In cultures that practice polygamy, where men are permitted to have multiple wives, the intra-sexual competition causes a greater level of crime, violence, poverty and gender inequality than in societies that practice monogamous marriage, suggests a study led by the University of British Columbia.

The study, exploring the global rise of monogamous marriage, suggests that monogamous marriage is replacing polygamy rapidly 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, published 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."

Their findings did not mimic the positive happy go lucky HBO series Big Love, which displays a great and loving family of a man and his three wives but rather the findings suggest that polygamy societies have significantly higher levels of rape, kidnapping, murder, assault, robbery and fraud.

"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 explained.

Contrary to polygamous marriages, the study found that monogamous marriage results in significant improvements in child welfare, including lower rates of child neglect, abuse, accidental death, homicide and intra-household conflict.

Henrich explained that monogamous marriage has largely preceded democracy and voting rights for women in the nations where it has been institutionalized.

They found that monogamous marriages “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.”