Men are more likely to want a committed relationship when women are scarce, anthropologists have found.
In a study of the Makushi people in Guyana, researchers from the University of Utah found there is a strong supply and demand principle in place when it comes to monogamy versus short-term flings.
Ryan Schacht and his wife Jacque spent 16 months living with the Makushi people in Guyana.
Around 13,000 Makushi people live near the southwest border of Guyana with Brazil. They hunt, fish and farm. Premarital sex is expected and men normally marry monogamously. Families normally live in villages of between 160 to 750 people, but migration has led to differing male to female ratios.
Over recent years, women have been moving towards urban areas where there are jobs for them, while men have been going towards mining, farming and logging outposts.
The couple interviewed men and women over the course of their stay to find out sensitive information about their intimate relationships.
Schacht said: "Commitment to a relationship is influenced by the availability of partners. So we can think of the number of men and women in a population as a potential mating market where the principles of supply and demand hold sway.
"When you are a member of the sex that is abundant, you must cater to the preferences of the rare sex. So the expectations are that men will be interested in short-term relationships when more women are available. But when women are difficult to find, they become valued resources, so men will attempt to attract and maintain a single partner because it is costly to lose a partner when partners are rare."
Published in the journal Royal Society Open Science, the authors wrote: "In short, men appear to increase their willingness to engage in uncommitted sex in response to a surfeit of women, and to become less keen to engage in uncommitted sex when men are abundant."
However, the study also found that women are indifferent in changes to the sex ratio, preferring commitment in whatever scenario they find themselves.
This, Schacht says, is likely a result of the judgment and gossip they face from other members of society, much like can be seen among western societies. "Women for example don't get the same pay offs from having short-term relationships as men," he told IBTimes UK.
"Women will face more gossip and social sanction than men – it is very similar to the West – from having multiple partners."
Women also appeared to have stronger social bonds with other women. In the Makushi society, women are largely responsible for the food production, and women work together and help one another.
"In some ways women compensate for the lack of male investment by relying on female kinship networks. But there would be consequences for a woman's reputation if she were to take multiple partners. If women in the community are competing with one another ... [this] might cause conflict and break down these very important social networks."
Schacht said that what their study tells us is that sex is not necessarily the driving factor in relationships. "It's not that sex doesn't matter, it's just that it is more complicated. It's something we can examine. Sex matters, but commitment to a relationship or relationship preferences are also going to be patterned by partner availability.
"We can take a very simple principle of supply and demand and find where sex can be more demanding as a consequence of the rarity. Sex should be responsive to the demands of the rarer sex.
"And if we're of a situation where there is an abundance of men and women are in demand, then men should realise that short-term uncommitted strategies are those that will essentially leave them without a partner, because women have many options and will leave partners that are unwilling to respond to their demands."