麻豆传媒

YOU can hardly open a newspaper or magazine or turn on a TV these days without stumbling on evolutionary explanations for any number of human behaviours: from what we seek in mates and why we are unfaithful, to why we love our children but not our stepchildren, why men and women differ, and even why husbands kill their wives. All this is put forward in the name of evolutionary psychology.

As an ardent and thoroughgoing evolutionist, I would like to endorse all of these explanations. But I don鈥檛. It鈥檚 not that I reject the idea of an evolutionary psychology. In the broadest sense, evolutionary psychology is simply a field of enquiry, defined not by any specific theories about human psychology but by the questions it investigates 鈥� and I definitely endorse investigating questions about the evolution of human behaviour and psychology.

What I don鈥檛 endorse are the doctrinaire answers to these questions deriving from an approach to the field of evolutionary psychology spearheaded by Steven Pinker (The Blank Slate) and David Buss (The Evolution of Desire and The Murderer Next Door). They and their followers define evolutionary psychology very narrowly, identifying it with their particular set of doctrines concerning the evolutionary history and current nature of the human mind.

To avoid confusion, I鈥檓 going to use the initials EP to describe the Pinker-Buss paradigm, reserving the phrase 鈥渆volutionary psychology鈥� for the broader field of enquiry. And remember: it is the EP camp which claims to have evidence for those headline-grabbing claims you鈥檝e seen.

In Adapting Minds, published this year, I argue that EP is wrong in almost every theoretical detail and that the evidence for its headlining 鈥渄iscoveries鈥� is inconclusive at best. The intention of this critique, however, is not to plough under evolutionary psychology but to weed it of the overgrown yet evidentially malnourished doctrines of EP.

In this essay I鈥檒l take up a couple of the defining doctrines of EP. One is that the human mind is massively modular, containing 鈥渉undreds or thousands鈥� of functionally specialised 鈥渕ental organs鈥� called 鈥渕odules鈥�, each of which evolved during the Pleistocene to solve a problem of survival or reproduction faced by our hunter-gatherer ancestors. Endowed with substantial innate knowledge about how to solve the problem in its area of expertise, each module is likened to a 鈥渟pecial-purpose minicomputer鈥� dedicated to solving a single problem. Thus the human mind allegedly achieved intellectual dominion over the beasts of the field by evolving a larger army of idiot savants than the beasts possessed, thereby enabling it to solve a wider array of problems.

A second defining doctrine of EP is that human minds remain adapted to a Pleistocene hunter-gatherer lifestyle, that psychologically we are living fossils of our Stone Age ancestors. Accordingly, the evolved nature of the human mind is allegedly discoverable by figuring out the adaptive problems our ancestors faced and then hypothesising the modules that evolved to solve them.

So what鈥檚 wrong with the EP claims here? Actually, a number of things. First, the EPists argue that the human mind must still be adapted to the Pleistocene conditions under which it evolved because the 10,000 years (or 400 human generations) since then is not enough time to evolve something as complex as the mind. But this argument contains a simple fallacy. Without doubt, evolution could not have built the human mind from scratch in the last 400 generations, but that doesn鈥檛 mean evolution didn鈥檛 significantly modify the human mind during that time.

Second, this doctrine greatly underestimates the rate at which natural and sexual selection can drive evolutionary change. Recent studies show selection can produce radical change in the life-history traits of a population in as little as 18 generations 鈥� for humans, roughly 450 years (麻豆传媒, 9 July, p 28). Of course, such rapid evolution can occur only with significant change in the selection pressures acting on a population. But environmental change since the Pleistocene has unquestionably altered the selection pressures on human psychology.

Agricultural and industrial revolutions have precipitated fundamental changes in the social structures of human populations, which, in turn, have altered the challenges humans face when mating, forming alliances or negotiating status hierarchies. Other human activities 鈥� from building shelters to preparing and preserving food, and from contraception to organised education 鈥� have also consistently altered the selection pressures on human psychology. And teaching techniques, whether skill or information-based, have continually altered the cognitive niches to which humans have had to adapt.

Third, the EPists fail to appreciate the significance of the fact that the principal driving forces in human psychological evolution have been the demands of competition and cooperation with fellow humans. This created an arms race in human psychological evolution in which every bit of evolution in human psychology changed the competitive and cooperative environments to which human psychology needed to adapt. As the Machiavellian among us evolved subtler techniques of manipulation and exploitation, we forthright and upright few were forced to evolve ever subtler techniques of suspicion and resistance. Such arms races accelerated the rate of human psychological evolution.

So there has undoubtedly been significant psychological evolution since the Pleistocene. We are not simply Stone Age hunter-gatherers struggling like Fred and Wilma Flintstone to survive and reproduce in evolutionarily novel suburban habitats.

The doctrine that our minds are adapted to the Pleistocene is partially a by-product of the doctrine that the mind is massively modular, a notion beloved of the EPists. If the mind does contain 鈥渉undreds or thousands鈥� of distinct psychological adaptations, then the odds are infinitesimally small that all of them have been independently upgraded by selection in the past 10,000 years, they argue. If the mind is massively modular, then no doubt much of it lags behind the rapid environmental changes to which most human populations have been exposed since the end of the Pleistocene.

But the doctrine of massive modularity is mistaken. The argument most frequently proffered in its support says that our ancestors faced diverse adaptive problems, and that each problem required its own unique solution. Since what you do to avoid inedible flora, for example, won鈥檛 help you form a social alliance, a distinct module must have evolved for each problem our ancestors faced.

This argument conflates behavioural solutions to problems (choosing a nutritious berry, say, and choosing a political ally) with the cognitive mechanisms that produce behaviour. And there is no reason to think that the same cognitive mechanism can鈥檛 produce diverse behaviours that solve distinct problems, in much the way that the same computer program can produce both spreadsheets and bar charts.

Moreover, environments that are complex (posing multiple and varied adaptive problems) and rapidly changing (involving evolutionary arms races) typically select for the capacity to produce more than one adaptive phenotype rather than a legion of separate adaptations. The 鈥減henotypic plasticity鈥� that is selected for under such conditions produces more than one anatomical form, physiological state, or psychological mechanism in response to highly variable environments. Research in developmental neurobiology suggests that the mechanisms governing neural development have a plasticity that produces, through interaction with the local environment, cognitive mechanisms that perform relatively specialised problem solving. And this neural plasticity persists throughout most of life.

So, contrary to the claims of EPists, humans did not need to conscript an army of modular idiot savants to solve the adaptive problems we faced throughout evolutionary history. In designing the human brain, evolution hit upon a different solution: a plasticity that allows the brain to adapt to highly variable and often rapidly changing environments. In this respect, the brain is very similar to the immune system, which manufactures antibodies to meet changing pathogenic demands.

EPists do not accept, however, that mechanisms of plasticity are capable of producing the complex cognitive abilities humans possess. In The Blank Slate, Pinker even claims that, as a way of explaining cognitive development, neural plasticity is tantamount to magic. The reason, he argues, has to do with simple facts about learning. The environment in which learning occurs contains a literal infinity of information. In order to acquire some complex cognitive ability, they claim, the brain must be innately equipped with hypotheses about which features of the world it must pay attention to in order to acquire the further knowledge necessary for that complex ability. Once focused on the right subset of information in the environment, the brain must know what to do with that information, which requires further innate hypotheses. And so on, for each stage of learning. Thus, EPists conclude, many of our cognitive abilities are so complex they can emerge only if their development is guided by ability-specific databases of richly detailed innate hypotheses.

But this argument is doubly problematic. First, if the argument were correct, we would be unable to acquire any complex cognitive ability for which we had no innate module. Yet thinking one鈥檚 way through a cricket or baseball game requires extremely complex cognitive abilities, which we obviously acquire without benefit of innate cricket or baseball modules. And these cognitive achievements are far more complex than being attracted to nubile females, an 鈥渁chievement鈥� for which EPists believe males require an innate module.

Second, the argument is identical to the argument for intelligent design, which claims some aspects of the structure of an organism are so complex that they couldn鈥檛 possibly emerge through an unguided natural process, and whose emergence must therefore have been guided by an intelligence. Indeed, to an intelligent-design creationist, appealing to evolution by natural selection to explain organic form is like appealing to magic. Ironically, although no EPist would accept the complexity argument as support for the notion of intelligent design, they accept an identical argument when they argue that complex cognitive abilities can emerge only if development is guided by an intelligence embodied in modules stocked with richly detailed innate hypotheses. Just as we should reject the complexity argument for intelligent design, we should reject EPists鈥� claim that the brain contains 鈥渉undreds or thousands鈥� of innate modules that guide learning. The fact that we may not fully understand how the brain acquires its complex abilities in no way diminishes the speciousness of their argument.

If my arguments are right, where do they leave evolutionary psychology? It remains a worthwhile field, though one lacking any kind of 鈥済rand unified theory鈥�. And that is one of the alluring features of EP: it offers an overarching theoretical framework that synthesises speculation about human psychological evolution with speculation about the current nature of the human mind. Unfortunately, this theoretical framework is held together by two false presuppositions 鈥� that there were stable and separate adaptive problems that drove psychological evolution, and that the mind is massively modular.

鈥淚 doubt that evolutionary psychology will ever have a successful grand unified theory鈥�

If we accept that human psychological evolution was driven by rapidly changing environmental demands, and that the adaptive response to these changing demands was neural plasticity, the prospects for identifying evolutionary problems and mapping them to separate psychological adaptations appear dim. Though no one knows what the future holds, I doubt that evolutionary psychology will ever have a successful grand unified theory of the sort the EPists envision.