SEX

You’ve read elsewhere about the sin of promiscuity. Let me tell you about the sin of self-restraint. Consider Martin, a generally prudent young man with a limited sexual history, who has been gently flirting with his coworker Joan. As last week’s office party approached, Joan and Martin silently and separately entertained the prospect that they just might be going home together. Unfortunately, Fate took a hand. On the morning of the party Martin happened to notice one of those ads touting the virtues of abstinence. Chastened, he decided to stay at home. In Martin’s absence, Joan hooked up with the equally charming but considerably less prudent Maxwell – and Joan got Aids.

When the cautious Martin withdraws from the mating game, he makes it easier for the reckless Maxwell to prey on the hapless Joan. If those ads are more effective against Martin than against Maxwell, then they are a threat to Joan’s safety.

If the Martins of the world would loosen up a little, we could slow the spread of Aids. When sexual conservatives increase their activity by moderate amounts, they do the rest of us a lot of good. Michael Kremer, a Harvard professor, estimates that the spread of Aids in England could be halted if everyone with fewer than 2.25 partners a year were to take additional partners more frequently. That would apply to three quarters of all British heterosexuals between 18 and 45.

A cautious guy like Martin does the world a favour every time he hits the bars. In fact, he does the world two favours. First, he improves the odds for everyone who’s out there seeking a safe match. The second favour is more macabre, but probably also more significant: if Martin picks up a new partner tonight, he just might pick up an infection as well. That’s great. Because then Martin goes home, wastes away in solitude, and eventually dies, taking the virus with him. If someone has to get infected tonight, I want it to be Martin rather than Promiscuous Pete, who would probably infect another 20 people before dying.

When you take a new sex partner you bear some costs and you reap some benefits. Those are your business. You also impose costs and benefits on others, and those are everyone else’s business. If you have a history of reckless promiscuity, that’s a cost. Everyone’s fishing for partners in a great communal stream and you’ve polluted that stream just by entering it. But if you’ve always been cautious and selective, you’re likely to raisethe average quality of the partner pool. Just by jumping into the stream, you make it purer.

Like any other communal stream, the stream of partners has too many polluters and too few volunteers to clean it up. If all you want to do is slow down the epidemic, Professor Kremer’s research says that more sex is a good thing. But if you want to maxi-mise the excess of benefits over costs, then even more sex is an even better thing.

So how do we encourage Martin (and others like him) to have more sex? We could pay people to have more sex with more partners. But that’s not ideal because we don’t want everyone to have more sex with more partners. Maxwell, for example, is oversexed enough as it is. The problem is to subsidise Martin’s sexual awakening without simultaneously subsidising Maxwell’s excess. We need a reward that’s of no value to Martin unless he actually has sex. And it should be something that the cautious Martin values more than the promiscuous Maxwell does.

I can think of only one reward that fits both criteria: free (or heavily subsidised) condoms. To reap the benefits of a free condom, Martin has to have sex. And Martin probably values a free condom much more than Maxwell does. Here’s why: Martin’s almost surely not infected yet, so a condom has a good chance of saving his life. Maxwell, by contrast, knows he might have the virus already, so a condom at this point is less likely to make a difference.

Subsidised condoms could be just the ticket for luring Martin out of his shell without stirring Maxwell to a new frenzy of activity. It’s often argued that subsidised (or free) condoms have an upside (they reduce the risk from a given encounter) and a down-side (they encourage more encounters). But that’s not an upside and a downside: it’s two upsides. Without the subsidies, people don’t use enough condoms, and without the subsidies, the sort of people who most value condoms don’t have enough sex partners. The main drawback to subsidising condoms is that they’re inexpensive to begin with: you can reduce their price to zero without having much impact on people’s sexual choices.

Our goal, then, should be to drive the price of condoms below zero, by rewarding people who use them. In other words, we should pay a bounty for used condoms. The best such bounty would be one that is more valuable to abstemious Martins than to promiscuous Maxwells.

With that in mind, the journalist Oliver Morton has suggested that if at least some abstemiousness is due to shyness and an inability to find partners (while the promiscuous have relatively little trouble in this regard), the answer might be to establish a government-funded dating service: bring us a used condom, and we’ll get you a date. The entire problem, along with the entire case for subsidies, would vanish if our sexual pasts could somehow be made visible so that future partners could reward past prudence and so provide appropriate incentives. Perhaps technology can ultimately make that solution feasible. (I can imagine the pornography of the future: “Her skirt slid to the floor and his gaze came to rest on her thigh, where the embedded monitor read ‘This site has been accessed 314 times’.”) Or, as someone once suggested to me, we could have an online service to record negative HIV test results. You’d type in your prospective partner’s name and get a response such as “Last negative test result 7/4/2006.” Or, to protect privacy, you’d type in an ID number provided by the partner. Your screen could show both a test result and a photo to avoid fake IDs. This strikes me as such a good idea that I can’t figure out why nobody’s doing it yet. Until then, the best we can probably do is to make condoms inexpensive – and get rid of those ads.

DAUGHTERS

If you want to stay married, three of the most ominous words that you will ever hear are: “It’s a girl.” All over the world, boys hold marriages together and girls break them up.

An American with one daughter is nearly 5 per cent more likely to get divorced than an American with one son. The more daughters, the bigger the effect: the parents of three girls are almost 10 per cent more likely to get divorced than the parents of three boys. In Mexico and Colombia the gap is wider; in Kenya it’s wider still. In Vietnam it’s huge: parents of a girl are 25 per cent more likely to get divorced than parents of a boy.

Two economists, Gordon Dahl and Enrico Moretti, gathered these numbers from more than three million census observations. Correlation, of course, need not imply causation. But in this case it probably does. Here’s why: Take three million people, invite them to toss coins and divide them into two groups according to how their coins land. Then the two groups will look statistically identical – same average income, same average intelligence, same average height. That’s called the law of large numbers, and it works for two reasons – first, the sample size is huge, and second, coin-tossing is random.

Now do the same thing, dividing your three million people according to the gender of their last-born child. The same thing happens – parents of boys are statistically identical to parents of girls because you’ve still got a huge sample size and because the sex of a child is as random as a coin toss. The two groups will have the same average financial stresses, the same average emotional distance, and the same average incidence of infidelity.

All that’s left to explain the difference in divorce rates is the gender of the children. Children do affect divorce decisions. And to a small but not negligible extent, it appears that girls cause more divorces than boys.

The next question is why. Children of divorce usually stay with the mother, so the question is: why do fathers stay in a marriage for sons when they won’t do so for daughters? (Or, alternatively, why do mothers stay married so that their sons can have a father when they won’t do so for their daughters?) Do fathers prefer the company of sons? Do parents think a boy needs a male role model? Do they worry that boys cope less successfully with the emotional consequences of divorce? Or do they believe that an emotionally devastated daughter is somehow less of a tragedy than an emotionally devastated son?

Dahl and Moretti believe that boys hold marriages together because parents prefer boys. This raises the question: is it true that parents prefer boys?

Here is Dahl and Moretti’s first bit of evidence: divorced women with girls are substantially less likely to remarry than divorced women with boys. Not only do daughters lower the probability of remarriage; they also lower the probability that a second marriage, if it does occur, will succeed. Apparently, daughters are a liability in the remarriage market, which suggests that potential husbands prefer male stepchildren.

Or maybe not. Maybe it just tells us that mothers prefer not to expose their daughters to a potentially predatory stepfather. So though the remarriage statistics are suggestive, I’m not at all sure what they’re telling us.

But there’s more striking evidence, based on shotgun marriages. Take a typical unmarried couple expecting a child. Suppose they have an ultrasound scan, which more often than not reveals the child’s sex. It turns out that those couples are more likely to get married if the child is a boy. Apparently, for unmarried fathers, the prospect of living with a wife and a son is more alluring than the prospect of living with a wife and a daughter.

Finally, Dahl and Moretti observe that parents of girls are significantly more likely to try for another child than parents of boys, which suggests that there are more parents hoping for sons than for daughters.

Once again, the effect is strong in the US, but even stronger elsewhere. In the US, Colombia and Kenya, a couple with three girls is about 4 per cent more likely to try for another child than a couple with three boys; in Mexico it is closer to 9 per cent, and in Vietnam it is 18 per cent. In China, before the national one-child policy was imposed in 1982, the number was an astounding 90 per cent.

© Steven E. Landsburg 2007 From More Sex is Safer Sex: The Unconventional Wisdom of Economics (Simon & Schuster; £17.99), available from Times BooksFirst for £16.19, including p&p; 0870 1608080.