The rise of monogamy - and the decline of polygamy - has historically led to greater gender equality, the spread of democracy and economic prosperity, an expert testified Thursday in a British Columbia court.
In contrast, Prof. Joseph Henrich of the University of British Columbia told a judge examining Canada's polygamy laws that multiple marriage is linked to increases in crime, substance abuse, child mortality and discrimination against women.
Prof. Henrich, whose research combines evolutionary psychology, anthropology and economics, is the lead expert witness for the B.C. government, which is arguing the harms associated with polygamy justify keeping it illegal.
Prof. Henrich's testimony pointed to numerous studies indicating that societies that abandon polygamy do better.
"There's a lot of research increasingly showing that amongst modern, westernized democracies, societies that are more equal … have a whole bunch of better social outcomes," Prof. Henrich said.
"So in many ways, monogamy is the first effort to create equality."
Prof. Henrich said humans and other primates are genetically predisposed to favour polygamy - specifically, the form of polygamy in which one man has multiple wives.
Indeed, he said monogamy is a relatively recent phenomenon for humans, tracing its history back to ancient Greece and Rome, which in turn influenced Christianity and eventually spread into Europe.
The rise of monogamy led to a different kind of evolution: cultural evolution. Societies that became monogamous, he said, became more advanced and prospered, while those that remained polygamous did not.
"It spreads because society benefits," he said. "It maintains internal harmony, it reduces crime, it increases solidarity."
Prof. Henrich named a long list of social problems he said are associated with polygamy.
When men have multiple wives, they require younger women to meet the demand. Prof. Henrich said that leads to teenage brides, with young girls marrying much older men.
That also creates a pool of men, usually of lower economic and social status, with no one left to marry. Those men are more likely to commit crimes, including rape and murder, and abuse drugs and alcohol, Prof. Henrich said.
Because of the competition for wives, he said, men are more likely to exert control over women, leading to increases in domestic violence and abuse.
And children suffer in polygamous societies. Prof. Henrich said men are less likely to invest time and resources in child rearing because they father so many children and are constantly focused on finding new wives. That strain on resources also hinders the greater society's economic performance.
Prof. Henrich said studies of polygamous societies around the world bear those theories out, and the problems even affect monogamous people within those societies.
He also said because of the genetic predisposition he noted earlier, it's "plausible" that legalizing polygamy in Canada would lead to an increase in the number of people who take up the practice - eventually.
"One of the problems with our thinking is we tend to think, 'This couldn't happen tomorrow, it couldn't happen next week,"' Prof. Henrich said.
"But [if]you're going to look ahead 50 years, it doesn't seem implausible that we could lose ground on gender equality" if polygamy were legalized.
The constitutional reference case was prompted by the controversy over Bountiful, B.C., a polygamous commune in the southeastern part of the province. Two leaders in Bountiful were charged last year with practising polygamy, but those charges were later thrown out on technical legal grounds.
The residents of Bountiful follow the teachings of the Fundamentalist Church of Jesus Christ of Latter Day Saints, or FLDS, a fundamentalist Mormon sect that believes in polygamy. The mainstream Mormon church renounced polygamy more than a century ago.
While Prof. Henrich's testimony didn't focus on Bountiful in particular, he cited the work of an anthropologist who is studying FLDS communities in Arizona and Utah. That research found the shortage of eligible women in those communities leads to intergenerational conflict, including the expulsion of boys who can't find wives, as well as forced marriages and the need for young women to marry much older men.
"I was amazed, you could read this same kind of thing in polygamous societies in Africa, the same dynamics are taking place," said Prof. Henrich.
"It gives you a sense that the same social dynamics that play out in other societies also seem to work in North America."