An illustration depicting the evolution of a man. The last image of the man depicts him kneeling with his head in his hands
N. CARY/SCIENCE

Have humans developed natural defenses against suicide?

While co-organizing a symposium a few years ago, a distinguished evolutionary psychologist named Nicholas Humphrey sought an expert to explore a mystery dating back to the time of Charles Darwin. "Natural selection will never produce in a being anything injurious to itself," Darwin wrote in On the Origin of Species.

But in humans, natural selection apparently did exactly that. Suicide is the leading cause of violent death, striking down about 800,000 people worldwide each year—more than all wars and murders combined, according to the World Health Organization.

Humphrey, an emeritus professor at the London School of Economics, knew that a handful of evolutionary thinkers had offered ways to resolve this paradox. But he couldn't find an explanation he thought fit most instances of suicide. So he decided to explore the topic and give the presentation himself.

Applying an evolutionary eye to epidemiological data and human cultures, Humphrey concluded that suicide was likely the tragic byproduct of a vital adaptation: the sophisticated human brain. While publishing a paper on his work after the conference, he found that another researcher had similar ideas. A psychotherapist named Clifford Soper, now in private practice in Lisbon, had done Ph.D. work concluding that the ravages of suicide are a consequence of human intelligence and have shaped our minds and cultures.

Such arguments may clash with the medical view that suicide is driven chiefly by psychiatric illness. And some clinicians may worry that people at risk could misinterpret the ideas as suggesting suicide is "natural." In fact, Humphrey and Soper propose that if what makes us human has put us at risk, it has also saved us. They argue that, faced with the persistent threat of suicide, humans have developed a set of defenses, such as religious beliefs, that are crucial elements of our culture and psychology.

"Humans very rarely die by suicide because we are superbly designed to deal with anything life throws at us, but our antisuicide defenses are not fail-safe either," Soper says. He suggests that those hypotheses may help explain why suicide is often impulsive, and also focus attention on effective prevention strategies.

The ideas are shaking up the field, says evolutionary psychologist Todd Shackelford of Oakland University in Rochester, Michigan. By shifting the focus to natural defenses against suicide, Soper has "completely flipped everything on its head," says Shackelford, who was an outside reader of Soper's thesis. "He may be dead wrong. But he's pushing the field in challenging new directions."

TERESA PETRINI

Soper was a part-time psychotherapist in 2014, living in Gloucestershire in the United Kingdom, when he found himself wanting to understand the origins of suicide in order to help people bereaved by it. As he combed through the scientific literature, he was captivated by the power of evolutionary thinking, which "seemed more useful than anything else," he says.

Soper eventually earned a Ph.D. from the University of Gloucestershire's School of Natural and Social Sciences in Cheltenham in 2017. "What I was thinking about was so strange that I knew I needed peer review," he says. "I needed people to talk back."

Soper calls his model pain and brain: When faced with agonizing pain, a sophisticated mind can think of death as an escape. In developing the model, he started with familiar facts. All organisms feel pain, which is vital to avoiding threats. But humans are arguably unique in our big brains, which allow us to have complex social lives, culture, and an awareness of death.

Humphrey followed similar reasoning. He and Soper say the combination could explain why suicide is both unique to and widespread among humans, having been reported in all types of human society, as varied as hunter-gatherer groups and industrialized nations. A 4000-year-old Egyptian poem mentions suicide, as do historical records from every era since. Humphrey also notes that suicidal thoughts and behaviors are vastly more common than the act itself. In the United States in 2017, about 4% of all adults, or nearly 10 million people, thought seriously of suicide, according the National Survey on Drug Use and Health. That's more than 200 times the number who died by suicide that year.

In contrast, Humphrey and Soper find no convincing evidence that other animals intentionally end their lives. And although suicide rates are rising among U.S. adolescents, suicide remains exceedingly rare among children: Of the more than 47,000 people in the United States who died by suicide in 2017, only 17 were ages 8 to 10; no such deaths were reported at younger ages.

Although suicide appears across human cultures, it is also a rare event. Soper and Humphrey point out that many cultures attempt to counter suicide by stigmatizing it or making it unthinkable. All major religions prohibit at least some forms of suicide, Soper says, as do many tribal customs. For example, Soper notes that the Baganda people of Uganda destroy the homes of people who die by suicide and banish their kin, as other scholars have documented.

Such defenses are perhaps easiest to see when they break down. Physicians and others have long worried about suicide contagion—for example, after the widely publicized death of a celebrity. Actor Robin Williams's death was associated with 1800 additional suicides in the following months, according to a study in PLOS ONE last year. Research cited by both Soper and Humphrey reported shockingly high annual suicide rates, as high as 1.7%, in certain villages on the island of Palawan in the Philippines. There, anthropologists reported that suicide was discussed casually and that villagers had little belief in an afterlife. And in a 2014 longitudinal study, researchers found that U.S. adolescents exposed to suicide in friends and family are more likely to experience suicidal thoughts and, sometimes, attempts. "There are existing cultural protections against suicide, and you can see them when they weaken," says sociologist Anna Mueller of Indiana University in Bloomington, who co-wrote that study in the American Sociological Review.

Religion's role can be complex, cautions sociologist Bernice Pescosolido, also of Indiana University. Her studies in the United States suggest that whatever a religion's doctrine, it may be most likely to protect against suicide when it nurtures close-knit social networks.

Most controversially, Soper proposes that mental illness itself can be a safeguard against suicide. He suggests certain mental disorders are associated with suicide because they were designed by natural selection to be last-line defenses against it. For example, he argues that the lack of initiative that accompanies depression may help prevent suicidal acts.

That argument is plausible for some kinds of depression, says Riadh Abed, who chairs the Royal College of Psychiatrists's evolutionary psychiatry group in London. But he and some other psychiatrists are skeptical of Soper's arguments about other disorders—for example, that compulsive drug use and addiction can sometimes dull unbearable pain and so may reduce suicides.

Even Shackelford says such ideas need more testing, and other scholars find them fringe. One critic is psychologist and leading suicidologist Thomas Joiner of Florida State University in Tallahassee. Joiner's interest deepened during his graduate school years, when his father died by suicide, and he has explored suicide's evolution because, like Soper, he thought the resulting understanding could help patients. But Joiner vehemently disagrees that suicidal behavior is a result of a natural human condition.

Rather, he suggests suicide may represent a misfiring of altruistic and self-sacrificing behaviors, something akin to honey bees that voluntarily sting intruders and die to protect the nest. Some suicidal people may mistakenly think they are reducing a burden on their loved ones. To Joiner, "it couldn't be clearer" that those deaths are driven by psychiatric illness and that suicidal impulses must be treated as such.

Evolutionary psychiatrist Randolph Nesse of Arizona State University in Tempe says he's "intrigued, but by no means convinced," by Soper's idea that humans have been shaped by tendencies to avoid suicide. Nesse emphasizes that suicide "is a behavior that can have many possible causes and motives," so no unified evolutionary theory can adequately explain it.

Despite such critiques, Humphrey hopes this work may help patients. "I like to think that it might help a person if she could see why she has evolved to have a brain that is vulnerable to opting for this disastrous, short-term solution," he says. "A person who can see through the logic behind her suicidal impulses may be best placed to resist them."

For help, call 1-800-273-8255 for the National Suicide Prevention Lifeline, or visit https://www.speakingofsuicide.com/resources.