Evolutionary Psychology Primer

The following is a short essay on Evolutionary Psychology and some criticisms against it. It was originally written to appear in my thesis, but has now been discarded. Seems a waste to let it gather digital dust, so figured I’d post it here in case it’s of interest to anyone.

In 1992, anthropologists, Jerome Barkow and John Tooby, and psychologist, Leda Cosmides, jointly edited a book of commissioned papers called The Adapted Mind, which proved to be a turning point in the study of human behaviour as seen through the lens of evolutionary biology (Webster, 2007).

The book brought together researchers from a range of fields who came together to “clarify how this new field, by focusing on the evolved information-processing mechanisms that comprise the human mind, supplies the necessary connection between evolutionary biology and the complex, irreducible social and cultural phenomena studied by anthropologists, sociologists, economists, and historians” (Barkow & Cosmides & Tooby, 1992, page 3).

In many ways this new discipline of Evolutionary Psychology[1] is a development of sociobiology, in that they both have a conceptual foundation in evolutionary theory, although Evolutionary Psychology focuses its attention primarily on human psychology and human behaviour rather than upon animal behaviour as a whole.

Thus, where sociobiology was broad-brush project – attempting to link biology with behaviour and large-scale social phenomena – Evolutionary Psychology is more of a narrow-scale programme that seeks to understand the proximate psychological mechanisms that direct behaviour. What both disciplines have in common is an acceptance that evolution has influenced our psychology at the proximate level, with evolution imbuing us with a number of faculties, or ‘modules’, which have evolved to solve particular adaptive ‘problems’ from our evolutionary past. According to Cosmides and Tooby, the discipline is defined as:

The long-forestalled scientific attempt to assemble out of the disjointed, fragmentary, and mutually contradictory human disciplines a single, logically integrated research framework for the psychological, social, and behavioral sciences – a framework that not only incorporates the evolutionary sciences on a full and equal basis, but that systematically works out all of the revisions in existing belief and research practice that such a synthesis requires.

The  long-term scientific goal toward which evolutionary psychologists are working is the mapping of our universal human nature. By this, we mean the construction of a set of empirically validated, high-resolution models of the evolved mechanisms that collectively constitute universal human nature. (Tooby & Cosmides, 2005, page 5)

They contrast the Evolutionary Psychology approach from what they call the ‘Standard Social Science Model’ (SSSM), which is:

The consensus view of the nature of social and cultural phenomena that has served for a century as the intellectual framework for the organization of psychology and the social sciences and the intellectual justification for their claims of autonomy from the rest of science. Progress has been severely limited because the Standard Social Science Model mischaracterizes important avenues of causation, induces researchers to study complexly chaotic and unordered phenomena, and misdirects study away from areas where rich principled phenomena are to be found. (Cosmides & Tooby, 1992, page 3)

They criticise the SSSM for being largely unproductive in providing comprehensive explanations of human behaviour, from individual to institutional levels, and tying them together with a consistent theoretical underpinning – something they suggest evolutionary approaches to behaviour can achieve.

The Environment of Evolutionary Adaptedness

There are a number of theses that together form the foundation of this particular approach to Evolutionary Psychology. The first is that the brain is a specialised organ shaped by natural selection, just like any other organ in the body, such as the heart, the kidney or the intestines. And in the same way that evolution can shed light[2] on the particular design features of our organs, evolution can also shed light on the design of the brain and mind.

Like the other organs, the brain serves a specific function – where the heart’s function is to pump blood around the body, and the kidney’s function is to filter and purify the blood – the brain functions as an information processing device, designed to take input from the environment and produce a behavioural response appropriate to that environment (Cosmides & Tooby, 1997). By ‘appropriate’, Evolutionary Psychologists mean ‘fitness enhancing’ because, just like other phenotypic traits can be adaptive, so too can behaviour. Thus, if genes play a role in encouraging certain behaviours, and those behaviours are adaptive, then those genes ought to increase in their frequency within a population.

As the concept of adaptation is a central notion in Evolutionary Psychology, it’s important to know something of the adaptive problems that were faced by our ancestors in order to get some understanding of the solutions that evolved to solve those problems. To this end Cosmides and Tooby invoke the notion of the Environment of Evolutionary Adaptedness, or EEA. This is the environment in which the our brains were shaped by evolution:

The environment of evolutionary adaptedness (EEA) refers jointly to the problems hunter-gatherers had to solve and the conditions under which they solved them (including their developmental environment). (Cosmides & Tooby, 2005, page 22)

Cosmides and Tooby stress that the EEA shouldn’t be thought of as a particular place or particular time, but instead as the statistical aggregate of those environments and selection pressures that shaped our psychology by pushing the “alleles underlying an adaptation systematically upward in frequency until they became species-typical or reached a frequency-dependent equilibrium” (ibid.):

Characterizing an adaptation involves characterizing the ancestral conditions and selection pressures – the adaptive problem – that the adaptation solves. The “environment of evolutionary adaptedness” (EEA) is not a place or a habitat, or even a time period. Rather, it is a statistical composite of the adaptation-relevant properties of the ancestral environments encountered by members of ancestral populations, weighted by their frequency and fitness-consequences. (Tooby & Cosmides, 1990, page 387)

However, Evolutionary Psychologists often do identify a particular stretch of time as being particularly significant in shaping contemporary human psychology, the Pleistocene, which is the epoch starting around 2.5 million and ending around 10,000 years ago. This is the period in which homo sapiens evolved into our present form and, contend Evolutionary Psychologists, it is the adaptive problems faced by early human populations during this period that the mind evolved to solve. During this time, humans lived largely as hunter-gatherers, and it’s the adaptive problems faced by individuals living this kind of lifestyle that has largely shaped the human mind.

The reason the EEA extends only to around 10,000 years ago is this is when humans began to engage in agriculture, a practice that within a relatively short period of time rapidly changed the living conditions of many humans, and thus the adaptive pressures they faced. And as this period represents only a very small fraction of the time since the beginning of the Pleistocene – less than one percent – and evolution takes many generations to make substantial alterations to an entire population, Cosmides, Tooby and Barkow argue that “it is unlikely that new complex designs – ones requiring the coordinated assembly of many novel, functionally integrated features – could evolve in so few generations” (Barkow, Cosmides & Tooby, 1992, page 5).

This is not to say that humans are no longer evolving; there is ample evidence that evolution has continued to affect humans (Sabeti et al., 2006; Hawks et al., 2007; Templeton, 2010). One example of humans responding to localised selection pressures can be seen by in the incidence of sickle-cell anaemia and thalassaemia in populations where malaria is endemic. This is because the genes that cause sickle-cell anaemia and thalassaemia in homozygotes can also lend a measure of protection against the Plasmodium falciparum parasite that causes malaria in heterozygotes. As such, the greater selective advantage enjoyed by heterozygotes – because they’re able to reliably survive long enough to produce more offspring than homozygotes, or those lacking the genes that cause sickle-cell anaemia and thalassaemia altogether, when in environments where risk of malaria infection is high – perpetuates the polymorphism at this locus on the genome, a phenomenon known as overdominant selection. As such, one scientist has noted that malaria might be “the strongest known force for evolutionary selection in the recent history of the human genome” (Kwiatkowski, 2005).

Cultural evolution can also influence the human genome – a phenomenon called niche-construction. This is where some cultural development alters the selection pressures present on a population, thus influencing the fitness and frequency of genes in that population. One example is the case with lactose tolerance found in populations of humans that began domesticating cattle. The presence of a new protein-rich food source – milk – introduced a positive selection pressure in favour of a mutation that lent lactose tolerance (Laland et al., 2010), resulting in a relatively rapid uptake of that gene. Lactose tolerance can then, in turn, influence cultural practices, such as can be seen by flourishing dairy traditions in cultures with a high proportion of lactose tolerant individuals, such as in western Europe, compared to an absence of such traditions in cultures with low lactose tolerance, such as east Asia.

Many of these examples of recent human evolution are either monogentic traits, which are traits that are influenced by a change in a single gene, or involve only a small number of genes. The mind, according to Evolutionary Psychologists, is substantially more complex, and changes take a correspondingly relatively longer time to emerge. According to Cosmides, Tooby and Barkow (1995), given that the 10,000 years since the advent of agriculture represent only around one per cent of the time humans spent as hunter-gatherers during the Pleistocene, it’s insufficient time for us to evolve complex new psychological faculties. As such, they argue, the last several thousand years are largely irrelevant to the study of the design of the human mind.

However, not everyone agrees with this conclusion. David Buller, for example, argues that Evolutionary Psychologists make the error of assuming that evolution must take the form of entirely new mental modules (which will be discussed below) or faculties, rather than incremental changes or changes in the relative proportion of particular alleles within a population.

Without doubt, selection could not build a human mind from scratch in a mere four hundred generations. But, from the fact that a “new complex design” could not have evolved since the Pleistocene, it doesn’t follow that the psychological adaptations of contemporary humans are identical to those of our Pleistocene ancestors. For the issue is whether old complex designs, which evolved during the Pleistocene, could have been modified by selection in the last 10,000 years. Since the argument doesn’t address this possibility, it fails to show that our psychological adaptations must have remained adapted to Pleistocene conditions. (Buller, 2005, page 108)

However, even if Buller’s criticism is warranted – and I’m inclined to believe that it is – this doesn’t necessarily mean that the EEA is identical to the environmental circumstances that our near ancestors found themselves in. It simply implies that Evolutionary Psychologists may have incorrectly bounded it at the end of the Pleistocene. We may concede that psychological adaptations have emerged in the past 10,000 years while still admitting that many adaptations have remained as vestiges of our Pleistocene hunter-gatherer past. This does complicate the original story of the Evolutionary Psychologists – that we need only look to the adaptive problems that faced hunter-gatherers to find the source of our evolved mental faculties – but it doesn’t undermine the broader programme of considering the role of biology or genes in our psychology.

However, if some of our adaptations emerged in response to adaptive problems faced by hunter-gatherers, these adaptations might prove to be maladaptive in today’s environment, particularly as our environmental conditions have changed radically over the 10,000-odd years since humans began engaging in agriculture, and especially over the past 200-odd years since widespread industrialisation. As Cosmides and Tooby put it: “our modern skulls house a stone age mind” (Cosmides and Tooby, 1997).

An example of maladaptive behaviour that is often cited by Evolutionary Psychologists is the catalogue of common human fears, including “heights, storms, large carnivores, darkness, blood, strangers, confinement, deep water, social scrutiny, and leaving home alone” (Pinker, 1997). Pinker adds that “these are the situations that put our evolutionary ancestors in danger.” Yet, humans don’t exhibit the same fear responses to contemporary dangers:

Strikingly absent from this catalog of human fears are the things humans should be afraid of in contemporary environments. The sight of a car or a gun, for example, should strike far more fear into the heart of a modern human than does the sight of a snake, for cars and guns kill far more people than do snake bites. From an evolutionary standpoint, argues Evolutionary Psychology, this should be expected. For there have been too few generations since the invention of guns and cars for selection to proliferate any recently emerged genotype for a fear of guns or cars. (Buller, 2005, page 61)

The term ‘maladaptive’ is important in this context. One sense of maladaptive suggests that behaviour that was once adaptive is no longer so, such as having a strong fear response to snakes when few snakes exist in urban environments. Another sense of maladaptive is that there are behaviours that would be adaptive today, such as donating to sperm banks in order to propagate one’s genes as widely as possible for a very low cost, that don’t register as being highly desirable to many individuals. A third sense is that maladaptive behaviour is something that can harm an individual’s survival or reproductive prospects in modern environments, such as in eating sweet and fatty foods to excess. I’ll return to maladaptive behaviour in the context of our evolved moral sense in a later chapter.

Massive Modularity

Another central tenet of the Cosmides and Tooby brand of Evolutionary Psychology is that the brain isn’t just a general-purpose information processing device, like a computer. Instead, it is made up of multiple modules, or “information-processing mechanisms”, that are “functionally specialized” adaptations that each evolved to solve a particular adaptive problem, such as “mate selection, language acquisition, family relations, and cooperation” (Tooby & Cosmides, 1992, page 4).

Jerry Fodor sums up the theory such:

Much of the substance of C&T’s [Cosmides and Tooby’s] “massive modularity” theory of cognitive architecture is that there are distinct, specialized computational mechanisms for solving problems in each of several different cognitive domains. Each such mechanism becomes active when a problem is recognized as belonging to its proprietary domain, with the consequence that quite different computational procedures may be applied to structurally identical problems depending on domain variables. In particular, the computational treatment of problems that are similar or identical in “logical form” may be quite different depending on the “content” of the materials. (Fodor, 2008, page 137)

Or, as stated by Russil Durrant and Bruce Ellis:

Its central assumption is that the human brain is comprised of a large number of specialized mechanisms that were shaped by natural selection over vast periods of time to solve the recurrent information-processing problems faced by our ancestors. These problems include such things as choosing which foods to eat, negotiating social hierarchies, dividing investment among offspring, and selecting mates. (Durrant & Ellis, 2002, page 1)

This notion of ‘massive modularity’ appears in much Evolutionary Psychology literature, and it builds on cognitive models of the mind as being made up of a number of compartmentalised units that function according to particular rules to perform various cognitive tasks (Fodor, 1983). Part of this notion is that many of these modules are domain-specific, meaning they are specialised to solve highly specific problems rather than being domain-general modules that can respond to a wide spectrum of situations. This thesis also allows for the mind to possess domain-general faculties, such as generalised reasoning or general learning features such as operant and classical conditioning, which can be used to produce a very broad range of behaviours. But one of the intentions of Evolutionary Psychology is to tease out situations where domain-specific modules are employed rather than domain-general ones in the belief that the former represent cognitive modules that have evolved to solve specific adaptive problems.

One example is the cheater detection module (Cosmides & Tooby, 2008), which was proposed to explain a well-known discrepancy in performance by many subjects in the Wason selection task when the task involved situations concerning social exchange. In the Wason selection task, subjects are presented with two (or more) situations that involve the same logical structure – ‘if p then q’ – but with different contexts, and are then asked questions that challenge their understanding of the conditional. Cosmides and Tooby, and others, have conducted many experiments that demonstrate that subjects make many errors when the context is very general – such as “if a person has Ebbinghaus disease, then that person will be forgetful” – and their performance is significantly better when the context involves social exchange – such as “if you someone is drinking alcohol, then they must be over 18”. From this, Cosmides and Tooby argue that we must have a mental module that is attuned to processing the conditions of social contracts, particularly for detecting violations of those contracts, i.e. cheaters or free riders, who would have presented a serious threat to cooperative ventures in our evolutionary past.

Another example – which also relates to the notion that evolution has influenced our moral psychology – is an ‘incest-avoidance’ module proposed by Lieberman, Cosmides and Tooby (2003). They suggest that humans have evolved a heuristic for kin-recognition that both promotes altruistic behaviour, as predicted by kin-selection, but which also discourages mating with kin and the deleterious genetic effects such breeding incurs. This incest-avoidance mechanism is primed by co-habitation as infants, with co-habitation predicting “the strength of opposition to incest, even after controlling for relatedness and even when co-residing individuals are genetically unrelated” (Lieberman et al., 2003). This is supported by studies undertaken by the anthropologist, Edward Westermarck in the late 19th century and which were published in his book, The History of Human Marriage (1921), which found that individuals of the opposite sex who had been raised together since infancy were unlikely to enter into sexual relationships when adults – the so-called ‘Westermarck effect.’

Lieberman et al. argue that a substantial part of our moral beliefs and behaviour concerning incest stem from this innate incest-avoidance mechanism (Lieberman et al., 2008, page 165). However, we don’t just rely on this incest-avoidance sentiment to regulate sexual relationships between closely related individuals; we also possess norms prohibiting incest which appear to be universal (Brown, 1991). Yet, if our innate incest-avoidance mechanism was sufficient to prevent incest, why would we also have norms to reinforce the notion? Lieberman et al. suggest a number of reasons communities of humans might have developed norms relating to incest in addition to possessing the innate incest-avoidance mechanism, such as that the evolved mechanism is prone to error and is far from foolproof. Furthermore, in some circumstances, incest may be fitness enhancing, particularly if compared to not breeding at all. Finally, I would suggest that the incest-avoidance mechanism only exists because we have another very potent mechanism that encourages sexual attraction between members of the opposite sex; the incest-avoidance mechanism has to resist the very powerful urges to breed, and in not every circumstance will the incest-avoidance mechanism win out in this struggle. Thus, it is plausible that, with an innate predisposition against incest fuelled by the incest-avoidance mechanism, yet seeing the fallibility of such a mechanism, it might have been reasonable to concoct an enforceable moral norm prohibiting incest in order to better prevent inbreeding.

The massive-modularity thesis has drawn criticism, however. Two prominent critics are from David Buller (2005) and Jerry Fodor (Fodor, 2000). Fodor, who’s interests primarily lie in the computational theory of mind, interprets the massive modularity thesis is a fairly extreme way, suggesting that it implies there is no central general-purpose cognitive faculty, only specialised modules. This, however, he argues cannot possibly be how the mind works, as summarised by Susan Schneider (2007):

Fodor offers the following argument against MM [massive modularity]. If MM is true then there will be modules that are not merely in the sensory “periphery”; there will be modules that are less perceptual, and which take more sophisticated inputs. Consider, for example, Cosmides and Tooby’s Cheater Detection Modules [sic], which is supposed to have evolved to detect cheaters in the context of social exchanges. Obviously some processing is needed to determine what the inputs for the module are, for there are no “transducers” as exist in the case of sensory modules. And, given MM, this processing must be modular too. But it should be less domain specific than the cheater detection module, for it will need a broader range of inputs. And similarly, it appears that the new module would itself need a module, with a broader range of inputs, ad infinitum (MDW [The Mind Doesn’t Work That Way], pp. 71-78).

Fodor is also concerned about how information flows between these specialised modules. Being a proponent of the computational theory of mind, Fodor resorts to a computational metaphor in articulating how these modules might be formed. One central problem that has occupied much of Fodor’s writing is that of relevance, and how a computational system can get over the ‘frame problem’, which threatens to drive the system into infinite regress as it seeks to exclude irrelevant information for the task at hand[3]. He thus talks of modules as being necessarily ‘information encapsulated’, meaning they only have access to information within the boundaries of the module. Encapsulated modules like this would prevent the ‘relevance’ problem by being attuned to very particular information processing situations rather than forcing a centralised domain-general information processing system from having to sort out the relevant information from the mountains of irrelevant information.

Fodor and Buller also suggest that Cosmides and Tooby’s experiments fail to demonstrate that “problems that are similar or identical in ‘logical form’ may be quite different depending on the ‘content’ of the materials” because they fail to discriminate between two different logical forms – namely descriptive or indicative conditionals (“if p then q”) with deontic conditionals (“obligatory if p then q”). So when Cosmides and Tooby show a difference in performance by subjects, they assume that the only variability in tasks is in content, whereas there may also be a difference in logical form that might account for the difference in performance (Buller, 2006, p174; Fodor, 2008, p138).

However, Cosmides and Tooby reply that they have performed or cited a range of experiments that tease out this distinction in logical form, and they still find a difference in performance that is dependent on context, and thus argue that this remaining difference in performance is explicable by their evolved massive modularity thesis (Cosmides & Tooby, 2008, p151).

The debate over massive modularity is ongoing, although the outcome – either positive or negative – won’t dramatically impact the core thrust of this essay, for it’s quite possible that the massive modularity thesis is false yet it’s still entirely plausible that evolution has played a role in shaping our brains and minds. Even critics of massive modularity such as Buller acknowledge that the ‘paradigm’ of the Comides-and-Tooby brand of Evolutionary Psychology could be false, but a broader programme of exploring the evolutionary underpinnings to psychology and anthropology might be a highly rich a rewarding field (Buller, 2006, p12).

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[1] I follow the convention established by David Buller (2005, page 12) of distinguishing between two approaches to evolutionary psychology. The first, “evolutionary psychology” (lower case), is the broad application of evolutionary theory to the study of psychology and/or behaviour – which might be as little as admitting the evolution has endowed humans with a generalised cognitive faculty capable of classical and operant conditioning, and no more. The second is the ‘paradigm’ of “Evolutionary Psychology” (upper case), which is the approach pioneered by Leda Cosmides, John Tooby, David Buss, Robin Dunbar, Steven Pinker, Jerome Barkow, Martin Daly and Margo Wilson, amongst others.

[2] To appropriate a phrase coined by evolutionary biologist, Theodosius Dobzhansky: “nothing in biology makes sense except in the light of evolution”. This suggests that evolution is the central notion that can make sense of the disparate elements of biology, and can help explain why certain phenotypes are they way they are rather than being otherwise. Likewise, evolution plays a similar role in Evolutionary Psychology by explaining why the brain and mind function the way they do rather than otherwise.

[3][3] “Most philosophers consider the Frame Problem to be the problem of how to locate a computational theory of how humans determine what is relevant. (In light of this, it is sometimes called ‘The Relevance Problem’. Herein, I’ll use this expression to avoid confusion with AI’s Frame Problem, which differs in important ways). The Relevance Problem is often conveyed in the following way. If one wanted to get a machine to determine what is relevant, it would need to walk through virtually every item in its database, in order to determine whether a given item is relevant or not. This is an enormous computational task, and it could not be accomplished in a quick enough way for a system to act in real time. Of course, humans make quick decisions about relevance all the time. So, it looks like human domain general thought (i.e., the processing of the central systems) is not computational.” (Schneider, 2007)