The modern environment—whether it’s the social environment or other aspects such as the kind of the foods we eat—differs from the original human environment, that of hunter-gatherers. This was the environment in which human traits evolved, and humans lived in for most of their evolutionary history i.e. tens of thousands of generations. Now, they live in different social conditions and this may lead to a mismatch, which can affect mental and physical health. Schizophrenia is one example, and eating disorders are another.

Now, I’ll talk a bit about my theory regarding schizophrenia, which you asked me about. My theory is that mismatch, particularly concerning modern social conditions, plays a significant (but is not the only factor) in causing or contributing to the biological roots of schizophrenia. First, there’s an observation: there was a common but mistaken belief that the rates of schizophrenia were the same worldwide and across all societies, with 1% of people affected. This is incorrect. Studies now show that the rates of schizophrenia vary from one society to another, and that environmental conditions of societies impact the incidence of schizophrenia. Another observation is that immigrants have much higher rates of schizophrenia than native-born populations.

Can we mention some figures about the incidence rates among immigrants and refugees, or the 1% rate you mentioned—what’s the current rate?

It varies widely. Recent studies show that the difference in schizophrenia rates across societies can vary as much as 30 times from the lowest to the highest. This means that environmental conditions in some societies increase schizophrenia rates, while in others, they result in much lower rates. For example, take the West Indians, residents of the Caribbean islands. Many migrated to Britain in the mid-20th century seeking job opportunities after the war, when there was a need for unskilled labour, and migration from the Caribbean to Britain was encouraged. Now, there are second and third generations of these immigrants, and the rates of schizophrenia in these communities is several times higher than among native (white) London residents.

The rate of schizophrenia among West Indian communities—who are, of course, of African descent, taken centuries ago as slaves to work in sugar plantations and whose descendants later migrated to Britain—is very high, exceeding four to five times the rate among native White Londoners. More importantly, the schizophrenia rate among West Indians in the Caribbean, where they form the majority, is a fraction of the rate among those in London—ranging from one-tenth to one-fifth of the rates. This means that immigrants who came to Britain, along with their children and grandchildren, have schizophrenia rates five to ten times higher than those who remained in the Caribbean.

This increase in rates of schizophrenia is consistent across multiple studies and is now an established fact, even down to the neighbourhood or street level where someone lives. If you are a minority in that neighbourhood or street, your schizophrenia rate increases; if you are part of the majority, it decreases. This phenomenon is now understood and termed “ethnic density effect” referring to the population density of immigrants in a given area and its relation to schizophrenia rates among those immigrants.

My theory, which I co-authored with an Iraqi colleague who trained under me and is now a consultant in Leicester, named Mohammed Juma Abbas, was published in 2011 and is called “The Outgroup Intolerance Hypothesis”. This theory suggests that the risk of schizophrenia is partly—not entirely, we don’t claim it’s the sole cause—due to a mismatch between certain psychological traits that are strongly or abnormally triggered by the presence of people different from you, whom we can call “strangers.” Living among strangers, a phenomenon largely absent from the original human social environment where people lived in small, tight-knit communities and knew each other intimately, perhaps for a lifetime, was rare. Mixing with strangers was very uncommon, and living among them was unknown. Even if someone encountered strangers, through cohabitation, they would become familiar.

Compare that to living in modern cities—we all live in a sea of strangers, not just strangers today but strangers forever. In a big city, you’re often surrounded by people you don’t know and will never know, because your interaction with them is nothing more than superficial, temporary, and fleeting. This is a mismatched condition absent from human history, and some people are more sensitive to it than others. We published—I don’t know if you’ve come across this paper in your review of my work—a paper with a couple British colleagues in early 2024 titled “The Modified Aberrant Salience Hypothesis,” where we tried to explain not just the social mismatch but also the factors increasing schizophrenia risk within the brain itself. Sorry, did you want to ask a question?

Cities as Environments of Alienation

Yes, several questions. You mentioned brain and genetic factors, and these questions weren’t planned, but I find them more important than others. The second question might come from a psychology rather than psychiatry perspective: What are the limits of this alienation and estrangement? How do we diagnose it scientifically? Someone might say that Dr. Riadh Abed came to the UK a long time ago, now holds a position in academic institutions, and has lived for decades in a certain area—does the concept of alienation still apply? At the same time, someone from Wales or Scotland might move to London—would alienation apply to them as much as it does to someone from the Caribbean or someone like me from Iraq?

Excellent questions, and I’ll address them briefly because these topics could be discussed for hours. Absolutely, alienation applies to everyone. I gave the Caribbean immigrants as an example because they’ve been extensively studied, and we know exactly what the rates and risks of schizophrenia are in that specific community. But that doesn’t mean someone from Wales or Scotland wouldn’t have increased rates—no, their does also increase. Let me clarify an important point: studies now clearly show that living and growing up in cities increases the likelihood of schizophrenia compared to living and growing up in rural areas, globally.

Omar: So, can we say cities are environments of alienation?

Yes, exactly. We must remember something: cities are a modern phenomenon, not exceeding a few thousand years. The earliest cities emerged in the Fertile Crescent about six to seven thousand years ago. Homo sapiens has been around for 300,000 years, and the Genus Homo, from which Homo sapiens emerged, is about 2.6 million years old. If we go back to just the Genus Homo—not the common ancestor with chimpanzees— from which about eleven human species emerged, that’s 2.6 million years ago. How long have cities existed? No more than seven thousand years, or ten thousand at most. What’s that as a percentage of 2.6 million years? A tiny fraction.

That fraction—and we’re talking about cities that emerged less than ten thousand years ago—what kind of cities were they? They didn’t exceed a few thousand inhabitants, small or even tiny by today’s standards. For example, we know that ancient Rome was one of the first cities to reach nearly a million inhabitants, around 2,100 to 1,700 years ago at its peak. That was a rare exception among cities that reached such a size. So, cities are an aberration and an anomaly in human history, an unusual condition causing numerous mismatches, not just socially.

I’ll explain some facts listeners might find interesting: life in cities was terrible until the 19th century. Why? Because of poor sanitary conditions. Until the causes of epidemics were discovered—which weren’t known—cities saw sudden outbreaks of plagues, cholera, smallpox, killing tens of thousands. In many periods of human history, city dwellers were forced to leave and live in the countryside temporarily to escape death from epidemics. Cities were unhealthy places. If you read Baghdad’s history, it’s a history of epidemics. I refer to Ali Al-Wardi’s books, who detailed the plagues in the 18^th and 19th centuries, mentioning that at one stage there were only 2,000 survivors left. People would regularly abandon Baghdad and go to live in the desert until the plague or cholera subsided, because they didn’t know what caused these epidemics.

After the causes of diseases were discovered in Europe in the 19th century, sanitation and public health measures were taken especially with regard to sewage disposal and water purification. However, these measures did not become widespread in the rest of the world until well into the 20^th century.

Back to the environment and mismatch: a person growing up, regardless of their ethnic or racial origin, in modern cities today, anywhere in the world, has a higher schizophrenia rate than if they lived in rural areas or villages—places with low population density. High population density increases the risk of schizophrenia. I hope I’ve answered your question somewhat. It’s a very big topic that one could elaborate on extensively but we need to cover other subjects.

Yes, a really fascinating topic—I didn’t expect all these dimensions. Some might be philosophical, and some might prompt people to rethink life decisions. If someone imagines moving to another city or environment might contribute to more sadness, loneliness, or even schizophrenia, they might reconsider such a decision. But I’d like to ask quickly, based on your knowledge of this range of studies and the impact of alienation, does it affect other conditions like depression or other mental health issues? Before moving to another topic, I noticed your focus—or part of your research—on eating disorders and anorexia and their connection to evolutionary psychology. So, my first question: Were there other disorders? If you answer that, let’s move to eating disorders.

Okay, thank you for that question. The answer is yes—mismatch relates to a vast number of psychological disorders or mental illnesses, including depression, addiction, eating disorders, and more. Even dementia has a connection to mismatch, although in a different way. Environmental conditions can even affect dementia rates. Mismatch, as a distinctly evolutionary concept, is neglected—frankly, it’s neglected by modern medicine in a way that is hard to comprehend, because modern medicine generally ignores or doesn’t care about Darwinian or evolutionary concepts.