts. This would have allowed them to thrive in certain cultures.

You also found this genetic link between certain forms of intelligence and bone density. What do you make of that?

It’s hard to explain except for the fact that you cannot detach certain systems from other systems, as I mentioned earlier. A good example of this is domestication syndrome. Darwin found that domestic animals tended to have similar features compared to their wild counterparts, even when the animals were totally unrelated. They tended to have floppy ears, a floppy tail, spots on their fur. They tended to be smaller, with duller teeth. Darwin and other scientists wondered why. There was an experiment in the Soviet Union where someone tried to domesticate foxes. Over generations, they found the same pattern. Other people tried to explain why that happened, and they found that there are certain developmental cells called neural crest cells that are the precursors of certain cells in the hypothalamic-pituitary-adrenal axis, which regulates your fight-or-flight response, your immune response, and your aggressiveness.

When you select for animals that are more tame, then you’re regulating something in the system. And when you regulate that part of an animal that controls how aggressive it is toward humans, you also change expression of genes that govern these other things: how the bones grow, how the fur grows. We don’t know the mechanism, but this is genetic pleiotropy.

There have been some concerns over the years about genome-wide association studies. Have those concerns been addressed?

One problem is that the global north was over-represented in these studies. Most of the genetic data and other health information came from people with European ancestry. It’s still like that, but there are some efforts to collect more diverse samples. One of the reasons that Europeans are so heavily represented was that the effort and resources required to do a huge study, and recruit thousands of people, were mostly accessible to the richest countries in the world, which tend to be in the global north. But now there are consortia that are trying to gather research from different labs in different countries, to try to have more humans with African ancestry, with Asian ancestry, also Latinos.

But it’s also more complicated to study humans that are highly mixed, like Latinos or Africans. African ancestries are extremely diverse. Africa has the most diverse population in the world because they’ve been there for a long time. My study, for example, only makes conclusions about people of European ancestry. In order to do this study in people of Latino or African ancestry, I would have had to wait at least 10 or 15 years until we had enough data.

Do the findings from your study suggest any new or different ways to treat mental illness?

No, we’re not there yet. Maybe in the future. We’re focusing on broad statistical associations. The nervous system is extremely complex, and we’re still very far from understanding what is going on with these disorders.

Do your findings tell us anything about the dangers of using genetics to create designer babies?

Yes, let’s say you give parents the option to select embryos that don’t have a high score for schizophrenia. Maybe you’re going to be less likely to have a child with schizophrenia, but maybe you’re also going to be less likely to have a child with high creativity. There could be unintended consequences. We only use one single score that focuses on just one trait. We forget that all those traits are related to other traits that we care about. Maybe these parents would also like to have a very creative child. I think this paper tells us, “Okay, we should be more careful about this.”

Do you think the findings could help change how people think about the stigma attached to mental illness?

I think so. Of course, people suffer from mental illnesses. And that’s why they’re called illnesses. But it’s not that everything is bad. My motivation is to explore why these diseases are still with us. We don’t just have bad genes or good genes.

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