Rational decision theory predicts that decreasing the riskiness of a activity will tend to increase the prevalence and degree of risky activity. As paragliding is made safer, one expects more people to be engaged in paragliding and those who are engaged in it to do it more intensely. But of course increasing the prevalence and degree of behavior will tend to increase the prevalence of occurrences of the negative outcome that one was decreasing. This is the phenomenon of risk compensation: decreasing the risk of a negative outcome of an activity is to some degree—maybe sometimes completely—compensated for by an increase in the prevalence and degree of engagement in the risky activity. For instance, taxi drivers who have antilock brakes tend to follow the vehicle in front of them more closely.
Suppose that in some case the risk compensation to some safety measure is complete: i.e., the prevalence of the relevant negative outcome (say, crashes or fatalities) is unchanged, due to the compensating increase in the prevalence and degree of the risky behavior. One might think that at this point the safety measure was pointless.
Whether this conclusion is correct depends, however, on what values the risky behavior itself has when one brackets the risk in question. If the risky behavior has positive value when one brackets the risk, the safety measure does in fact achieve something good: an increase in prevalence and degree of valuable but risky behavior with no increase in negative outcome. Paragliding is (I assume) a pleasant way to enjoy the beauty of the earth and to stretch the limits of human ability. An increase in the prevalence of paragliding without an increase of negative outcomes is all to the good.
When the behavior is completely neutral, then the safety measure, however, is simply a waste given complete risk compensation.
Finally, if the risky behavior has negative value even when one brackets the particular risky outcome, then in the case where the risk compensation is complete, the safety measure is counterproductive. It does not decrease the negative outcome it is aimed at, but by increasing the prevalence of an otherwise unfortunate activity it on balance has a negative outcome. For instance, suppose bullfighting is an instance of immoral cruelty to animals. Then apart from the risks to the bullfighter, the activity has negative value: it harms the animal and damages the soul of the person. If a safety measure for prevention of goring then were compensated for by an increase in prevalence, the safety measure would have on balance a negative outcome: there would be no decrease in gorings but there would be an increase in immoral and harmful activity.
Moreover, in cases where the risky behavior has independently negative value, a safety measure can have on balance negative effect even when the risk compensation is quite modest. Suppose that (I am making up the numbers) gorings occur in 10% of bullfights and cruelty to bulls in 80%. Suppose, further, that cruelty to bulls is at least as bad as goring (since it not only harms the bull but more importantly it seriously damages the soul of the cruel person). Then a safety measure that decreases the probability of goring by a half but results in a modest 10% increase in the prevalence of bullfighting will have on balance negative effect. For suppose that previously there were 1000 bullfights, and hence 100 gorings and 800 instances of cruelty. Now there will be 1100 bullfights, and hence (at the new rate) 55 gorings and 880 instances of cruelty. We have prevented 45 gorings at the cost of 80 instances of cruelty, and that is not worth it.
Much of the public discussion of risk compensation and safety measures centers on sex, and particularly premarital sex. We should typically expect some behavioral change in the direction of risk compensation given a safety measure. If one thinks premarital sex to be itself typically valuable, then even given total risk compensation one will think the safety measure to be worthwhile. If one thinks premarital sex to be value-neutral, then as long as the risk compensation is incomplete (i.e., the decrease in the risks due to the safety measure is not balanced by the increase in prevalence), one will think the safety measure to be worthwhile (at least as long as the costs of the safety measure are not disproportionate). But if one thinks premarital sex to have negative moral value, then one may well think a safety measure to be counterproductive even if the risk compensation is incomplete—as in my imaginary bullfighting cases.
I think public discussion of things like condoms and sex education could be significantly improved if participants in the discussion were all open and clear about the fact that we should expect some degree of risk compensation—that's just decision theory[note 1]—and were mutually clear on what value they ascribe to the sexual activity itself, independently of the risks in question.
In these kinds of cases, it sounds very attractive to say: "Let's focus on what we all agree on. Being gored, getting AIDS and teen pregnancy are worth preventing." But a public policy focused successful at improving the outcomes we have consensus on can still be on balance harmful, as my (made up) bullfighting example shows.