The cooperation theory was proposed by the early anthropologist Edward B. Tylor and further elaborated by Leslie A. White and Claude Lévi-Strauss. It emphasizes the value of the incest taboo in promoting cooperation among family groups and thus helping communities to survive. As Tylor saw it, certain operations necessary for the welfare of the community can be accomplished only by large numbers of people working together. To break down suspicion and hostility between family groups and make such cooperation possible, early humans developed the incest taboo to ensure that individuals would marry members of other families. The ties created by intermarriage would serve to hold the community together. Thus, Tylor explained the incest taboo as an answer to the choice “between marrying out and being killed out.”59
The idea that marriage with other groups promotes cooperation sounds plausible, but is there evidence to support it? There are, of course, societies such as the Gusii in which marriage is often between hostile groups. But is that society an exception? And does marriage itself promote cooperation? Because people in all recent societies marry outside the family, we cannot test the idea that such marriages promote cooperation more than marriages within the family. We can ask whether other kinds of out-marriage, such as marriage with other communities, promote cooperation with those communities. The evidence suggests not. There is no greater peacefulness between communities when marriages are forbidden within the community and arranged with other communities than when they are not.60
Even if marriage outside the family promoted cooperation with other groups, why would it be necessary to prohibit all marriages within the family? Couldn’t families have required some of their members to marry outside the family if they thought it necessary for survival, but permitted incestuous marriages when such alliances were not needed? Although the incest taboo might enhance cooperation between families, other customs can also promote alliances. The need for cooperation does not adequately explain why the incest taboo exists in all societies. In particular, the cooperation theory does not explain the sexual aspect of the incest taboo. Premarital and extramarital sex is common and tolerated in many societies. Incestuous sex could likewise theoretically be allowed, as long as children were required to marry outside the family.