Maladaptive By-Product Hypothesis
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In evolutionary psychology, maladaptive theory holds that some human behaviors are the result of past selective forces that are no longer operative, usually because of changes in the physical and social environment, resulting in behaviors that are no longer beneficial for fitness.
A maladaptation is a feature of an organism that, despite being genetically encoded or influenced, hinders the biological fitness of the individuals that bear it. Rarely is it straightforward to determine that a given trait is unambiguously maladaptive. Indeed, as noted by Crespi (2000), “Conditions for unambiguously identifying maladaptation are considerably more stringent than those for demonstrating adaptation.” Nevertheless, the existence of maladaptive physical and behavioral traits is undeniable and reminds us of the limits of adaptationist explanations for human behaviors.
At its very origin [pardon the pun], modern evolutionary theory has acknowledged the limits of natural selection in explaining every single feature of an organism. Darwin’s insight that, “many structures now have no direct relation to the habits of life” remains a valid, if oft-forgotten qualifier (Darwin 1859). Despite the emergence of the neutral theory of variation (Kimura 1983) and the identification of so-called evolutionary “spandrels” (Gould and Lewontin 1979), the ghosts of adaptationism persist, particularly in popular science writings, and more specifically in the just-so stories that dominate popular evolutionary psychology. While the tendency to pin every quirk of modern human behavior on adaptation to Pleistocene Africa makes for sciency entertainment, serious theoretical work in evolutionary psychology requires a higher standard.
By definition, a maladaptation reduces reproductive success in some way and therefore calls out for an explanation as to its emergence and/or persistence in a population. This explains both the interest in and controversy of the topic. Perhaps the most frequent explanation for maladaptation is something now called mismatch (Gluckman and Hanson 2004). A mismatch trait is one that was adaptive in the ancestral milieu, but is maladaptive in the present, most often due to a change in the environment. A commonly cited human behavioral example is our appetite for sweet foods. For millions of years prior to the dawn of agriculture, human ancestors encountered few foods that were high in mono- and disaccharides, the natural dietary ligands most often responsible for the perception of sweet (Eaton 2006). Those few were mostly fruits, so it is thus inferred that the human sweet tooth was at least partly an adaptation driving us to seek the specific foods that provide an essential nutrient found in many fruits: vitamin C (Breslin 2013). However, beginning with the cultivation and subsequent artificial selection of carbohydrate-rich cereal crops, along with the selection of larger and sweeter fruits, the production of honey, maple syrup, cane sugar, and so on, humans have tremendously increased dietary intake of simple sugars which have been almost completely decoupled from vitamin C (Eaton 2006). Given the mounting evidence that excessive consumption of simple sugars correlates with a variety of poor health outcomes, the human appetite for sugar may fairly be called maladaptive under present conditions (Jew et al. 2009).
Mismatch, an environmental change that has yet to be addressed adaptively by an organism, is but one possible cause of maladaptation. Others include genetic drift, the fixing of a maladaptive trait by chance; heterozygote advantage, the persistence of a harmful allele, even when a nonharmful alternative exists in the gene pool, due to the advantage that hybrids have over either form of homozygote; genetic linkage, the connectedness of a maladaptive trait with an adaptive one through either pleiotropy or close chromosomal association; and gene flow, the mixture of alleles and traits from two populations that have adapted to different environments. In these cases, with the possible exception of heterozygote advantage, continued selection could presumably correct the maladaptation, though unambiguous examples of this are lacking.
General theories of maladaptation have remained elusive in part because the very concept is strictly context-dependent. This is best illustrated by example. Humans have substantially less hair and substantially more sweat glands than chimpanzees per unit area (Rogers et al. 2004; Zihlman and Cohn 1988). The prevailing view is that both of these traits are adaptations to the much dryer air of the grasslands, compared to that of the rainforest, the habitats of our more proximal and distant ancestors, respectively. Central to this hypothesis is the notion that Pleistocene hominins were persistence hunters, an activity in which the advantage of both hairlessness and copious sweating has been demonstrated (Bramble and Lieberman 2004). However, as archaic humans and other hominins migrated northward, the dearth of body hair represented a distinct disadvantage in the colder, wetter climates that greeted them. Hairlessness, then, can convey an advantage or disadvantage based on context. (It is worth noting that adaptationist interpretations of modern hair distribution are not without challenges. See Morris 1994.)
As difficult as it is to apply adaptive and maladaptive interpretations to matters of anatomy and physiology, it is incredibly more so with complex behaviors. The current teleology of a behavior is difficult enough to assess on its own, especially cross-culturally, and the phylogenetic analysis is even more so. Indeed, the substantial behavioral discontinuity between humans and their extant relatives dooms many phylogenetic analyses within human evolutionary psychology. Notwithstanding that caveat, behavior, particularly reproductive behavior, may be the richest area of maladaptation given how directly sexual selection can act on reproductive success, particularly in highly social animals with marked intrasexual competition, e.g., humans and our close relatives. For example, frequency-dependent sexual selection, the advantage enjoyed by uncommon mating strategies, is inherently fickle. Insofar as they are genetically based, a sexually selected mating strategy can switch from an adaptation to a maladaptation in as little as a single generation (Lande 1980; Pomiankowski 1987).
Suicide as an Adaptive or Maladaptive Behavior
Recently, suicide has come under scrutiny as a possible maladaptive behavior. Tragically, suicide has long been a significant cause of death among humans, especially males. In 2016, 45,000 people died by suicide in the United States, more than double the number of homicides, making this the second leading cause of death of those age 10–34 and fourth most common cause of death among those aged 35–54 (CDC 2017). To call this a conundrum for an adaptationist view of evolutionary psychology would be an understatement, especially given that the peak age of suicide attempts correlates closely with the peak reproductive years in both males and females.
The notion that suicide may provide adaptive benefits in some contexts is controversial but has both theoretical and experimental support (Gyllenberg and Parvinen 2001; McAndrew 2002). In theory, if self-removal from the gene pool results in a net increase in the fitness of close relatives that is greater than the loss of personal fitness, the principle of inclusive fitness holds that suicide could emerge as an adaptive behavior. In this model, kin-selective altruistic suicide could be driven by self-interested genetic elements. In support of this theory, those who survive a suicide attempt very frequently report an overwhelming feeling of being burdensome to their families, which is even more common among those who are beyond prime reproductive age or who suffer from major chronic illness, a known risk factor for suicide, and are of low reproductive potential more generally (though these are minority of total suicide attempts, successful or not) (Kovacs and Garrison 1985; Juurlink et al. 2004; Gould et al. 2003). Further supporting this notion is the otherwise paradoxical statistic that suicidality shows considerable heritability (Brent and Mann 2005; Voracek and Loibl 2007).
One example of adaptive suicide is found in traditional Inuit culture. During times of severe famine, elderly members of a family have been known to walk as far away from the camp as they can until they collapse from exhaustion or hypothermia where they are left to die (Leighton and Hughes 1955). Because this spares family resources for their children and grandchildren, and because they are beyond reproductive age themselves, the connection to inclusive fitness is direct.
The myriad differences between the modern world and the environment of evolutionary adaptedness could explain suicide as an adaptive-turned-maladaptive behavior. In other words, suicidality may be a mismatch disease due to modern population densities that are orders of magnitude higher than the ones our species experienced for millions of years prior to urbanization. Indeed, a connection between suicide rates and population density has been observed under certain conditions (Galle et al. 1972; Levy and Herzog 1974; Yamasaki et al. 2008). This is not to suggest, however, that suicide is a form of quorum sensing and population control. Indeed, that hypothesis would rest entirely on group selective theory, a shaky foundation.
There is at least one additional adaptationist theory of suicide that warrants mention although full exploration of which is outside of the scope of this entry. The bargaining hypothesis holds that suicide attempts are a form of honest costly signaling designed to help an individual in an otherwise untenable situation recalibrate various social relationships and cultural realities (McAndrew 2002; Henrich 2009). This falls under the “cry for help” paradigm of understanding suicidality and is buttressed by the fact that the overwhelming majority of suicide attempts are both publicly observable and unsuccessful.
Sex differences in suicidality may be relevant to both of these theoretical frameworks because death by suicide is much more common among men, while among women, suicide attempts are more often unsuccessful (Oquendo et al. 2001). It is possible that gender differentially activates the two frameworks of suicide discussed here in ways related to other gender-influenced behaviors such as impulsivity, risk-taking, and sexually selective costly signaling.
Honor Killings as Maladaptive Behavior
While suicide is often considered the most direct affront to one’s personal biological fitness, the murder of one’s children is a close second. Nevertheless, so-called honor killings of children by their parents are a noted phenomenon in several different cultures. These murders are wrought upon young women far more often than upon young men and are most often conducted by the father, usually with the assent of the mother, and sometimes with the aid of his brothers and/or sons, the victim’s uncles and brothers. Shockingly, these murders are rarely concealed and may even be conducted in public. Regions in which this enigmatic behavior is observed with some regularity include the Middle East, North Africa, and the Indian subcontinent (Chesler 2010). While its occurrence in Muslim communities, especially those in the West, has attracted the most media attention, honor killings have also been observed in contemporary Hindu and Jewish contexts and the upper classes of Medieval and early modern Europe (Bowman 2007).
Recently, Robert Trivers has invoked kin selection and inclusive fitness theory to attempt to explain the phenomenon of honor killings (Trivers 2018a, b). Importantly, most of the modern cultures in which familial honor killings occur have a high degree of first-cousin marriage. In a society with strong borders and low gene flow, frequent first-cousin marriage markedly raises the degree of genetic relatedness between individuals and their second-degree relatives, e.g., nieces and nephews. If identical twins show a genetic relatedness (r) of 1, while genealogical nonrelatives show an r of 0, in a society in which first-cousin marriage is rare, siblings have an r of 0.5, as do parents and children, and second-degree relatives have an r of 0.25. However, as rates of first-cousin marriage rise, the r among genealogical relatives increases gradually, can surpass 0.5 within a few generations, and continues rising asymptotically to 1. Within-family genetic diversity thus drops precipitously.
The exception that proves the rule is traditional Hindu culture. Although first-cousin marriage is not common among Hindus, marriage customs have adhered to the caste system since antiquity, which permits only endogamous marriage. This has the same long-term evolutionary genetic effect as frequent first-cousin marriage, raising the genetic relatedness and reducing the genetic diversity within castes.
Under a system of high genetic relatedness, if the murder of an offspring preserves the reproductive potential of the victim’s siblings and cousins, it can boost the biological fitness of the murdering father or sibling, despite the total loss of the reproductive potential of the murdered individual. In all of the honor-based cultures listed above, shame carries substantial costs to reproductive potential and passes freely among first- and second-degree relatives, including first cousins. If a young person behaves in a way that brings shame upon her family, and that shame is broadly recognized in the community, the father of the shameful actor may indeed be acting in the interest of his own inclusive fitness when he murders his daughter. While the tight correlation between the two phenomena of honor killings and first-cousin marriage is compelling, a mechanistic explanation has yet to be suggested.
This behavior is very likely maladaptive in industrialized societies. Firstly, the social-reproductive costs of vicarious shame are rapidly attenuating around the world. Secondly, in the West and in developed regions of the East, the modern state imposes criminal penalties for murder with no official amelioration of punishment for honor-motivated killings. (There is some evidence that, the “culture defense” resulted in lighter sentences for honor killings in the United Kingdom through the 1990s, but this does not appear to the case post-2001 (Gill 2009).) This punishment extends well beyond the adjudication of the murder, as the family of a convicted murderer will experience substantial negative consequences as well, which may very well be reflected in reduced biological fitness, although this has yet to be examined.
Parental adoption involves the substantial investment of one’s resources into non-kin children at the expense of, or even sometimes instead of, one’s own genetic progeny. As such, adoption generally confers negative biological fitness. While altruistic and group selective theories could be marshaled to attempt to explain adoption (Rushton et al. 1984), such explanations collapse quickly in the light of the increasing frequency of international and/or transracial adoption (Sahlins 1976). From the view of biological fitness, adoption of non-kin children, particularly by apparently fertile individuals, can be seen as a clear, if highly simplified, maladaptive behavior. (Let it be noted that the author of this entry has himself adopted non-kin children and is unconcerned with the apparent maladaptiveness.)
The evolution of maladaptive behaviors, while difficult to demonstrate unequivocally, often highlights just how dynamic the adaptive milieu can be and the imprecision with which selective pressure is applied to behavior. This is tremendously exacerbated in modern humans in whom cultural evolution now dwarfs biological evolution in shaping behavior, a reality made crystal clear by the phenomenon of reproductive choice. In just two generations, technologies for birth control and abortion of unplanned pregnancies have caused the birth rate to plummet in Western cultures as increasing numbers of individuals choose small families or opt-out of reproduction altogether. Since these behaviors dramatically reduce biological fitness, they are technically maladaptive. This example shows both the fickle nature of selective pressures on reproductive behaviors and the limits of adaptive explanations for human behavior more broadly.
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