In cultures that permit men to take multiple wives, the intra-sexual competition that occurs causes greater levels of crime, violence, poverty and gender inequality than in societies that institutionalize and practice monogamous marriage.
That is a key finding of a new University of British Columbia-led study that explores the global rise of monogamous marriage as a dominant cultural institution. The study suggests that institutionalized monogamous marriage is rapidly replacing polygamy because it has lower levels of inherent social problems.
"Our goal was to understand why monogamous marriage has become standard in most developed nations in recent centuries, when most recorded cultures have practiced polygyny," says UBC Prof. Joseph Henrich, a cultural anthropologist, referring to the form of polygamy that permits multiple wives, which continues to be practiced in some parts of Africa, Asia, the Middle East and North America.
"The emergence of monogamous marriage is also puzzling for some as the very people who most benefit from polygyny – wealthy, powerful men – were best positioned to reject it," says Henrich, lead author of the study that is published today in the journal Philosophical Transactions of the Royal Society. "Our findings suggest that that institutionalized monogamous marriage provides greater net benefits for society at large by reducing social problems that are inherent in polygynous societies."
Considered the most comprehensive study of polygamy and the institution of marriage, the study finds significantly higher levels rape, kidnapping, murder, assault, robbery and fraud in polygynous cultures. According to Henrich and his research team, which included Profs. Robert Boyd (UCLA) and Peter Richerson (UC Davis), these crimes are caused primarily by pools of unmarried men, which result when other men take multiple wives.
"The scarcity of marriageable women in polygamous cultures increases competition among men for the remaining unmarried women," says Henrich, adding that polygamy was outlawed in 1963 in Nepal, 1955 in India (partially), 1953 in China and 1880 in Japan. The greater competition increases the likelihood men in polygamous communities will resort to criminal behavior to gain resources and women, he says.
According to Henrich, monogamy's main cultural evolutionary advantage over polygyny is the more egalitarian distribution of women, which reduces male competition and social problems. By shifting male efforts from seeking wives to paternal investment, institutionalized monogamy increases long-term planning, economic productivity, savings and child investment, the study finds. Monogamy's institutionalization has been assisted by its incorporation by religions, such as Christianity.
Monogamous marriage also results in significant improvements in child welfare, including lower rates of child neglect, abuse, accidental death, homicide and intra-household conflict, the study finds. These benefits result from greater levels of parental investment, smaller households and increased direct "blood relatedness" in monogamous family households, says Henrich, who served as an expert witness for British Columbia's Supreme Court case involving the polygamous community of Bountiful, B.C.
Monogamous marriage has largely preceded democracy and voting rights for women in the nations where it has been institutionalized, says Henrich, the Canadian Research Chair in Culture, Cognition and Evolution in UBC's Depts. of Psychology and Economics. By decreasing competition for younger and younger brides, monogamous marriage increases the age of first marriage for females, decreases the spousal age gap and elevates female influence in household decisions which decreases total fertility and increases gender equality.
View the study, The Puzzle of Monogamous Marriage, at: http://rstb.royalsocietypublishing.org/content/367/1589/657.full.pdf
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