In modern culture, it is not considered socially acceptable for married people to have extramarital sexual partners. However, in some Amazonian cultures, extramarital sexual affairs were common, and people believed that when a woman became pregnant, each of her sexual partners would be considered part-biological father.

Now, a new University of Missouri study published in the journal Proceedings of the National Academy of Sciences has found that up to 70 percent of Amazonian cultures may have believed in the principle of multiple paternity.

"In these cultures, if the mother had sexual relations with multiple men, people believed that each of the men was, in part, the child's biological father," said Robert Walker, assistant professor of Anthropology in the College of Arts and Science. "It was socially acceptable for children to have multiple fathers, and secondary fathers often contributed to their children's upbringing."

Walker says sexual promiscuity was normal and acceptable in many traditional South American societies. He says married couples typically lived with the wife's family, which he says increased their sexual freedom.

"In some Amazonian cultures, it was bad manners for a husband to be jealous of his wife's extramarital partners," Walker said. "It was also considered strange if you did not have multiple sexual partners. Cousins were often preferred partners, so it was especially rude to shun their advances."

Previous research had uncovered the existence of multiple paternity in some Amazonian cultures. However, anthropologists did not realize how many societies held the belief. Walker's team analyzed ethnographies (the branch of anthropology that deals descriptively with cultures) of 128 societies across lowland South America, which includes Brazil and many of the surrounding countries. Multiple paternity is reported to appear in 53 societies, and singular paternity is mentioned in 23 societies. Ethnographies for 52 societies do not mention conception beliefs.

Walker's team has several hypotheses on the benefits of multiple paternity. Women believed that by having multiple sexual partners they gained the benefit of larger gene pools for their children. He says women benefited from the system because secondary fathers gave gifts and helped support the child, which has been shown to increase child survival rates. In addition, brutal warfare was common in ancient Amazonia, and should the mother become a widow, her child would still have a father figure.

Men benefitted from the multiple paternity system because they were able to formalize alliances with other men by sharing wives. Walker hypothesizes that multiple paternity also strengthened family bonds, as brothers often shared wives in some cultures.

Walker collaborated with Mark Flinn, professor in the MU Department of Anthropology, and Kim Hill, professor in Arizona State University's School of Human Evolution and Social Change.