What do the numbers say? A cost-benefit analysis of love and sex

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How's your marriage portfolio doing?

In her book Dollars And Sex: How Economics Influences Sex and Love, University of British Columbia economics professor Marina Adshade suggests that almost every option, decision and outcome in love and sex is better understood when you look at matters through an economic lens.

Adshade reveals how economics trickle down to our most intimate moments, affecting everything from teen pregnancy rates to child care. While people don't consciously do the "promiscuity math" every time they fall into bed with someone, she reveals how economic factors shift consequences in our lives, and how this shapes the way men and women behave. Adshade spoke to The Globe and Mail from Vancouver.

Is it politically incorrect to view sex and love via economics – unless you're talking about straight-up prostitution, where value is easily measured? It makes everything people do seem opportunistic.

Somebody said to me the other day that this is a very cold perspective. The value of the economic approach is to offer some clarity on the decisions that we ourselves make. These are decisions that we can measure: how people match on income or education or political beliefs and how long those marriages last and how happy those people are.

Let's turn to some of your counterintuitive findings: Birth control has actually increased unintended pregnancy rates outside of marriage. How?

The rate planet-wide is surprisingly high, given how much technology we have to control our own fertility. The unintended consequence of access to contraceptives is that you get more unintended pregnancies because social norms have evolved in a way that allow people to more freely express their sexuality. When more people have sex outside of marriage, you're bound to get more pregnancy and child birth outside of marriage.

A study of 48 nations found that Finland was the most promiscuous and Taiwan the least. Finland enjoys a robust national income and high gender equality. How has that encouraged promiscuity?

The relationship between national income, gender equality and promiscuity has to do with what makes developed countries developed: We live in a country that values equality and personal freedom. Promiscuity is just a side effect of the social norm that values personal freedoms – to do with bodies as people wish, without punishment.

The idea that modern marriage is mostly an opportunity to consume more, is that truly the allure for people?

For a long time when we talked about marriage, it was economist Gary Becker's story about people getting married so they could specialize, and by that Becker meant men who were better earners could go out and work and women who earned less could stay home. This is clearly not our world any more, and it wasn't even our world when Becker wrote A Treatise on the Family in the 1990s. Now, people marry more because you have so many opportunities to share consumption when you're married. For me to travel to Italy in the middle of summer on my own is significantly more expensive than to do that as part of a shared couple: hotel rooms, renting cars. It's true for every area of life: housing and even buying food in the home – it's less expensive per person when there are more of you. The economic story behind marriage has changed from Becker's idea that marriage is like a mini-firm where the optimal way to run the firm is with a man and a woman. Now marriage is more about consuming, and valuing that consumption.

How is marriage like bargaining?

If you're married, you're constantly negotiating. In economics, we have a formal way of thinking about the distribution of bargaining power: When you have a disagreement over a decision, what's the probability that you will get your own way? That's really been redistributed over time. Women have more outside options because we can earn in the marketplace, we're much better educated now than many men are and women are confident living alone and raising children alone. That gives us a lot more bargaining power and say in our families. Some people think this is a bad thing for men, but it cannot have been fun to be the one person in the house making all the decisions. In the past, there was a lot of burden on men to do that. The shared decision-making should be making marriages better for men and for women.

Why is marriage now mostly something richer people do?

Less-educated women are marrying sooner, but they have very high divorce rates and they're much less likely to remarry. A women with a Grade 12 education, if she marries early, she's more likely to end up single before she turns 30 and she's likely to stay single. If you go back to the 1970s, a man who had a high-school education would have been able to earn enough money to support a family. Now, we have a drop in wages for people on the low end of education distribution and it's much harder for those people to support a family. People aren't marrying when they can't afford a home or to have children.

What are the downsides when more-educated women start marrying less-educated men?

I know couples where the wife works and has a very high-paying job and the husband stays home. The wives make a lot of the important decisions. This one couple I know, he needed a new car and she decided what he was getting. He likes staying home and taking care of the children; it makes him happy. Men who value these things over sinking themselves into careers, these men will select themselves into relationships with women who have higher incomes. They're making that choice. I think marriage is moving to a much, much better place.

You've described hookup culture on college campuses as a "buyer's market," where men outnumber women and so traditional dates go out the window. That makes it sound like these women have zero agency.

Women are much freer to go out and have the type of relationships they want when they're young without worrying later on that it will affect their future marriage prospects. There are women who want to have casual relationships, but we have this idea that women have to be compensated for sex because they really don't like it – they're always the suppliers and men are always the demanders. It despairs me that my [male] students think this, still.

On the other side, we have this idea that older women are just clamouring to find a man and all the older guys are playing the field. A lot of older women don't necessarily want to have the responsibility of a long-term committed relationship and they're also willing to enter into more casual relationships with men.

What about the economics of handing birth control to men?

From an economic perspective, when you introduce new technologies like the male birth control pill, that shifts the bargaining power in a couple away from the female. It reduces a woman's bargaining power to say to the guy, "I really want you to use a condom." Up until now, the protection has been taken by the person who pays the biggest cost in the case of a pregnancy. I'm not sure that women will relinquish that control.

What economic factor is most likely to shift social norms in our current love story?

The important trend to watch is the educational divide between men and women. Here in Canada, the 2010 Census records suggest that for women between the ages of 24 and 45, for every 100 men who have a university degree there are 125 university-educated women. This has far-reaching impact. People are changing their expectations of marriage partners' incomes. Those women who don't want to change their expectations are staying single, while other women are marrying younger men if they can't find someone with the educational level they want in their own age pool.

Today, there is much evidence that men who marry live longer and have happier, healthier lives. In the past, women went to university to try to find husbands – they went to get their "MRS." It's possible that men will start to going to university because it opens opportunities for better relationships.

This interview has been condensed and edited.