Dual inheritance theory
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Dual inheritance theory, (or DIT), in sharp contrast to the notion that "culture overrides biology," posits that humans are products of the interaction between biological evolution and cultural evolution. DIT assumes that culture, (including cultural transmission and cultural evolution), is both influenced by and constrained by genes via psychological adaptations and that culture, in turn, contributes to selection pressures on genes. The results of these interactions can be a mix of both adaptive and maladaptive traits within a population. Another way of conceiving DIT is as an approach that integrates evolutionary theory, cultural theory, and learning theory.
 Evolution and Populations
Organic evolutionists began to use mathematical models to investigate the properties of evolution in the first quarter of the 20th century. The aim of the effort was to take the micro-scale properties of individuals and genes, scale them up to a population of individuals and deduce the long run evolutionary consequences of the assumed micro-level processes. Empiricists have a handle on both the micro-scale processes and the long-run results, but not on what happens over many generations in between. Moreover, human intuition is not so good at envisioning the behavior of populations over long spans of time. Hence mathematics proved an invaluable aid.
 Evolution and Culture
Beginning with the pioneering work of Luigi Cavalli-Sforza and Marcus Feldman (1981) in the early 1970s, these methods were adapted to study cultural evolution. The problem is considered analogous to that of genetic evolution.
- People acquire information from others by learning and teaching.
- Cultural transmission is imperfect, so the transmission is not always exact.
- People invent new cultural variants, making culture a system for the inheritance of acquired variation.
- People also pick and choose the cultural variants they adopt and use, processes that are not possible in the genetic system (although in the case of sexual selection individuals may choose mates with the objective of getting good genes for their offspring).
Social scientists know a fair amount about such things, enough to build reasonable mathematical representations of the micro-level processes of cultural evolution. The theory is of the form:
- + effects of forces
where p measures something interesting about the culture of a population, for example the fraction of employees who are earnest workers. Teaching and imitation, all else equal, tend to replicate culture. The fraction of workers in a culture who are earnest tends to remain similar from generation to generation. Earnest workers model earnest behavior for others to imitate and try to teach earnestness to new employees. The same can be said for slackers.
Typically, several processes we call forces will act simultaneously to change culture over time. For example, management may find it difficult to discover and sanction slacking. Earnest workers may experiment with slacking and find that there are seldom any adverse consequences. Hence, some earnest employees may become slackers. New employees may observe that some people slack and some work hard. They may tend to prefer the easier path.
At the same time, firms with a high frequency of slackers will tend to fail while those with many earnest workers may prosper. Prosperous firms will have the opportunity to socialize many more new workers than those that fail prematurely. The overall quality of the economy's work force in the long run will be determined by the balance of forces favoring slacking versus those favoring earnestness.
Theorists are interested in the abstract properties of such evolutionary models. Empiricists are interested in finding the models that best describe actual evolving systems. Real world practitioners are interested in predicting the outcomes of policies that might improve or harm the quality of a firm's or an economy's work force.
 Topics of interest in dual inheritance theory
Substantive questions that have interested dual inheritance theorists include:
- the adaptive costs and benefits of culture
- the apparent rarity of cultural learning mechanisms in nature
- the cognitive processes underlying cultural learning and transmission, (i.e., social learning)
- the influence of gene-culture coevolution on human psychology and the histories of human societies
- rates of different kinds of cultural evolution
- the evolution of symbolic systems
- the role of culture in the evolution of cooperation
 Genes influence cultural evolution
Many of the analyses involve the coevolution of genes and culture (hence the term dual inheritance or gene-culture coevolution theory). Genes have an impact on cultural evolution via psychological predispositions that bias what people imitate, teach, or learn for themselves. Hence, a physically awkward type of tool is liable to be modified or abandoned in favor of one that better suits the human hand and arm. The facts that sex is pleasurable, that sweet things taste good, and that being cold and wet is miserable suggest how the structure of our nervous system will have an impact on such things as marriage customs, cuisine, and the construction of shelters.
 Culture influences biological evolution
However, the opposite is also true. Cultures create environments that in turn may select for genes that succeed in the cultural environment. One of the best worked out cases is adult lactose absorption. In populations with a long history of dairying, such as African and northern Europeans cattle-keeping societies, most adults retain the ability to break down and hence digest the milk sugar lactose. Societies with no history of dairying, such as east Asians and Amerindians, retain the primitive mammalian genotype in which the body shuts down lactose production shortly after the normal age of weaning.
According to some cultural evolutionists our social psychology was extensively remodeled by a long period of life in tribal scale social systems whose culturally transmitted rules encouraged much cooperation with non-relatives due to group selection on cultural variation. Darwin first proposed hypothesis much like this in The Descent of Man.
Contemporary work in the dual inheritance/gene-culture coevolution tradition includes empirical studies designed to test ideas, (e.g. simulations, cross-cultural studies), derived from the mathematical theory.
 See also
- Boyd, Rob & Richerson, Peter J. (1985). Culture and the Evolutionary Process. Chicago University Press.
- Boyd, R. & Richerson, P.J. (2001). Built For Speed, Not for Comfort: Darwinian Theory and Human Culture. In History and Philosophy of the Life Sciences 23: 423-463.
- Cavalli-Sforza, L. L., & Feldman, M. W. (1981). Cultural Transmission and Evolution: A Quantitative Approach. Princeton, NJ: Princeton University Press.
- Henrich, J., & McElreath, R. (2003). The Evolution of Cultural Evolution. Evolutionary Anthropology, 12, 123-135.
- Smith, Eric Alden (1999). Three Styles in the Evolutionary Analysis of Human Behavior in Lee Cronk, Napoleon Chagnon and William Irons Adaptation and Human Behavior: An Anthropological Perspective, 27-48, New York: Aldine de Gruyter.
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