Dear Economist,

My boyfriend and I have always practised safe sex, but now we’re talking about using just the pill rather than condoms. What concerns me is the risk of catching something. I expect my boyfriend slept with other girls before we started dating, but I feel fairly sure that he wouldn’t have done anything risky.

Am I right?

Cecilia Larson, Bristol

P.S. My boyfriend is an economist.

Dear Cecilia,

Oh dear. It was all looking so promising. Unprotected sex produces a classic negative externality. Someone who decides to have unsafe sex gets to enjoy all the pleasure but only part of the risk: if he contracts an infection, he will suffer from it himself but also risk passing it on to his future partners, and their partners’ partners. The only reason you have been using condoms at all is that you know other people haven’t bothered.

Your boyfriend knows this perfectly well. He may also know that some sexually transmitted diseases, such as chlamydia, have more serious effects in women than in men. Unsafe sex has benefits as well as risks; as an economist, he may well have decided that the personal risks are worth running.

Do not lose hope, though.

As a rational being, your boyfriend will have avoided the most unsafe practices, such as sharing needles and having unprotected intercourse with sex professionals. So your main risk is that he has had unprotected sex with a large number of ordinary women like you. But how likely is this? Such delights are likely to lie well outside his feasible consumption set: there is not usually a queue to jump into bed with economists.

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