Dual inheritance theory (DIT), also known as gene-culture coevolution or biocultural evolution, was developed in the 1960s through early 1980s to explain how human behavior is a product of two different and interacting evolutionary processes: genetic evolution and cultural evolution. In DIT, culture is defined as information and/or behavior acquired through social learning. One of the theory's central claims is that culture evolves partly through a Darwinian selection process, which dual inheritance theorists often describe by analogy to genetic evolution.
'Culture', in this context is defined as 'socially learned behavior', and 'social learning' is defined as copying behaviors observed in others or acquiring behaviors through being taught by others. Most of the modeling done in the field relies on the first dynamic (copying) though it can be extended to teaching. Social learning at its simplest involves blind copying of behaviors from a model (someone observed behaving), though it is also understood to have many potential biases, including success bias (copying from those who are perceived to be better off), status bias (copying from those with higher status), homophily (copying from those most like ourselves), conformist bias (disproportionately picking up behaviors that more people are performing), etc.. Understanding social learning is a system of pattern replication, and understanding that there are different rates of survival for different socially learned cultural variants, this sets up, by definition, an evolutionary structure: Cultural Evolution.
Because genetic evolution is relatively well understood, most of DIT examines cultural evolution and the interactions between cultural evolution and genetic evolution.
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Theoretical basis
DIT holds that genetic and cultural evolution interacted 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:
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.
Culture evolves
Social learning processes give rise to cultural evolution. Cultural traits are transmitted differently from genetic traits and, therefore, result in different population-level effects on behavioral variation.
Genes and culture co evolve
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. 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.
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View of culture
People have defined the word "culture" to describe a large set of different phenomena. A definition that sums up what is meant by "culture" in DIT is:
Culture is socially learned information stored in individuals' brains that is capable of affecting behavior.
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 a view of culture as a superorganic entity to which individuals must conform. This view's main advantage is that it connects individual-level processes to population-level outcomes.
Genetic influence on cultural evolution
Genes affect 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 acquire and store culture. Genes may also endow individuals with certain types of transmission bias (described below).
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 first for meat and later for milk led to selection for genetic traits for lactose digestion. Recently, analysis of natural selection on the human genome suggests that civilization has accelerated genetic change in humans over the past 10,000 years.
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.
Natural selection
Cultural differences among individuals can lead to differential survival of individuals. The patterns of this selective process depend on transmission biases and can result in behavior that is more adaptive to a given environment.
Random variation
Random variation arises from errors in the learning, display or recall of cultural information, and is roughly analogous to the process of mutation in genetic evolution.
Cultural drift
Cultural drift is a process roughly analogous to genetic drift in evolutionary biology. 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"). These fluctuations might cause cultural variants to disappear from a population. This effect should be especially strong in small populations. A model by Hahn and Bentley shows that cultural drift gives a reasonably good approximation to changes in the popularity of American baby names. Drift processes have also been suggested to explain changes in archaeological pottery and technology patent applications. Changes in the songs of song birds are also thought to arise from drift processes, where distinct dialects in different groups occur due to errors in songbird singing and acquisition by successive generations. Cultural drift is also observed in an early computer model of cultural evolution.
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.
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. Transmission biases occur when some cultural variants are favored over others during the process of cultural transmission. Boyd and Richerson (1985) 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.
Content bias
Content biases result from situations where some aspect of a cultural variant's content makes them more likely to be adopted. 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. Content biases are sometimes called "direct biases."
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.
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. 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.
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" or "anti-conformist" bias.
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. While other species have social learning, and thus some level of culture, only humans, some birds and chimpanzees are known to have cumulative culture. 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. 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. 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)).
Cultural group selection
Although group selection is commonly thought to be nonexistent or unimportant in genetic evolution, 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. Based on an earlier model by Cavalli-Sforza and Feldman, Boyd and Richerson show that conformist biases are almost inevitable when traits spread through social learning, implying that group selection is common in cultural evolution. Analysis of small groups in New Guinea imply that cultural group selection might be a good explanation for slowly changing aspects of social structure, but not for rapidly changing fads. The ability of cultural evolution to maintain intergroup diversity is what allows for the study of cultural phylogenetics.
Historical development
The idea that human cultures undergo a similar evolutionary process as genetic evolution goes back at least to Darwin In the 1960s, Donald T. Campbell published some of the first theoretical work that adapted principles of evolutionary theory to the evolution of cultures. 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 effect 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. These models were to form the basis for subsequent work on DIT, heralded by the publication of three seminal books in the 1980s.
The first was Charles Lumsden and E.O. Wilson's Genes, Mind and Culture. 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 effect on the further development of DIT. Some critics felt that their models depended too heavily on genetic mechanisms at the expense of cultural mechanisms. Controversy surrounding Wilson's sociobiological theories may also have decreased the lasting effect of this book.
The second 1981 book was Cavalli-Sforza and Feldman's Cultural Transmission and Evolution: A Quantitative Approach. 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. This book presents the now-standard mathematical models of 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.
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. 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."
Kevin Laland and Gillian Brown attribute this lack of attention to DIT's heavy reliance on formal modeling.
"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."
Economist Herbert Gintis disagrees with this critique, citing empirical work as well as more recent work using techniques from behavioral economics. These behavioral economic techniques have been adapted to test predictions of cultural evolutionary models in laboratory settings as well as studying differences in cooperation in fifteen small-scale societies in the field.
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, thus far there have been little ethnographic fieldwork designed to explicitly test these hypotheses.
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. 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.
Relation to other fields
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. Cultures are defined by a suite of common traits shared by a large group of people. DIT theoriests argue that this doesn't sufficiently explain variation in cultural traits at the individual level. By contrast, DIT models human culture at the individual level and views culture as the result of a dynamic evolutionary process at the population level.
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. 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.
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.
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. 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.
Criticisms
A number of criticisms of DIT have been put forward. From some points of view, use of the term 'dual inheritance' to refer to both what is transmitted genetically and what is transmitted culturally is technically misleading. Many opponents cite horizontal transmission of ideas to be so "different" from the typical vertical transmission (reproduction) in genetic evolution that it is not evolution. However, 1) even genetic evolution uses non-vertical transmission through the environmental alteration of the genome during life by acquired circumstance: epigenetics, and 2) genetic evolution is also affected by direct horizontal transmission between separate species of plants and strains of bacteria: horizontal gene transfer. Other critics argue that there can be no "dual" inheritance without cultural inheritance being "sequestered" by the biotic genome. Evidence for this process is scarce and controversial. Why this is a demand of critics, however, can be considered unclear as it refutes none of the central claims laid down by proponents of DIT.
More serious criticisms of DIT arise from the choice of Darwinian selection as an explanatory framework for culture. Some argue, cultural evolution does not possess the algorithmic structure of a process that can be modeled in a Darwinian framework as characterized by John von Neumann and used by John Holland to design the genetic algorithm. Forcing culture into a Darwinian framework gives a distorted picture of the process for several reasons. First, some argue Darwinian selection only works as an explanatory framework when variation is randomly generated. To the extent that transmission biases are operative in culture, they mitigate the effect of Darwinian change, i.e. change in the distribution of variants over generations of exposure to selective pressures. Second, since acquired change can accumulate orders of magnitude faster than inherited change, if it is not getting regularly discarded each generation, it quickly overwhelms the population-level mechanism of change identified by Darwin; it 'swamps the phylogenetic signal'. DIT proponents reply that the theory includes a very important role for decision-making forces. As a point of history, Darwin had a rather sophisticated theory of human cultural evolution that depended on natural selection "to a subordinate degree" compared to "laws, customs, and traditions" supported by public opinion. When critics complain that DIT is too "Darwinian" they are falsely claiming that it is too dependent on ideas related to the neodarwinian synthesis which dropped Darwin's own belief that the inheritance of acquired variation is important and which ignored his ideas on cultural evolution in humans.
Another discord in opinion stems from DIT opponents' assertion that there exists some "creative force" that is applied to each idea as it is received and before it is passed on, and that this agency is so powerful that it can be stronger than the selective system of other individuals assessing what to teach and whether your idea has merit. But if this criticism was valid then it would be comparatively much easier to argue an unpopular or incorrect concepts than it actually is. In addition, nothing about DIT runs counter to the idea that an internally selective process (some would call creativity) also determines the fitness of ideas received and sent. In fact this decision making is a large part of the territory embraced by DIT proponents but is poorly understood due to limitations in neurobiology (for more information see Neural Darwinism).
Related criticisms of the effort to frame culture in Darwinian terms have been leveled by Richard Lewontin, Niles Eldredge, and Stuart Kauffman.
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