Looking at drug use from an evolutionary perspective

Ed Hagen, Associate Professor of Anthropology

For 15 years, Ed Hagen has been challenging the conventional theory that drug addiction is the consequence of a “hijacked” reward mechanism in the brain—the idea that people become addicts because taking drugs accidentally triggers brain circuits that make them feel good.img_5238

Studying hunter-gatherer and developing societies, Hagen, associate professor of anthropology at Washington State University Vancouver, and his frequent collaborator, Roger Sullivan, an anthropologist at California State University, Sacramento, believe that there are evolutionary reasons for drug use and addiction. Drug plants such as tobacco and marijuana contain natural neurotoxins, he said, and they might serve an evolutionary purpose in protecting people from certain infectious diseases and parasites.

“One of our main hypotheses is that we might have a taste for drugs as a form of self-medication against pathogens—it isn’t conscious, but maybe we sample some toxic plants and find that some seem to have medicinal effects, so we consume them when we get the chance,” Hagen said.

Hagen, Sullivan and others have published widely on evolutionary psychology, which is an approach that sees continuity in the evolution of behavior and cognition in both animals and humans. Evolutionary psychology raises the possibility that some forms of drug use could be an adaptation.

Hagen and Sullivan began studying drugs from a mental health perspective. Knowing that people with schizophrenia and depression tend to use more drugs, he said, “we thought we should start from the perspective that the origins of these drugs were as plant-defensive chemicals and are pretty potent neurotoxins, and see where that got us. Fifteen to 20 years later, I think it has gotten us somewhere.”


While the reward theory has a solid empirical basis, he said, “we still don’t have any really good treatments for drug addiction. The mainstream model has not yielded the result we had hoped for. It’s time to consider other theoretical approaches.”

In an article published in 2013 in the journal Frontiers in Psychiatry, Hagen and collaborators write: “We accordingly challenge the popular idea that the rewarding and reinforcing properties of drugs ‘hijack’ the brain, and propose instead that the brain evolved to carefully regulate neurotoxin consumption to minimize fitness costs and maximize fitness benefits.

“Why would anyone want to consume a toxic substance?” Hagen asked. “There is increasing evidence that non-human animals seek out toxic plants when they have an infection. Pretty much the same parasites and pathogens that attack plants attack us. If a plant has evolved something effective against its parasites, it’s probably effective against our parasites.”

Drug toxicity appears to explain many patterns of drug use. For example, children avoid drugs, and pregnant women use them much less than men, perhaps because of potential harm to the fetus.

“These plants might be particularly toxic to kids because their brains and organs are still developing,” Hagen said. “There are good reasons for kids to be cautious of toxic plants. They taste bitter, don’t look like they would taste good, don’t smell good, so they give off all kinds of cues that should tell kids that’s something I don’t want to eat. The same is true for pregnant women.”

Of course, culture and social learning also play a role—children and pregnant women are often warned away from harmful substances.

Growing evidence

Drug use is just one line of research for Hagen, who also studies depression, suicide, child development and evolutionary models of leadership, as well as evolutionary approaches to ontogeny, cognition and behavior.

He started drug research casually, Hagen said, “but the more we worked on it, the more we felt it was an important contribution to the literature on substance use. So when I got here, I decided to devote a lot of time to actually running studies to see if this perspective was fruitful. That was one reason I came here.”

The research team

The research team

Hagen has conducted studies among the Aka hunter-gatherers of the Congo Basin with WSU Vancouver anthropology professor Barry Hewlett. The Aka are heavy users of tobacco and marijuana.
“The Aka were a perfect population,” he said. “They use lots of plant drugs and are heavily infected with intestinal worms and other parasites. Maybe that’s not a coincidence. We were very interested in measuring that. We started one of the only systematic investigations of drug use in traditional hunter-gatherer populations,” testing saliva, urine, and feces for evidence of parasites and correlating the results with smoking.

“It looks like heaviest smokers have fewer worms,” Hagen said. “If you treat people for worms, it looks like they smoke less.”

Aka camp

Aka camp

While evolutionary studies may hold clues to the drug use puzzle, Hagen says reward studies also are important, and he sees a long road ahead. “I’d say our results on parasites are enough for us to keep working, but not strong enough to compel other folks to change their minds—yet,” he said.

Anthropologists and global public health researchers share many interests, and one potential area of promise lies in global smoking prevention. Women in the developing world rarely smoke, and therefore represent a potential huge new market for tobacco companies, but if those women currently avoid tobacco use to protect their fetuses, a good time to reach them with an anti-smoking message might be the age when they are considering marriage and children. “If we can tie the tobacco decision to other decisions that are more immediate in their lives and maintain a cultural tradition”—that is, heeding the advice of mothers and grandmothers, he said, it might help prevent a huge increase in tobacco-related illnesses and also empower women to take more control over their lives.

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