What is a Human Universal? Human Behavioral Ecology and Human Nature Elizabeth Cashdan, University of Utah In Arguing About Human Nature, S. M. Downes and E. Machery, eds. Routledge. 2013 In 1991 the cultural anthropologist Don a function of exposure to sunlight (tanning) is a Brown bucked anthropology's tabula rasa norm of reaction, as is the percentage of male and tradition by identifying over 300 \human female leopard geckos hatched at different incu- universals"|individual and sociocultural traits bation temperatures (see figure 1). Although that are found in every known human society gecko families vary in the the strength of their (Brown 1991; universals enumerated in Pinker response to temperature, the patterning of the 2003). The items he identified include both psy- response is similar: it is part of leopard gecko chological traits (e.g., wariness around snakes, nature for more males to be born when the tem- sweets preferred, sexual jealousy) and sociocul- perature is warmer, within this range of temper- tural ones (e.g., territoriality, females do more atures. Figure 2 shows another example, from a direct childcare, food sharing). Because they are brilliant study of soapberry bug mating behav- universal, it is plausible that these traits have ior by Carroll & Corneli (1999). A male soap- a biological basis and that they are evolved fea- berry bug may stay attached to his mate after tures of a universal human nature. mating in order to guard her against other suit- What, then, of the many domains where cul- ors. If there are many more males than females tures differ? The assumption is often made that (high sex ratio), this makes adaptive sense: he human nature is found solely in its universals { gains more by keeping other males away than in the traits found in every society. A trait that he loses in forgone mating opportunities. But is found in some societies and not others is then if eager suitors are less numerous (lower sex ra- assumed to be culturally constructed and with- tio), he gains little by mate-guarding and should out an evolutionary foundation. instead leave after mating and search for an- other mate. The upper figure shows the mate- Behavioral ecologists hold a different view: guarding response of male Oklahoma soapberry because human nature evolved to be flexible in bugs (each line is a family of half-sibs) to exper- predictable ways, the task of understanding hu- imental changes in the sex ratio. Mate-guarding man nature requires that we understand how increases as the sex ratio increases. Although evolution shaped that variation. The assump- each family has a slightly different reaction norm, tion is not just that we evolved to respond flex- they are similar enough to indicate a general ibly, but that selection shaped the nature and feature of what we might call Oklahoma soap- direction of that flexibility. To a behavioral ecol- berry bug nature. An observer will see differ- ogist, then, the predictable, patterned nature of ent behaviors in different environments, but the that response is the universal we must under- responsiveness|the shape of the reaction norm stand. In this view, we cannot understand our curve|is \universal." At least, it is univer- universal human nature without understanding sal in Oklahoma. The lower figure shows that the variability in its expression. soapberry bug families from Florida do not al- The concept is clarified by viewing variation ter their mate-guarding appreciably, irrespective as a norm of reaction|the pattern of expression of experimental changes in the sex ratio. Why of a genotype across a range of environments. not? In Florida, the climate, hence the sex ra- The increase in a person's skin pigmentation as 1 Figure 1: Sex ratio reaction norms as a function of incubation temperature in leopard geckos. Each line connects the sex ratio for a half-sib family (offspring sired by a single male) that was divided between two incubation temperatures. (Rhen et al., 2010) tio, is less variable than in Oklahoma. In Ok- This perspective on human nature derives lahoma, a facultative (plastic, flexible) response from the anthropologists' knowledge of human makes adaptive sense, because an individual bug variation and the ecological focus of the par- could find himself in a variety of environments ent discipline of behavioral ecology. Most hu- and he will reproduce better if he can respond man behavioral ecologists were trained as cul- to those changes. For the Florida bugs, living tural anthropologists, which gives them an un- in their equable environment, there is no advan- equaled knowledge of the breadth and regulari- tage to such flexibility, and in that population, ties of human variation. They know, better than the flexible response did not evolve (Carroll & anyone, that people living in developed, industri- Corneli, 1999). alized states (the usual subjects of human social science) represent only a very small part of the Soapberry bugs occupy a range of environ- range of human variation, and that those soci- ments, but the range is nothing compared to that eties are in many respects quite unusual. The na- of humans, who live in every part of the planet ture of the parent discipline of behavioral ecology and whose environments include novel ones that also shapes their perspective of human nature. they constructed for themselves. The principle Human behavioral ecology's modus operandi is illustrated in the previous example, therefore, is to model optimal outcomes by considering the even more important when discussing human na- costs and benefits of different strategies and how ture. To a human behavioral ecologist, then, hu- they trade off against one another. Doing this man nature is not limited to human universals, forces an explicit consideration of the ecological as that phrase is usually understood. The hu- factors that shape those costs and benefits, and man universal is the shape of the response, and how they vary across environments. the task is to understand how selection pressures shaped it. In what follows I will consider examples 2 Figure 2: Mate guarding reaction norms as a function of sex ratio in soapberry bugs in Oklahoma (top) and Florida (bottom). (Carroll & Corneli, 1999) 3 which illustrate the following implications of this havioral ecologists who work with humans also view of human nature: (1) human and non- typically hold the working assumption (not be- human animal behavior can be understood us- lief) that the human behavioral differences they ing the same evolutionary theoretical perspec- observe are the facultative expression of a largely tive, and, in some cases, models (hence human shared genotype. nature is seen as part of the evolved natural Finally, human behavioral ecologists as- world), (2) viewing behavioral variation as a re- sume that human nature is adaptive (fitness- action norm provides guidance on how policy can enhancing), and that selection will lead to op- address the darker side of human nature, and timal outcomes. These optimal outcomes are (3) the individual-maximizing process of natu- modeled as the best outcome possible given re- ral selection has created a remarkably altruis- source constraints and tradeoffs between com- tic, cooperative human nature. I will not, in peting demands. The assumption of optimality this essay, attempt to review the field of human is less a matter of belief about human nature behavioral ecology generally, which has been than it is a useful working assumption. Behav- done admirably elsewhere (Borgerhoff Mulder, ioral ecologists know, as well as anyone, that peo- 2003; Cronk, 1991; Laland & Brown, 2011; Smith ple sometimes do maladaptive things (although et al., 2001; Winterhalder & Smith, 2000). How- they do not expect maladaptive outcomes to be ever, several points should be made first about common), and they also know the reasons why the assumptions under which behavioral ecolo- evolution sometimes leads to sub-optimal out- gists operate. comes. But it is a reasonable and powerful work- Human nature, broadly speaking, encom- ing assumption in generating and testing hy- passes the ways in which people think, feel, and potheses about the functions of, and selection act. However, thoughts and feelings are them- pressures on, human behavior|hence how hu- selves \invisible" to natural selection, since they man nature came to be. This is best demon- can only affect survival or reproduction by mo- strated by example, so we will consider several, tivating behavior. An emotion, no matter how beginning with mating and marriage. strongly felt, is irrelevant to evolution if it does not cause an observable change. For this rea- Mating and Marriage: Biological mod- son, human behavioral ecologists are largely un- els can explain a lot about human cul- concerned with psychological mechanisms, and tural behavior focus instead on the behavioral outcomes that selection can act upon. Marriage is universal, but variable, across human For somewhat different reasons, human be- societies, and our understanding of what forms havioral ecologists also are largely unconcerned are natural or normal have both moral over- with genetic mechanisms. Although biologists tones and policy implications. The ethnographic studying the behavioral ecology of other species database makes it clear that human nature en- are increasingly interested in the genetic basis of compasses marriages that are both monogamous flexible responses, these are difficult to study in and polygynous, and even, under very rare and people, and we know very little about the ge- special circumstances, polyandrous. What be- netics underlying human nature and behavior. havioral ecology adds to this pluralistic view of Human behavioral ecologists therefore typically mating and marriage is the specification that adopt a \phenotypic gambit" that assumes there men and women will adjust their mating and has been sufficient genetic variation and time for marriage choices to environmental circumstances competing selection pressures to have resulted in in predictable ways, and that those choices will the evolution of better-adapted phenotypes.