We have all seen the bumper stickers insisting that marriage "is between one man and one woman," but throughout most of human history, that hasn't really been the case.
Anthropologists say 83 percent of societies they have studied traditionally permitted polygyny — marriage with multiple wives. (The more common term "polygamy" has the broader definition of having multiple spouses.) Just 17 percent insisted on monogamous marriage.
So how did social monogamy, which has spread in the past few centuries thanks to the influence of Euro-American culture, come to be?
Most evolutionary biologists think monogamy originated as a form of social leveling that reduced male competition for mates, fostered cooperation and led to the rise of successful nation-states.
But Italian anthropologist Laura Fortunato sees a deeper logic. She believes monogamy arose from differing reproductive strategies among men and women in the face of changing modes of subsistence.
"In anthropology people hardly ever felt the need to explain monogamous marriage," observes Fortunato, an Omidyar Fellow at the Santa Fe Institute in New Mexico. Monogamy was simply taken as a given by early European and American researchers who went off to study societies different from their own.
The usual evolutionary explanation is suspect, because social monogamy actually "long predates the establishment of large nation states," Fortunato wrote in a 2009 paper she co-authored with biologist Marco Archetti and published in the Journal of Evolutionary Biology.
Evolutionary accounts of marriage tend to focus simply on male reproductive success (success being measured by whether an organism passes on its genes to its offspring).
In mammals — including humans — the ease with which males produce sperm cells suggests they will put their effort into frequent mating as a reproductive strategy, while females, with a finite number of eggs and the need for a long period of internal gestation (followed by lactation), tend to put more effort into parenting.
According to this simple math, it makes sense for a man to take multiple wives, and it probably explains why polygyny was so widespread throughout human prehistory, Fortunato says. But only prosperous men take multiple wives in such societies, while their less wealthy brethren content themselves with just one mate — creating de facto monogamy, she says.
Polygyny is common among people who practice horticulture, clearing small plots of land to raise a variety of food crops, Fortunato says. Even today it is widespread in Africa, where horticulture is common.
But fathering many children (hoping some of them will survive to adulthood) is not the only possible male reproductive strategy, Fortunato writes. Transmitting wealth to the next generation is a way of improving the odds of a child's survival — and of one's reproductive success. Here, monogamy makes sense because it "channels a man's property to the offspring of a single wife," rather than having it dissipated among the children of multiple wives.
Fortunato argues from the historical record that monogamy first arose in Eurasian societies just as true agriculture was taking hold — about 12,000 years ago. Agriculture involves intensive cultivation of large tracts of land, often requiring ploughing, irrigation, fertilization and other soil improvements.
As agriculture spread, arable land became scarcer — and valuable. Land ownership became critical to reproductive success, driving a new form of marriage in which males were assured of investing resources in their heirs, thus improving the odds that their genes would be successfully passed on.
But in Fortunato's telling, that isn't the end of the story. She challenges the notion that women are just along for the ride in these varying versions of male reproductive strategy. "Women are not these passive players," Fortunato says. "They have some kind of say in what their husbands are doing."
"We've basically extended this model to say that if males allocate resources strategically, then females might allocate paternity strategically," Fortunato says. "This wasn't included in the previous model."
A couple arrives at a shared (if unspoken) understanding: If the man commits to passing on his resources to her children, the woman will commit to mating monogamously — meaning the children are likely his offspring. Fortunato sees in this contract the roots of elaborate social norms regarding female sexual behavior.
Fortunato cites historical examples in support of her argument. In ancient Rome, where monogamous marriage was the norm, men regularly had children with women other than their wives, she says.
"Why did these societies have a norm promoting monogamy when they in fact condoned polygamous mating? The explanation which we give is that marriage strategies have more to do with how property is transferred across generations than they have to do with mating and the production of children."
It all sounds depressingly unromantic. Fortunato points out that in the history of marriage, "[t]his association between love and marriage in Western societies is a very recent thing." Early marriage contracts "really did sound like a contract," focusing on an exchange of goods and obligations.
Some of Fortunato's other research focuses on which societies include doweries and bridewealth as part of the marriage ritual.
She has, for example, created phylogenetic trees comparing societies speaking Indo-European languages to show how cultural practices have been conserved as social groups branched off from one another in their descent from a common ancestor.
Assuming that cultural traits usually descend vertically — through time — instead of being borrowed from neighboring groups, Fortunato examines which practices these cultural cousins share in common and how they differ.
Dowry, in which wealth is transferred to a bride or the newlywed couple, was found exclusively in European and Asian societies, Fortunato notes. "You see that dowries tend to occur in societies that are monogamous," she says. "One of the explanations is that dowry is a way that allows women to compete for access to men where polygyny is not allowed."
It sounds a little like the plot of a Jane Austen novel: Men are, in effect, the scarcer resource, says Fortunato. "It makes wealthy men very attractive."
She acknowledges that much of this work is speculative, and the answers often depend on the categories used to frame the questions. "I see this as a first step. This tries to explain what we see. It is a more crude look at what is going on. We're trying to paint a broader picture."