CHAPTER 38 Dual-inheritance theory: the evolution of human cultural capacities and cultural evolution Joseph Henrich and Richard McElreath 38.1. Introduction means that a child can have multiple fathers, who share paternity according to the number of times In the early 1930's Wintrop and Luella Kellogg they had sex with the mother prior to birth (1933) began co-rearing their 10.5-month-old (in anthropological parlance, 'partible paternity'). son, Donald, with a 7.5-month-old female In response to this cultural belief, women in chimpanzee named Gua. The Kelloggs expected many of these societies actively seek out extra­ that Gua, with the chimpanzee's popular reputa­ marital copulations, often to provide their child tion for aping, would acquire numerous behaviors with extra fathers. And, while male jealously and practices via imitation from both Donald from the husband is sometimes a problem, it is and themselves. Unexpectedly, however, while regarded as socially inappropriate and thus Gua did finally acquire a few human patterns suppressed. Detailed statistical analyses from two (e.g. combing his hair), Donald was the one such societies, the Bad of Venezuela (Beckerman who began to imitate the chimpanzee in some et aI., 2002) and Ache of Paraguay (Hill and dramatic ways. Following Gua, Donald acquired Hurtado. 1996), show that the optimal number the habits of knuckle walking (which he contin­ of fathers for a child's survival is more than one. ued well after achieving full bipedality), chewing These 'other fathers' (non-husbands of mom) on shoes, scraping his teeth against interior provide resources, in the form of fish and meat, walls, and hard biting. Donald even adopted to their offspring and the mothers, both during some stereotypical chimpanzee food grunts, pregnancy and while the child is growing up. barks and hoots, using a particular bark as the Interestingly, since much of the sex associated word for orange. Thus, it was the human who with 'extra fathers' occurs after conception, did most of the aping. many of these social fathers cannot be the People in many small-scale societies believe genetic fathers. Culturally transmitted beliefs in that a human fetus is formed by many repeated partible paternity have been recorded in various ejaculations of sperm into the womb. This belief linguistically unrelated societies across lowland 556 . CHAPTER 38 Dual-inheritance theory South America, as well as in New Guinea, are theirs. Evidence indicates that we use a variety by multiple researchers over the last 75 years of cues to identify our kin, including phenotypic (Beckerman and Valentine, 2002). similarity and scent (DeBruine, 2002; Thornhill These examples illustrate two key points about et al., 2003), but humans also apparently use humans. First, while chimpanzees do show some their culturally transmitted beliefs about kinship capacities for imitative learning (Horner and and reproduction. More generally, there is also Whiten, 2005; Whiten et aI., 2005), their cultural evidence that culture influences our spatial transmission shows substantially lower degrees cognition, perception of visual illusions (Segall of fidelity, frequency and internal motivation. et al., 1966), judgement (Nisbett, 2003), risk Compared to chimpanzees, humans are "imita­ preferences (Henrich and McElreath, 2002), and tion machines" (Tomasello, 1999). More gener­ notions of fairness or preferences for equity ally, while only limited social learning abilities (Henrich et aI., 2004). are found elsewhere in nature, social learning in Given all this, we think that a proper evolution­ our species is high fidelity, frequent, internally ary framework for studying human psychology motivated, often unconscious, and broadly and behaviour needs to reckon with our species' applicable. Humans learn, via observation of heavy reliance on cultural learning and cultural others, everything from motor patterns to goals evolved adaptations. In providing such a frame­ and affective responses, in domains ranging work, dual-inheritance theory (Cavalli-Sforza from tool-making and food preferences to altru­ and Feldman, 1981; Boyd and Richerson, 1985; ism and suicide. We will refer to this form ofsocial for similar approaches also see Durham, 1991; learning, which may be particular to humans, Laland et al., 2000) aims to incorporate these as cultural learning. and other aspects of human culture under the The combination of both the high fidelity Darwinian umbrella by focusing on three key and frequency of social learning in our lineage concepts: has generated cumulative cultural evolution, which 1. Cultural capacities as adaptations. Culture, may exist to any significant degree only in our cultural learning and cultural evolution arise lineage: this is the process through which learning from genetically evolved psychological adap­ builds a body of culturally transmitted informa­ tations for acquiring ideas, beliefs, values, tion (behaviour, practices, beliefs, etc.) in a practices, mental models, and strategies from population in such a way that locally adaptive other individuals by observation and infer­ aspects aggregate over time, with the accumula­ ence. Thus, the first step is to use the logic of tion of successful additions and modifications. natural selection to theorize about the evolu­ Cumulative cultural evolution builds adaptive tion and operation of our cultural learning practices, tools, technique, and bodies of knowl­ capacities. edge (about animal behaviour, medicinal plants, etc.) that no single individual could figure 2. Cultural evolution. Our cultural learning out in their lifetime, and that can only be under­ mechanisms give rise to a robust second stood as products of cultural evolutionary system of inheritance (cultural evolution) processes. Paleoarchaeology suggests that sub­ that operates by different transmission rules stantial cumulative cultural evolution has likely than genetic inheritance, and can thus pro­ been occurring for at least the last 280 000 years duce phenomena not observed in other, less (McBrearty and Brooks, 2000), and is thus a cultural, species. Theorizing abou t this key element in understanding human genetic process requires taking what we know about evolution. human cultural learning and human cogni­ Our second point is illustrated by societies tion, embedding these into evolutionary mod­ with partible paternity: culturally acquired els that included social interaction, and beliefs can shape how we understand the world studying the emergent properties of these in ways that influence decisions, including deci­ models. This approach allows researchers to sions arising from essential aspects ofour evolved cobble up from psychology and individual cognition. To invest in their offspring, for example, decision-making to sociology and population­ males need to figure out which offspring level phenomena. Concept 1: evolved psychological mechanisms for cultural learning . 557 3. Culture-gene coevolution. The second system Such cultural learning mechanisms can be of inheritance created by cultural evolution categorized into (i) content biases and (ii) COlt­ can alter both the social and physical envi­ text biases. Content biases, or what Boyd and ronments faced by evolving genes, leading to Richerson (1985) have called direct biases, cause a process termed culture-gene coevolution. us to more readily acquire certain beliefs, ideas For example, suppose that the practice of or behaviours because some aspect of their con­ cooking meat spread by social learning in tent makes them more appealing (or more likely ancestral human populations. In an environ­ to be inferred from observation). For example, ment of 'cooked meat', natural selection may imagine three practices involving different addi­ favour genes that shorten our energetically tives to popcorn: the first involves putting salt costly intestines and alter our digestive on popcorn, the second favours adding sugar, chemistry. Such a reduction of digestive tis­ and the third involves sprinkling chalk dust sue may have freed up energy for more 'brain on the kernels. Innate content biases that affect building'. In this way, human biology is cultural transmission will guarantee that chalk adapting to culturally transmitted behaviour. dust will likely not be a popular popcorn additive in any human society. Both salt and sugar have positive innate content biases for sensible evolu­ 38.2. Concept 1: evolved tionary reasons: foods with salty or sugary flavours psychological mechanisms were important sources of scarce nutrients and for cultural learning calories in ancestral human environments. Thus. natural selection favoured a bias to acquire a Our approach to understanding culture begins by taste for salty and sweet foods so that we would considering what kinds of cognitive learning abil­ be motivated to acquire and eat them. Of course, ities would have allowed individuals to efficiently if you grew up in a society that only salts its pop­ and effectively extract adaptive ideas, beliefs, corn, you may steadfastly adhere to your salting and practices from their social worlds in the preference even when you find that sugar is the changing environments of our hunter-gatherer standard popcorn seasoning in other societies. ancestors. This approach diverges from main­ Thus, human food preferences are simultane­ stream evolutionary psychology in its emphasis on ously culturally learned and influenced by innate the costly information hypothesis and on the evolu­ content biases. tion of specialized