Wednesday, October 8, 2008

The Evolutionary Biology of Suicide: Is Suicide Adaptive?

See also "How People Die By Suicide," my review of Thomas Joiner's book, Why People Die By Suicide, challenging Joiner's refusal to consider an adaptive model for suicide and attempted suicide.

Suicide, like filicide, seems upon first consideration to be a ludicrous act, viewed from the perspective of evolutionary biology. However, the act of filicide (the killing of one's child or children) is clearly adaptive in many cases. Not only that, but it can be shown through statistical evidence that actors seem to differentiate between adaptive and non-adaptive filicides when they "decide" to commit filicide (as well as other homicides). What about suicide?

We can define an act to be adaptive when the act increases the genetic fitness of the actor - that is, when the act's benefits - in terms of survival, procreation, or nepotistic distribution of resources to one's genetic relatives - exceed the act's costs, in the same terms. When considering adaptiveness of acts or traits, it is important to consider not only the particular acting organism and its existing offspring, but all its genetic relatives, including possible future offspring and non-offspring relatives. (A judgment about whether an act is adaptive or not implies no moral judgment. An adaptive act may be praiseworthy, horrible, or neither.)

Filicide is adaptive when the resources to be expended raising a particular child would be better spent on others, in terms of benefit to genetic relatives, present or future. If a child is likely to die or otherwise fail to reproduce, or if raising the child will adversely affect the parent's ability to have future children that may be a better genetic "bet," then filicide - while certainly a gruesome act - must be said to be adaptive. Perhaps even more obviously, filicide of a partner's offspring that is not one's own genetic offspring is almost always adaptive.

Are people more likely to kill their children under circumstances where the act is adaptive? A major body of work in evolutionary psychology suggests that this is so. For instance, stepparents are much more likely to kill their stepchildren than birth parents are to kill their genetic children. People are much more likely to kill babies than older children, and younger mothers are more likely to kill their babies than older mothers. It seems that not only is filicide sometimes adaptive, but that humans possess mechanisms to prevent themselves from committing filicide when it is not adaptive, and to allow themselves to commit filicide when it is adaptive.

Does the same hold true for suicide? There are two related questions: first, is suicide ever adaptive? Second, if suicide is sometimes adaptive, do humans appear to possess mechanisms to limit suicide to cases where it is adaptive?

As to the first question, it can be clearly demonstrated, at least in the abstract, that suicide is sometimes adaptive. The easy case is one in which a person sacrifices her life so that genetic relatives may live. Such cases must be rare, and are so different from the usual connotations of "suicide" as to barely be considered suicide at all. The more common case where suicide is adaptive is this: one's total expected future contribution to one's genetic fitness is exceeded by one's total expected drain on the resources of one's genetic relatives.

To put this in more concrete terms, there are many cases - old age, crippling disability - where all measures of genetic fitness approach zero. Once one may no longer reproduce, and is no longer an effective nepotistic distributor of resources (including wisdom), one's expected contribution to one's own genetic fitness is likely to be nil. However, as long as one survives in this condition, he not only contributes nothing to his own genetic fitness, but also likely drains the resources of his genetic relatives. His continued survival is contrary to his genetic interests. Therefore, suicide, in this situation, must be said to be adaptive.

The second question is: given that suicide is sometimes adaptive, do human beings tend to commit suicide in circumstances where it is adaptive?

Unfortunately, I lack the quantitative tools and study design knowledge to answer this question. But I will point to two conflicting bodies of evidence on this question.

In support of the idea that humans tend to commit suicide when it is adaptive is data demonstrating that the suicide rate increases dramatically for elderly people. Using age as a rough estimate of expected genetic contribution, the suicide rate (inversely) tracks genetic fitness. Excluding children, the suicide rate is lowest for those with the highest expected evolutionary fitness - those ages 15-34. The suicide rate climbs from there. In 1950, before the age of nursing homes, the suicide rate for those ages 75-84 was more than double the average suicide rate for the country. In addition, suicides by the elderly are more likely to be planned, with less likelihood of warning prior to the act.

In addition to data on suicide among the elderly, suicide is particularly likely in elderly people with poor health, and particularly those with low vision - both conditions that, in the environment of evolutionary adaptedness, might be expected to indicate low expected contribution to the fitness of one's genetic relatives.

Contrary to this is the data on familicide. Familicide is the killing of one's spouse and children. It is almost the exclusive province of males. There are two key features of familicide that are relevant to the discussion of the adaptiveness of suicide: a familicide is much more likely to kill his biological children than a mere filicide, and a familicide is much more likely to commit suicide than a simple filicide or uxoricide (wife killer).

Familicide seems to be an extremely counter-adaptive act, especially since the killer's biological children are commonly involved, compared to simple filicides, which are more likely to involve stepchildren. The fact that suicide is much more likely in a familicide, as opposed to a simple uxoricide or filicide, suggests, by association, that suicide is frequently a counter-adaptive act.

Note: the question of whether suicide is adaptive is an entirely different question from whether it is rational, in the lifetime-utility-maximizing sense.

6 comments:

  1. Very thought-provoking post. Considering that suicide may at times be counter-adaptive, one wonders if it is, perhaps counterintuitively, a uniquely rational act--in that it's uniquely human.

    It seems to me only humans have the logical chops to make a cost-benefit analysis of life itself, measuring its worth against that of non-existence. Are there any other animals that choose suicide, that are able to transcend the daily exigencies of simple survival to escape their own personal vale of tears?

    Evolutionary psychology is fascinating, but I think it's important to remember that biology is not necessarily destiny. Obviously, not every mother or father who could further his or her own genes' survival by killing his or her offspring (by murdering stepchildren or sacrificing one child to benefit another) does so. They make ethical and rational choices to handicap their own genetic prospects. Similarly, suicide might be a choice that is not "rational" from the perspective of genetic survival but, ironically, serves a higher rationality.

    That said, I think suicide becomes difficult to justify even within a strictly utilitarian ethical framework. The emotional burden it puts on those around one likely far exceeds what comes with ordinary death. In that sense, it may indeed be rational to the suicide while nevertheless remaining highly unethical.

    Now, whether one has the right to suicide is another question entirely.

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    1. Putting someone in a position where he or she wants to commit suicide is, in the antinatalist view, the primary unethical act. But if you ignore the imposition of birth and the suffering that life carries with it, suicide definitely causes suffering to one's friends and family. (So does natural death, but we don't blame the person for that - we blame nature.)

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  2. There is another mechanism by which suicide is adaptive.

    If a person is valuable to the tribe, his suicide would harm the tribe. If his death would reduce the tribe's ability to survive significantly enough, it can be used as a threat to gain relative proportional resources.

    This is why you see overt suicidal behavior in teenagers more than in other ages. They have not *yet* been useful, but they are an investment, and it would be a waste to just get rid of this investment.

    Now, why does this result in actual suicides, rather than "mere" threats of suicide? Game theory holds the answer. For a threat to work, it must be credible. You wouldn't believe anyone was about to commit suicide if it never happened. However, if everyone who threatened to commit suicide was actually going to do it, it would be very costly and there would soon be people taking advantage of the free resources given to suicide threats. "Cries for help" suicide threats, as 'twere. It turns out the evolutionary stable strategy is a mix of genuine suicide threats with non-genuine resource-grabbing (cry for help) suicide threats.

    None of this happens consciously or even unconsciously. It just happens as a result of which game theoretical strategy produces the best inclusive genetic fitness.

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    1. The reason I hold to this theory even though I've never seen an evolutionary psychologist or evolutionary biologist defend it is because I have seen it independently invented by many people who are familiar with both evolutionary psychology and game theory (but without degrees), and because it explains not only why there are suicides at all, but why there are true suicide threats and false suicide threats, why both are taken seriously, and why suicide is a low-status, degradable thing. It is only natural that a selfish resource-grabbing strategy that harms the tribe will be looked down upon.

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    2. There's an interesting paper to the effect that people who attempt suicide but survive see an increase in income afterwards - the more serious the attempt, the bigger the boost in income. (I discuss this in the post on Joiner's book.) Also relevant: Attempted Suicide as a Signal, in which I argue that thwarting suicide attempts encourages insincere people to make them, at great cost to people who don't really want to die and people who want to die alike.

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    3. This is why you see overt suicidal behavior in teenagers more than in other ages.

      Also, this is used to explain why (in our country, not in others) women attempt suicide more than men but men succeed more. Someone (maybe it's Posner) has a paper on choice of suicide method for optimal outcome, balancing the desire to survive with the desire for perceived lethality enough to trigger resource-influx. Women are more valued, the argument goes, so they can choose less-lethal methods and still get attention.

      Doesn't work for China and India, but it's an interesting idea. Joiner blames Confucianism for the China/India thing, but you could see it as a consequence of lower status of women in general. Still pretty ad hoc. I bet you could do better quantitatively.

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