In public discourse we are constantly being bombarded with lectures about the safe way to behave. We are reminded that abstinence is the only surefire way to avoid pregnancy/STDs, that recreational drugs could have unknown harmful effects, and that no level of smoking is safe. Unlike most of the other messages we are exposed to these seem to sneak in under our defenses. Even people with liberal sexual attitudes do not usually object to the claim that abstinence is the only surefire way to avoid pregnancy or advise a teen that it’s absolutely impossible to get pregnant in some fashion. In effect these sorts of warnings carry a stealth moral payload that bypasses our standard defenses by exploiting the ambiguity between the literal claim that something is risk free and the conversational practice of rounding sufficiently small probabilities to zero. Unfortunately this effect frequently allows puritanical, luddite or other narrow minded groups to exert inappropriate influence over society, particularly the way we educate our children.

This is best illustrated with an example. Unsurprisingly many teenagers are paranoid about pregnancy and wonder if they could get pregnant as a result of dry humping while wearing underwear. The answers tend to look like this:

Look, it’s like this: sperm can go through anything that has holes in it. Your clothes, though they may look solid, are not. Especially if they get wet with “juices.” The ONE thing sperm can’t go through is a condom. Just USE one. It’s not that hard. You can wear one even if you don’t have intercourse, and are just dry humping.

Even when the event is absurdly unlikely people aren’t willing to tell teens that they can’t get pregnant this way. On the other hand when people ask if they could get HIV by shaking hands or through other casual contact they get answers like this:

Remember that HIV cannot be transmitted through casual contact. A person cannot become infected with HIV from a handshake, a sneeze, a hug, or from sharing cups and dishes, tools, telephones, computer keyboards, bathroom facilities, or drinking fountains. You cannot simply “catch” HIV from air, food, water, insects, or animals.

My girlfriend is working as a bartender and she cut her self by cutting fruit. Then she shaked[sic] hand with some guy has AIDS and he had sweat[sic] hands. [Could she have gotten infected?]

Nope! Even if sweaty hands poz-guy had also picked his nose, peed on his fingers and upchucked a Big Mac all over his hands before the handshake, your girlfriend’s HIV risk remains completely nonexistent. (Tears, sweat, saliva, nasal mucous, urine, vomitus and feces do not transmit HIV, unless there is visible blood.)

But similar sorts of scenarios can be imagined for the transmission of an HIV virus as for a sperm. If we are going to worry over the limited number of sperm in a guy’s precum worming their way through the cotton mesh of a girl’s panties and sneaking all the way up her vagina we should certainly be worrying about unnoticed cuts or rogue HIV viruses that managed to sneak into mucus. Of course what we really should be doing is rounding down the risks in both cases to zero, as we usually do with sufficiently low probabilities. Interestingly if you start asking question about whether you can get HIV from certain kinds of sexual contact all the sudden the answers start to resemble those given about pregnancy suggesting that it is the cultural attitude to the ‘risky’ activity that is biasing the answers.

If this phenomena only occurred when talking about sex it would be troubling but of limited harm. Unfortunately we can see the same effect when talking about the risks of drug use, cigarette smoking, airline safety and posting personal information online. For instance compare the frequent claims that there is no safe level of secondhand smoke to the way we treat the safe level of mercury in fish or other types of non morally suspect exposure to potentially harmful chemicals. Or the way the potential of unknown harms from marijuana are treated differently than that of prescription medication. Making a broad oversimplification there seems to be a bias toward emphasizing the risks when the benefits of the activity are only pleasure or convenience while this bias is absent when it has more puritanically acceptable benefits.

Obviously this practice has the harmful consequences of creating unnecessary worry and helping demonize many perfectly reasonable behaviors as well as distorting the implicit cost/benefit analysis we make about policies like taking large tubes of toothpaste on airlines. Just because some people think teens shouldn’t be experimenting with sex doesn’t mean the rest of us should help keep teens unnecessarily worried about pregnancy or STDs. Even worse this practice impairs our ability to warn about real risks. When we won’t tell teens that dry humping wearing underwear is a surefire way to prevent pregnancy should it really be that surprising when they don’t heed warnings about pulling out being an ineffective method of birth control? Ultimately we would be much better to dispense with this bias and base our decisions on an unbiased estimation of probability and explicit value judgments about the underlying activities but what can we do to prevent this effect?

The best and easiest thing we can do is to be aware of this effect and refuse to fall victim to it ourselves. When someone asks if some activity could get them pregnant or if miniscule amounts of secondhand smoke are dangerous don’t just let yourself be pulled along by the ‘yes it’s possible’ crowd. Instead, explicitly compare the risk to familiar risks people routinely round to zero. Don’t say, “yes, it is possible but very unlikely you could get pregnant that way,” say, “yes it’s possible but you’re more likely to die in a car crash on your way over.” If you’re a bit more ambitious you might even want to point out when blogs and publications fall victim to this subtle, but dangerous bias. It’s probably unrealistic to think that we could ever eliminate such a deep seated bias from the larger culture. However, by making more people aware of the problem, particularly doctors, we can try to minimize the harms it causes.