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That spring felt dark and heavy. Outside, the light was bright, the air crisp and the tulips were in full bloom. But inside, I was a mess—sad, sluggish, tears rolling down my eyes for no good reason. Nothing was wrong, and yet I just couldn’t stop crying. "Why am I depressed, I wondered?"

A simple blood test ultimately identified the culprit—an underactive thyroid gland. After a couple of weeks on medication, I was totally fine. But the notion that a little nodule in my neck could wreak havoc on my entire mental life—on my feelings, thoughts, and sense of self—was shocking. How can a bodily organ control my mind?

This division between body and mind isn’t just the stuff of philosophy. People naturally think of themselves in these terms—across many cultures. Most of the time, we aren’t even aware of that distinction. And yet it affects countless aspects of reasoning.

When we experience emotional pain, we often wonder whether the cause is “psychological” or “biological,” as if the two were different. Science tells us that they are not—our psychological life and the body are one and the same; my psyche is my brain. This is why an underactive thyroid gland made me sad. But intuitively, this seems weird—an oxymoron. It’s almost like saying that scratching my ear can turn on the light bulb. It is for this reason that people wrongly believe that if a psychological disorder has “biological” causes then it cannot be alleviated by psychotherapy—only medication can help. But if it arises from “psychological” causes, such as childhood trauma, then here, only “talk therapy” can work; medication won’t. Unfortunately, even trained clinicians fall into this mind-body trap. And, to reiterate, science shows that this reasoning is faulty.

Mind-body divisions also infiltrate our courtrooms and classrooms. When judges and potential jurors are informed that the defendant’s actions are linked to a bodily cause—an abnormal brain scan, or some genetic risk factors—they view the defendant as less culpable, and the verdict is more lenient. This divide between bodies and minds further explains why people marvel at neuroimaging studies demonstrating that mental functions “show up” in a brain scan.

The illusion of the mind-body divide, then, is pervasive and costly—it can seriously derail how we view ourselves and how we judge others. But why do we think this way? Why do we view the mind as “ethereal,” distinct from the physical body?

Nurture—culture, especially religion, can, no doubt, play a role; we could have learned to think this way from our experiences. And yet, the nurture view cannot explain why the mind-body divide is so prevalent across distinct human cultures and societies.

Recent evidence suggests that nature is also to blame. The mind-body divide arises from the core mechanisms that make the human mind tick. It emerges because humans are endowed with distinct cognitive systems for reasoning about the physical world, on the one hand, and people, on the other.

For example, young infants believe that a stationary ball can only move upon contact with another ball—they tacitly believe that inanimate objects must obey the laws of physics. But infants aren’t the least surprised to see a person move spontaneously—they apparently assume that people’s actions can arise from what they know, believe and desire—their mental states. Because we apply different mechanisms for reasoning for physical objects and for “reading” the psychological states of people, we assume that psychological states aren’t physical. Mind, then, might naturally seem distinct from matter—this division could arise from how our mind works, innately.

Recent research from my lab supports this proposal. The logic is simple. Suppose the mind-body divide arises naturally, from “mind-reading” (the psychological system that guides our reasoning about people, as opposed to objects). If so, then individuals with stronger mind-reading abilities should show a stronger mind-body distinction. Conversely, those who struggle with mind-reading ought to consider bodies and minds alike.

There is good evidence that this is the case in males. Males score lower on tests of “mind reading” than females, and this is the case across cultures. To be clear, this generalization only holds on average; at the individual level, there are clearly gifted male psychologists, so sex is not destiny. Still, if stronger “mind-reading” naturally begets the contrast between minds and bodies, then, as a group, males ought to view the mind as more similar to the body than females. Indeed, they do.

For example, men are less prone to believe that one’s thoughts could persist without one’s body (e.g., in the afterlife), and they are more prone to believe that maintaining one’s body (e.g., in a futuristic body replica) will maintain one’s thoughts. Furthermore, the magnitude of the mind-body divide correlates with mind-reading ability—the more readily one can reason about the minds of others, the more likely one is to view the mind as ethereal, distinct from the body.

These results agree with a previous study from my lab, in which we compared the mind-body divide in neurotypical and autistic people—people with autism score lower on mind-reading tests, but higher on tests of intuitive physics, and indeed, they, too, are less likely to consider minds as distinct from bodies. So, in two different studies, we found that the illusion of the mind-body divide is attenuated in two distinct groups that are naturally weaker in “mind-reading.”

It is no wonder then, that, many years later, and despite the hindsight of science, in my gut, I’m still shocked to recognize that a bodily gland messed up my mind. I doubt this dualist mindset can be utterly erased. But perhaps recognizing this “mental bug” could help us live with it better and be happier.