I've been thinking about suicide. Not for myself, mind you, but because it's such an evolutionary puzzle. After all, even though evolutionary biologists know full well that no complex human behavior is rigidly and unilaterally determined by genes alone, it remains a well established article of faith that even complex human behavior has at least some underlying genetic component. (Is it an oxymoron, by the way, to suggest that an "article of faith" can be "well established," given thata reasonable definition of faith is belief without evidence? Oh well, that's another question, for another time!)

The suicide puzzle is simple enough to state: any genetically influenced phenotype including behavior that leads to elimination of the genetic factors themselves should be strongly selected against. And yet, suicide appears to be a cross-culturaluniversal. According to the World Health Organization it is the 10th leading cause of deathworldwide, responsible for nearly 1.5 million annual fatalities.

Freud was convinced that the answer, essentially, was "Thanatos," which was among his most crackpot and biologically ignorant theories. Although the word doesn't appear directly in his writings, "death drive" (Todestrieb) does, and is ostensibly opposed to "Eros," the "life drive." In Beyond the Pleasure Principle, Freud maintained that living things possess"an urge ...to restore an earlier state of things," specifically that inorganic simplicity from which all living things emerged. It is a force"whose function is to assure that the organism shall follow its own path to death." This "explanation" is total BS, reminiscent of Henri Bergson's notion that life is due to an "lan vitale," which Julian Huxley caricatured as being equivalent to explaining the movement of a railroad train by its "lan locomotif."

It is plausible that natural selection could favor suicide if, by doing so, predisposing genes were benefiting identical copies of the same genes residing in other bodies namely, in genetic relatives andthus operating by the well-established phenomenon of kin selection, or inclusive fitness. This process hasalready been demonstrated to be a powerful explanation for "altruism" in many animals, both nonhuman and human. But what about cases where this explanation doesn't apply, namely individuals whose death benefits no one, genetic relative or not?

Emile Durkheim, one of the founders of sociology, made an important contribution in his classicbook, Suicide. In it, he identified five distinct social explanations for self-killing: egoistic, altruistic, anomic, and fatalistic, each worth understanding, but none providing any reconciliation with evolution by natural selection.

Some biological coherence, on the other hand,appears to derivefrompathology. Many people die of heart disease or cancer, not because doing so is adaptive, but becausein various ways, our bodies arevulnerable, just like any organism. Similarly, there arealso mental pathologies,notably depresion, due to biochemical malfunctions, among other things. Powerful evidence for this and for not beingcavalier about suicide comes from the fact that antidepressant medications and/or psychotherapy often abolishsuicidality.

Beyond this, however, are cases in which suicide is not precipitated by depression per se. And that's where my hypothesis comes in. Start with pain,a biological warning signal that something isout of whack. Accordingly,living things, including people, have doubtless been selected to minimize painand to avoid it when possible. (For example, it is pain thathelps prevent us from embracing a hot stove.) Then add two consequences of our big, smart brains: knowledge of death, and of killing.

There is much debate about the evolutionary pressures that made us so clever, but no doubt that our species hasbecomevery, veryintelligent. And furthermore, it seems likely that at least some of ourmental abilities e.g., painting, composing symphonies, programming computers, playing the violin weren't selected for directly but arose as a by-product of a creative intellect that was favored because it conveyed other, more clear-cut benefits, such as social cooperation, complex communication, and so forth.

I suggest that as a byproduct, human beings perhaps alone among living things understand about death.(Shades of Ernest Becker.) We don't simply act in most cases to avoid death; our speciesunderstands that it means the cessation of life, and thus, of all sensation not just the end of pleasure, but also of pain. Finally, due once again to our big brains, we have figured out how to end life: not only the lives of prey or other people, but also, by simple extrapolation, our own.

Put these three observations together: (1) people are strongly predisposed to avoid pain, (2) our speciescomprehends that death meansthe end ofbodily sensation, and (3) we know how to kill, including how to kill ourselves. Couldn't the result then be that when individual Homo sapiens experience unrelenting and untreatable pain (physical or emotional)they, unique among living things, can be inclined to endthat pain? Other animals sometimes behave in a way that results in their death: Honeybees die when they sting an intruder to their hive, Pacific salmon die after spawning, and so forth. But there is no reason to think that they are intentionally killing themselves. We, on occasion, are different.

The above considerations are neither intendedto condone suicidenor to oppose it. Moreover,I am not proposing that myhypothetical triad explains more than a subset of suicide, which is doubtless multi-factorialand differs for different people. In addition, this difficult and complex topic is immense, such that perhaps thethree-part biologically based hypothesis presented here isn't even valid. Or new. But maybe it's at least worth thinking about.

David P. Barash is professor of psychology emeritus at the University of Washington. Among his recent books is Through a Glass Brightly: using science to see our species as we really are (2018, Oxford University Press)

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Thinking About Suicide: A Three-Part Hypothesis - Psychology Today