Conceptual Issues with Evolutionary Psychology: The Case of the ‘Mate-Killing Module’

In his book The Dangerous Passion (2000), evolutionary psychologist David Buss provides “an evolutionary explanation for mate killing,” (pg. 120) based on earlier work with his colleague, evolutionary psychologist Joshua Duntley. Buss writes that, “Duntley and I argue that men have evolved a mate-killing module, a psychological mechanism whose function is not threat or deterrence, but rather the literal death of a mate.” (pg. 122).

Now, at first glance, this may seem like a very extreme claim (and to be clear, I think it is), however when we read Buss’s reasoning, we can see the proposal is not that men have a kind of unchecked urge or drive to murder their partners, but rather that they have a “psychological mechanism” that leads them to do so in certain contexts, “According to this theory, over the long course of human evolutionary history, it has been reproductively advantageous for men in some circumstances to kill an errant partner, especially when the finality of her departure sinks in.” (pg. 123). But if indeed this is a behavior that men are expected to engage in only in select, specific contexts, it is hard to see the utility of the unspecified and overly general ‘mate-killing module’ explanation. Buss and Duntley are, without appearing to realize it, actually making a behavioral ecology argument about different socioecological contexts incentivizing different fitness strategies, and then adding on top of that a single ‘psychological module’ that supposedly explains this behavior across these contexts.

This is important because evolutionary psychology as a discipline was founded on opposition to certain key dimensions of human and animal behavioral ecological thinking, which generally considers behavior as reflecting fitness maximizing strategies that vary substantially depending on the socioecological environment. For example, one of the founders of evolutionary psychology, Donald Symons, in an influential 1989 paper that laid the intellectual groundwork for the discipline, writes that, “In DP [‘Darwinian psychology’], human activities in environments differing radically from the EEA [Environment of Evolutionary Adaptation] are as informative as human activities in more traditional settings, even though the former are likely to depart radically from the path of fitness maximization.”

In other words, if men really evolved a ‘mate-killing module’, then why is it that men are only supposed to express it in specific, fitness relevant and (supposedly) adaptive, contexts? Evolutionary psychologists often contest the socioecologically-contingent fitness maximization logic in other domains and emphasize the primacy of ‘psychological mechanisms’, which may be adaptive or mismatched in different environments. Buss, in the most recent volume of his textbook Evolutionary Psychology (2016), writes that,

An important point to keep in mind is that a mechanism that led to a successful solution in the evolutionary past may or may not lead to a successful solution now. Our strong taste preferences for fat and sugar, for example, were clearly adaptive in our evolutionary past because fat from meat and sugar from ripe fruits were valuable and scarce sources of calories. Now, however, with pizza places selling pies and sugar-laden soft drinks on every street corner, fat and sugar are no longer scarce resources. Thus, our strong taste for such substances now causes us to overconsume fat and sugar, which can lead to clogged arteries and heart attacks and thereby hinder our survival. This can be called a mismatch between ancestral and modern environments (see Li, van Vugt, & Colarelli, 2017). We will be examining other evolutionary mismatches later in the book, but the central point is that evolved mechanisms exist in the forms that they do because they led to success on average during the period in which they evolved [emphasis added] (pg. 44).

Note the final sentence: “success on average during the period in which they evolved,”—can one plausibly argue that killing one’s partner was, on average, adaptive? Putting aside the lack of real utility of the ‘mate-killing module’ explanation, as well as the lack of any evidence for it whatsoever, at the level of the brain, or the genetic level, or otherwise, let us consider the specific circumstances Buss and Duntley have in mind for when this ‘mate-killing module’ is supposed to express itself (see pgs. 122-123).

Buss writes, “First, in a polygynous mating context, where a man might have several wives, killing one wife as a result of an infidelity or defection could prevent other wives from cheating or leaving.”

Maybe. However, could this not just as easily induce other wives to attempt to flee when opportunity arises, or join up with an alternative male protector, in ways that hurt this hypothetical man’s fitness rather than help? —to say nothing of possible reprisal from the wife’s kin. In any case, this logic seems very dubious in most circumstances. Next,

Second, in some cultures, a man’s reputation would have suffered so extensively as a result of a wife’s infidelity that killing her would be the only means of salvaging lost honor. Killing an unfaithful wife sometimes restores a man’s honor. As Daly and Wilson note, “Not infrequently, men salvage some of their lost honor by killing an unchaste wife . . . and the male seducer. Shrinking from such vengeance may even add to their dishonor.”

I don’t think this is necessarily wrong, but note, even more so than with the polygyny example, we are dealing with a clearly *learned and culturally contingent* phenomena. We’re talking about particular cultures oriented around certain conceptions of honor which are socially learned and historically contingent. In this case the ‘mate-killing module’ is very obviously unnecessary to explain this. Further, it is not necessarily clear that such behaviors would increase one’s reproductive success, even if they may serve to in some cases protect one’s status (at least among the men or perhaps their lineage) within their society. They might, but just as with the polygyny example this hasn’t actually been demonstrated, Buss and Duntley are operating on the basis of plausibility rather than actual solid evidence. Let’s consider the next few examples together in relation to Buss and Duntley’s conclusion:

Third, a sexual infidelity may have inflicted such a severe cost on a man in the currency of paternity uncertainty and the associated misdirection of his investments, that killing the woman may have been a viable means of stanching the costs. If she is pregnant with another man’s child, he also hurts his rival’s reproductive success.

Our fourth argument hinges on the fact that one of the major triggers of mate killing is an irrevocable loss of the relationship. When a woman finally convinces her partner that she’s leaving for good, the loss may be so substantial that it pushes the man over the edge into entertaining homicidal thoughts.

The final end of the relationship, in sum, historically may have put the man in triple jeopardy in the currency of reproduction. He lost entirely his access to her reproductive capabilities. He suffered severe and possibly irreparable reputational damage as a result of the loss. And if the woman was at all desirable, it was likely that she would remarry, so that a man’s loss would have been his rival’s gain. His same-sex rival benefited in direct proportion to the original man’s loss (pg. 123).

I do think these last two examples are contexts where men across cultures do seem to be more likely to kill their spouses (see Daly & Wilson, 1988), although again it is unclear why the ‘mate-killing module’ is a necessary or useful explanation for this phenomena. Buss quotes evolutionary psychologists Margot Wilson and Martin Daly as arguing instead that such spousal homicides are more an unfortunate and often unplanned byproduct of mate-guarding strategies, ““Men . . . strive to control women . . . women struggle to resist coercion and to maintain their choices. There is brinkmanship and risk of disaster in any such contest, and homicides by spouses of either sex may be considered slips in this dangerous game.”” (pg. 121). This explanation need not necessarily be correct either, though notably it is more consistent with the limited data available and is not reliant on currently unsupported and likely unsupportable claims about an evolved ‘mate-killing module’.

Let me now give an example from a small-scale society of cultural incentivizes playing a role in a man killing his wife, as well as a specific case from the same society more in line with Daly and Wilson’s perspective than Buss and Duntley’s. In his work Naven (1936) discussing the Iatmul forager-horticulturalists of New Guinea, anthropologist Gregory Bateson writes that,

In general the fighting and killing was confined to the killing of foreigners, i.e. members of other villages, especially of villages against whom a feud existed. But even this rule was not too strictly interpreted; a woman, married into the village, might for purposes of head-hunting be considered a foreigner. I even came across one case in which a man wore a tassel for killing his own wife in revenge for a kill accomplished by members of the village from which she had come.

In one example of a man killing his wife, recounted to him by a man who was the classificatory brother of the victim, Bateson writes,

Laindjin killed Tualesh, his wife. She was a member of Wolgem clan and (therefore) my (classificatory) sister. Tualesh went to get water in a long bamboo. When she returned Laindjin said, “Why were you so long?” She said, “I was not long. What are you thinking?” Laindjin said, “I was only asking," and she said, “Other men don't ask, but you do. You are jealous.” Laindjin said, “Yes, I know how women carry on.” Then he jumped up and seized the water bamboo and bashed her with it and she died. They made incisions in her skin with a bamboo knife and used spells, but they could not cure her. Then Laindjin wept.

Notably, after this killing Laindjin was attacked by members of his wife’s clan, in a strike against his household which left his brother’s son injured. Afterwards, Laindjin further had to provide restitution to his wife’s clan in the form of, “one mother-of-pearl crescent, one tortoise-shell arm band, two Conus shell necklaces and three Turbo shells, in a string bag which was hung up in the Wolgem ceremonial house.”

There are two key points to keep in mind here which, while specific criticisms of the ‘mate-killing module’ hypothesis, are also worth thinking about when considering other proposed ‘psychological adaptations’, particularly ones that are expected to manifest only in particular socioecological contexts:

1) Buss and Duntley do not provide any evidence that mate killing increases a man’s reproductive success in any of the contexts that they are discussing. They simply propose that it does, on the basis of plausibility alone (see Gould & Lewontin, 1979). One way of evaluating this would be to look across different societies and show that men who do kill their wives have more children that survive to adulthood than men who don’t kill their wives. This at least would provide some support for the idea that killing one’s mate can be part of an effective fitness strategy, however…

2) Even if it were shown that killing one’s mate does increase a man’s reproductive success in certain socioecological contexts, in other words, that it can potentially be an adaptive behavioral strategy, this does not show that it is a behavior resulting from a specific psychological adaptation, such as the proposed ‘mate-killing module’.

To provide an example of how to investigate these kind of questions more carefully, anthropologists Richard Wrangham and Luke Glowacki’s work on the evolution of warfare offers a salient contrast to Buss and Duntley’s more simplistic adaptationist thinking. In their paper comparing patterns of intergroup conflict among chimpanzees and nomadic hunter-gatherer societies, Wrangham and Glowacki find some key similarities. As I wrote in a previous piece,

Glowacki and Wrangham have…shown that intergroup violence in both chimpanzees and among nomadic hunter-gatherers is often undertaken tactically; aggressors are more likely to attack when they have a numerical advantage and can ambush outsiders who encroach on their territory. Indiscriminate use of violence is not favored, but the tactical use of violence can represent a biologically and/or culturally adaptive strategy.

Importantly, Wrangham and Glowacki appropriately conclude their article by writing that,

In sum, nomadic hunter-gatherer warfare tends to conform to the chimpanzee model, but whether humans have also evolved species-specific adapations for war is uncertain. The chimpanzee model therefore appears to be a useful starting point for analyzing the cultural evolution of warfare, and possibly its biological evolution as well.

Wrangham and Glowacki have been carefully exploring this topic in a series of papers over the last few years. They have shown how cultural rewards are often key in incentivizing participation in warfare in small-scale societies. They also found that among the Nyangatom pastoralists of East Africa, elders who participated in more stealth raids against rival communities in their youth had more wives and surviving children. Even where they point to the role of various potentially evolved psychological mechanisms in warfare, they caution against boiling such a complex behavior down to a single propensity,

This strategy [warfare] is not the result of a single “instinct” for war, but is instead an emergent property resulting from evolved psychological mechanisms (such as xenophobia and parochial altruism). These mechanisms are sensitive to ecological and social conditions, such that the prevalence and patterns of warfare vary according to subsistence strategies, military technology, cultural institutions, and political and economic relations.

Napoleon Chagnon also warned against boiling complex behaviors down to particular ‘instincts’, writing that, “one cannot account for the engagement [in warfare] by citing psychological variables such as anxiety or in terms of innate aggression. To do so is to confuse effects with functions and reduce cultural phenomena to bio-psychological variables.”

Evolutionary psychologists are sometimes critical of human and animal behavioral ecologists for giving insufficient attention to the role of psychological factors (particularly putative psychological adaptations) in complex behavior, without recognizing that in many cases this is the only responsible way to handle the limited data available to behavioral scientists, and avoids unnecessary reductionism. As anthropologist E.A. Smith writes,

HBEs [Human Behavioral Ecologists] tend to focus on explaining behavioral variation as adaptive responses to environmental variation; they assume that this adaptive variation (facultative behavior, phenotypic response) is governed by evolved mechanisms that instantiate the relevant conditional strategy or decision rule. This assumption, which takes a “black-box” approach to the actual mechanisms involved, is part of what Grafen (1984) terms the phenotypic gambit. This means taking a calculated risk to ignore the (generally unknown) details of inheritance (genetic or cultural), cognitive mechanisms, and phylogenetic history that may pertain to a given decision rule and behavioral domain in hopes that these don’t matter to the end result.

The important thing to remember is that complex behaviors such as these emerge from innumerable factors sensitive to particular contexts. Boiling mate-killing, or warfare, or numerous other complex behaviors down to a single ‘psychological adaptation’ is not only often unjustified in light of the lack of neurological and genetic evidence available to evaluate such claims about adaptations, but it actually obscures the more immediately relevant socioecological factors that actually let you make predictions and test hypotheses about the phenomena in question.

Postscript

My friend anthropologist Cody Moser has a nice post on evolutionary psychology where he offers some constructive suggestions for investigating evolutionary psychology hypotheses:

  1. Emphasizing strict, cross-cultural and developmentally or biologically-constrained patterns over broader ones

  2. Moving to identify genetic mechanisms associated with these patterns

  3. Identifying modules associated with these mechanisms (in this case, coupling with the ongoing connectome project could be fruitful) OR moving away from modules entirely and moving to a broader neurophysiological or hormonal-affect state models

  4. Most importantly, emphasizing the room that spandrels might play in our current behaviors or how developmental scaffolds may build upon more baseline-level EP module-esque systems to build higher-order emergent properties (I think Tooby & Cosmides “computational theory of mind” partially covers this, but even so this seems to be less emphasized in EP than its other aspects)