Dual Inheritance Theory (DIT), also known as Gene-Culture Coevolution, was developed in the late 1970s and early 1980s to explain how human behavior is a product of two different and interacting evolutionary processes: genetic evolution and cultural evolution. DIT is a "middle-ground" between much of social science, which views culture as the primary cause of human behavioral variation, and human sociobiology and evolutionary psychology which view culture as an insignificant by-product of genetic selection. ^[1] In DIT, culture is defined as information in human brains that got there by social learning. Cultural evolution is considered a Darwinian selection process that acts on cultural information. Dual Inheritance Theorists often describe this by analogy to genetic evolution, which is a Darwinian selection process acting on genetic information.^[2]

Because genetic evolution is relatively well understood, most of DIT examines cultural evolution and the interactions between cultural evolution and genetic evolution.

[edit] Theoretical basis

DIT holds that genetic and cultural evolution interact in the evolution of Homo sapiens. DIT recognizes that the natural selection of genotypes is an important component of the evolution of human behavior and that cultural traits can be constrained by genetic imperatives. However, DIT also recognizes that genetic evolution has endowed the human species with a parallel evolutionary process of cultural evolution. DIT makes three main claims: ^[3]

[edit] Culture capacities are adaptations

The human capacity to store and transmit culture arose from genetically evolved psychological mechanisms. This implies that at some point during the evolution of the human species a type of social learning leading to cumulative cultural evolution was evolutionarily advantageous.

[edit] Culture evolves

Social learning processes give rise to cultural evolution. Cultural traits are transmitted differently than genetic traits and, therefore, result in different population-level effects. These effects can help explain human behavioral variation.

[edit] Genes and culture coevolve

Cultural traits alter the social and physical environments under which genetic selection operates. For example, the cultural adoptions of agriculture and dairying have, in humans, caused genetic selection for the traits to digest starch and lactose, respectively.^{[4][5][6][7][8][9]} As another example, it is likely that once culture became adaptive, genetic selection caused a refinement of the cognitive architecture that stores and transmits cultural information. This refinement may have further influenced the way culture is stored and the biases that govern its transmission.

DIT also predicts that, under certain situations, cultural evolution may select for traits that are genetically maladaptive. An example of this is the demographic transition, which describes the fall of birth rates within industrialized societies. Dual Inheritance Theorists hypothesize that the demographic transition may be a result of a prestige bias, where individuals that forgo reproduction to gain more influence in industrial societies are more likely to be chosen as cultural models. ^[10][11]

[edit] View of culture

People have defined the word "culture" to describe a large set of different phenomena.^[12][13] A definition that sums up what is meant by "culture" in DIT is:

Culture is information stored in individuals' brains that is capable of affecting behavior and that got there through social learning.^[14][15]

This view of culture emphasizes population thinking by focusing on the process by which culture is generated and maintained. It also views culture as a dynamic property of individuals, as opposed to the more standard social science view of culture as a superorganic entity to which individuals must conform.^[16] This view's main advantage is that it connects individual-level processes to population-level outcomes.^[17]

[edit] Genetic influence on cultural evolution

Genes have an impact on cultural evolution via psychological predispositions on cultural learning. Genes encode much of the information needed to form the human brain. Genes constrain the brain's structure and, hence, the ability of the brain to store culture. Genes may also endow individuals with certain types of transmission bias (described below).

[edit] Cultural influences on genetic evolution

Culture can profoundly influence gene frequencies in a population. One of the best known examples is the prevalence of the genotype for adult lactose absorption in human populations, such as Northern Europeans and some African societies, with a long history of raising cattle for milk. Other societies such as East Asians and Amerindians, retain the typical mammalian genotype in which the body shuts down lactase production shortly after the normal age of weaning. This implies that the cultural practice of raising cattle for milk led to a selection for genetic traits for lactose digestion.^[18] Recently, analysis of natural selection on the human genome suggests that civilization has accelerated genetic change in humans over the past 10,000 years.^[19]

[edit] Mechanisms of cultural evolution

In DIT, the evolution and maintenance of cultures is described by five major mechanisms, natural selection of cultural variants, random variation, cultural drift, guided variation and transmission bias.

[edit] Natural selection

Cultural differences among individuals can lead to differential survival of individuals, with those that survive more likely to have their cultural variants adopted by others. The patterns of this selective process depend on transmission biases and can result in behavior that is more adaptive to a given environment.

[edit] Random variation

Random variation arises from errors in the learning, display or recall of cultural information.

[edit] Cultural drift

Cultural drift is a process roughly analogous to genetic drift in evolutionary biology.^[20] In cultural drift, the frequency of cultural traits in a population may be subject to random fluctuations due to chance variations in which traits are observed and transmitted (sometimes called "sampling error").^[21] These fluctuations might cause cultural variants to disappear from a population. This effect should be especially strong in small populations. ^[22]

An example of cultural drift would be dialects of song birds. If a group of songbirds are culturally split (viz, no interactions with their songs), then distinct dialects will form from the original mating songs of the male birds. This is thought to happen due to errors in songbird singing and acquisition by successive generations, which, while still functional, do not maintain fidelity with the original song of the first generation.^[23]

[edit] Guided variation

Cultural traits may be gained in a population through the process of individual learning. Once an individual learns a novel trait, it can be transmitted to other members of the population. The process of guided variation depends on an adaptive standard that determines what cultural variants are learned.

[edit] Biased transmission

Understanding the different ways that culture traits can be transmitted between individuals has been an important part of DIT research since the 1970s.^[24][25] Transmission biases occur when some cultural variants are favored over others during the process of cultural transmission. ^[26] Boyd and Richerson (1985)^[27] defined and analytically modeled a number of possible transmission biases. The list of biases has been refined over the years, especially by Henrich and McElreath.^[28]

[edit] Content bias

Content biases result from situations where some aspect of a cultural variant's content makes them more likely to be adopted.^[29] Content biases can result from genetic preferences, preferences determined by existing cultural traits, or a combination of the two. For example, food preferences can result from genetic preferences for sugary or fatty foods and socially-learned eating practices and taboos. ^[30] Content biases are sometimes called "direct biases."^[31]

[edit] Context bias

Context biases result from individuals using clues about the social structure of their population to determine what cultural variants to adopt. This determination is made without reference to the content of the variant. There are two major categories of context biases: (1) model-based biases, and (2) frequency-dependent biases.

[edit] Model-based biases

Model-based biases result when an individual is biased to choose a particular "cultural model" to imitate. There are four major categories of model-based biases: (1) prestige bias, (2) skill bias, (3) success bias, (4) similarity bias. ^[32][33] A "prestige bias" results when individuals are more likely to imitate cultural models that are seen as having more prestige. A measure of prestige could be the amount of deference shown to a potential cultural model by other individuals. A "skill bias" results when individuals can directly observe different cultural models performing a learned skill and are more likely to imitate cultural models that perform better at the specific skill. A "success bias" results from individuals preferentially imitating cultural models that they determine are most generally successful (as opposed to successful at a specific skill as in the skill bias.) A "similarity bias" results when individuals are more likely to imitate cultural models that are perceived as being similar to the individual based on specific traits.

[edit] Frequency-dependent biases

Frequency-dependent biases result when an individual is biased to choose particular cultural variants based on their perceived frequency in the population. The most explored frequency-dependent bias is the "conformity bias." Conformity biases result when individuals attempt to copy the mean or the mode cultural variant in the population. Another possible frequency dependent bias is the "rarity bias." The rarity bias results when individuals preferentially choose cultural variants that are less common in the population. The rarity bias is also sometimes called a "nonconformist bias".

[edit] Social learning and cumulative cultural evolution

In DIT, the evolution of culture is dependent on the evolution of social learning. Analytic models show that social learning becomes evolutionarily beneficial when the environment changes with enough frequency that genetic inheritance can not track the changes, but not fast enough that individual learning is more efficient.^[34] While other species have social learning, and thus some level of culture, only humans, some birds and chimpanzees are known to have cumulative culture.^[35] Boyd and Richerson argue that the evolution of cumulative culture depends on observational learning and is uncommon in other species because it is ineffective when it is rare in a population. They propose that the environmental changes occurring in the Pleistocene may have provided the right environmental conditions. ^[36]. Michael Tomasello argues that cumulative cultural evolution results from a "ratchet effect" that began when humans developed the cognitive architecture to understand others as mental agents. ^[37] Furthermore Tomasello proposed in the 80s that there are some disparities between the observational learning mechanisms found in humans and great apes - which go some way to explain the observable difference between great ape traditions and human types of culture (see Emulation (observational learning)).

[edit] Cultural group selection

Although group selection is commonly thought to be nonexistent or unimportant in genetic evolution, ^[38][39][40] DIT predicts that, due to the nature of cultural inheritance, it may be an important force in cultural evolution. The reason group selection is thought to operate in cultural evolution is because of conformist biases (see above section on transmission biases). Conformist biases make it difficult for novel cultural traits to spread through a population. Conformist bias also helps maintain variation between groups. These two properties, rare in genetic transmission, are necessary for group selection to operate.^[41] Based on an earlier model by Cavalli-Sforza and Feldman, ^[42] Boyd and Richerson show that conformist biases are almost inevitable when traits spread through social learning, ^[43] implying that group selection is common in cultural evolution. Group selection can also only work if the rate of group formation is greater than the rate of group extinction. Analysis of the rates of formation and extinction of small groups in New Guinea implied that cultural group selection might be a good explanation for slowly changing aspects of social structure, but not for rapidly changing fads.^[44] The ability of cultural evolution to maintain intergroup diversity is what allows for the study of cultural phylogenetics. ^[45]

[edit] Historical development

The idea that human cultures undergo a similar evolutionary process as genetic evolution goes back at least to Darwin^[46] In the 1960s, Donald T. Campbell published some of the first theoretical work that adapted principles of evolutionary theory to the evolution of cultures. ^[47] In 1976, two developments in cultural evolutionary theory set the stage for DIT. In that year Richard Dawkins's The Selfish Gene introduced ideas of cultural evolution to a popular audience. Although one of the best-selling science books of all time, because of its lack of mathematical rigor, it had little impact on the development of DIT. Also in 1976, geneticists Marcus Feldman and Luigi Luca Cavalli-Sforza published the first dynamic models of gene-culture coevolution. ^[48] These models were to form the basis for subsequent work on DIT, heralded by the publication of three seminal books in 1980 and 1981.

The first was Charles Lumsden and E.O. Wilson's Genes, Mind and Culture.^[49] This book outlined a series of mathematical models of how genetic evolution might favor the selection of cultural traits and how cultural traits might, in turn, affect the speed of genetic evolution. While it was the first book published describing how genes and culture might coevolve, it had relatively little impact on the further development of DIT.^[50] Some critics felt that their models depended too heavily on genetic mechanisms at the expense of cultural mechanisms ^[51]. Controversy surrounding Wilson's sociobiological theories may also have decreased the lasting impact of this book.^[52]

The second 1981 book was Cavalli-Sforza and Feldman's Cultural Transmission and Evolution: A Quantitative Approach.^[53] Borrowing heavily from population genetics and epidemiology, this book built a mathematical theory concerning the spread of cultural traits. It describes the evolutionary implications of vertical transmission, passing cultural traits from parents to offspring; oblique transmission, passing cultural traits from any member of an older generation to a younger generation; and horizontal transmission, passing traits between members of the same population.

The next significant DIT publication was Robert Boyd and Peter Richerson's 1985 Culture and the Evolutionary Process.^[54] This book presents the now-standard mathematical models the evolution of social learning under different environmental conditions, the population effects of social learning, various forces of selection on cultural learning rules, different forms of biased transmission and their population-level effects, and conflicts between cultural and genetic evolution. The book's conclusion also outlined areas for future research that are still relevant today.

[edit] Current and future research

In their 1985 book, Boyd and Richerson outlined an agenda for future DIT research. This agenda, outlined below, called for the development of both theoretical models and empirical research. DIT has since built a rich tradition of theoretical models over the past two decades. ^[55] However, there has not been a comparable level of empirical work.

In a 2006 interview Harvard biologist E.O. Wilson expressed disappointment at the little attention afforded to DIT:

"...for some reason I haven't fully fathomed, this most promising frontier of scientific research has attracted very few people and very little effort." ^[56]

Kevin Laland and Gillian Brown attribute this lack of attention to DIT's heavy reliance on formal modeling, which doesn't attract the media attention of less rigorous approaches to human behavioral evolution, such as evolutionary psychology:

"In many ways the most complex and potentially rewarding of all approaches, [DIT], with its multiple processes and cerebral onslaught of sigmas and deltas, may appear too abstract to all but the most enthusiastic reader. Until such a time as the theoretical hieroglyphics can be translated into a respectable empirical science most observers will remain immune to its message."^[57]

Economist Herbert Gintis disagrees with this critique, citing existing empirical work as well as more recent work using techniques from behavioral economics ^[58]. These behavioral economic techniques have been adapted to test predictions of cultural evolutionary models in laboratory settings ^[59][60][61] as well as studying differences in cooperation in fifteen small-scale societies in the field. ^[62]

Since one of the goals of DIT is to explain the distribution of human cultural traits, ethnographic and ethnologic techniques may also be useful for testing hypothesis stemming from DIT. Although findings from traditional ethnologic studies have been used to buttress DIT arguments, ^[63][64] thus far there have been little ethnographic fieldwork designed to explicitly test these hypotheses.^[65][66][67] A major difficulty in using existing ethnographic data to test DIT is that, due to cultural anthropology's assumption of culture as a superorganic entity, ethnographic data tends to ignore individual and intragroup cultural variation and focus almost entirely on intergroup variation.

DIT has been viewed as having great potential for unifying diverse academic fields under one overarching theory. Mesoudi, et al. have identified DIT as the ideal way to build a comprehensive theory of cultural evolution to answer questions about human behavior at different temporal and spacial scales. ^[68] Along with game theory, Herb Gintis has named DIT one of the two major conceptual theories with potential for unifying the behavioral sciences, including economics, biology, anthropology, sociology, psychology and political science. Because it addresses both the genetic and cultural components of human inheritance, Gintis sees DIT models as providing the best explanations for the ultimate cause of human behavior and the best paradigm for integrating those disciplines with evolutionary theory. ^[69] In a review of competing evolutionary perspectives on human behavior, Laland and Brown see DIT as the best candidate for uniting the other evolutionary perspectives under one theoretical umbrella.^[70]

[edit] Relation to other fields

[edit] Sociology and cultural anthropology

Two major topics of study in both sociology and cultural anthropology are human cultures and cultural variation. However, Dual Inheritance theorists charge that both disciplines too often treat culture as a static superorganic entity that dictates human behavior. ^[71][72] Cultures are defined by a suite of common traits shared by a large group of people, without regard to variation in cultural traits at the individual level. This is in sharp contrast to DIT, which models human culture at the individual level and views culture as the result of a dynamic evolutionary process at the population level. ^[73][74]

[edit] Human sociobiology and evolutionary psychology

Human sociobiologists and evolutionary psychologists try to understand how maximizing genetic fitness, in either the modern era or past environments, can explain human behavior. To the human sociobiologist and evolutionary psychologist, culture is either trivial or so bound by genetic fitness that it is unimportant. When faced with a common and seemingly maladaptive trait, practitioners from these disciplines try to determine how the trait actually increases genetic fitness (maybe through kin selection or by speculating about early evolutionary environments). Dual Inheritance theorists, in contrast, will consider a variety of genetic and cultural processes in addition to natural selection on genes.

[edit] Human behavioral ecology

Human behavioral ecology (HBE) and DIT have a similar relationship to what ecology and evolutionary biology have in the biological sciences. HBE is more concerned about ecological process and DIT more focused on historical process. One difference is that human behavioral ecologists often assume that culture is a system that produces the most adaptive outcome in a given environment. This implies that similar behavioral traditions should be found in similar environments. However, this is not always the case. A study of African cultures showed that cultural history was a better predictor of cultural traits than local ecological conditions^[75].

[edit] Memetics

Memetics, which comes from the meme idea described in Dawkins's The Selfish Gene, is similar to DIT in that it treats culture as an evolutionary process that is distinct from genetic transmission. However, there are some philosophical differences between memetics and DIT.^[76] One difference is that memetics' focus is on the selection potential of discrete replicators (memes), where DIT allows for transmission of both non-replicators and non-discrete cultural variants. DIT does not assume that replicators are necessary for cumulative adaptive evolution. DIT also more strongly emphasizes the role of genetic inheritance in shaping the capacity for cultural evolution. But perhaps the biggest difference is a difference in academic lineage. Memetics as a label is more influential in popular culture than in academia. Critics of memetics argue that it is lacking in empirical support or is conceptually ill-founded, and question whether there is hope for the memetic research program succeeding. Proponents point out that many cultural traits are discrete, and that many existing models of cultural inheritance assume discrete cultural units, and hence involve memes.^[77]

[edit] See also

• Nature versus nurture
• Adaptive bias
• Cultural selection theory
• Memetics

[edit] References

[edit] Further reading

[edit] Books

• Lumsden, C. J. and E. O. Wilson. 1981. Genes, Mind, and Culture: The Coevolutionary Process. Cambridge, Massachusetts: Harvard University Press.
• Cavalli-Sfornza, L. L. and M. Feldman. 1981. Cultural Transmission and Evolution: A Quantitative Approach. Princeton, New Jersey: Princeton University Press.
• Boyd, R. and P. J. Richerson. 1985. Culture and the Evolutionary Process. Chicago: University of Chicago Press.
• Durham, W. H. 1991. Coevolution: Genes, Culture and Human Diversity. Stanford, California: Stanford University Press. ISBN 0-8047-1537-8
• Tomasello, M. 1999. The Cultural Origins of Human Cognition. Cambridge, Massachusetts: Cambridge University Press.
• Shennan, S. J. 2002. Genes, Memes and Human History: Darwinian Archaeology and Cultural Evolution. London: Thames and Hudson.
• Laland, K. N. and G. R. Brown. 2002. Sense & Nonsense: Evolutionary Perspectives on Human Behavior. Oxford: Oxford University Press.
• Boyd, R. and P. J. Richerson. 2005. The Origin and Evolution of Cultures. Oxford: Oxford University Press.
• Richerson, P. J. and R. Boyd. 2005. Not By Genes Alone: How Culture Transformed Human Evolution. Chicago: University of Chicago Press.

[edit] Reviews

• Smith, E. A. 1999. Three styles in the evolutionary analysis of human behavior. In L. Cronk, N. Chagnon, and W. Irons, (Eds.) Adaptation and Human Behavior: An Anthropological Perspective New York: Aldine de Gruyter.
• Henrich, J. and R. McElreath. 2003. The evolution of cultural evolution. Evolutionary Anthropology 12:123-135.
• Mesoudi, A., A. Whiten, and K. N. Laland. 2006. Towards a unified science of cultural evolution. Behavioral and Brain Sciences 29:329-383
• Gintis, H. 2006. A framework for the integration of the behavioral sciences Behavioral and Brain Sciences 30:1-61
• Bentley, R.A., C. Lipo, H.D.G. Maschner and B. Marler 2007. Darwinian Archaeologies. In R.A. Bentley, H.D.G. Maschner & C. Chippendale (Eds.) Handbook of Archaeological Theories. Lanham (MD): AltaMira Press.
• McElreath, R. & Henrich, J. 2007. Modeling cultural evolution. In R. Dunbar and L. Barrett, (Eds.), Oxford Handbook of Evolutionary Psychology Oxford: Oxford University Press.
• McElreath, R. & Henrich, J. 2007. Dual inheritance theory: the evolution of human cultural capacities and cultural evolution. In R. Dunbar and L. Barrett, (Eds.), Oxford Handbook of Evolutionary Psychology Oxford: Oxford University Press.
• Sterelny, Kim (2002). Review Genes, Memes and Human History. Stephen Shennan. London: Thames and Hudson. pp. 304. http://www.vuw.ac.nz/phil/staff/documents/sterelny-papers/tripod.pdf#search='dual%20inheritance%20theorypdf'. 
• Laland, K.N., Odling-Smee, J. & Myles, S. 2010. How culture shaped the human genome: bringing genetics and the human sciences. Nature Reviews: Genetics 11:137-148

[edit] Journal articles

• "Culture, Adaptation, and Innateness". The Innate Mind: Culture and Cognition. http://www.sscnet.ucla.edu/anthro/faculty/boyd/Innateness%20ver%204.1.pdf. 
• "Built for Speed, Not for Comfort: Darwinian Theory and Human Culture". History and Philosophy of the Life Sciences (23): 425'465. 2001. http://www.sscnet.ucla.edu/anthro/faculty/boyd/Built%20for%20Speed,%20Not%20for%20Comfort.pdf. 

[edit] External links

[edit] Current DIT researchers

• Rob Boyd, Department of Anthropology, UCLA
• Marcus Feldman, Department of Biological Sciences, Stanford
• Joe Henrich, Departments of Psychology and Economics, University of British Columbia
• Richard McElreath, Anthropology Department, UC Davis
• Peter J. Richerson, Department of Environmental Science and Policy, UC Davis

[edit] Related researchers

• Herb Gintis, Emeritus Professor of Economics, University of Massachusetts & Santa Fe Institute
• Kevin Laland, School of Biology, University of St. Andrews
• Ruth Mace, Department of Anthropology, University College London
• Michael Tomassello, Department of Developmental and Comparative Psychology, Max Planck Institute for Evolutionary Anthropology