Risky diagnoses

When you're cautious in taking sexual risks, you help both yourself and your partners. The former effect can be purely selfish optimisation. The latter could be due to other-regarding preferences in relationships where you care about the other person, or just a positive externality.

Why does this matter? Consider what happens if more HIV testing is funded. If you're behind the veil and don't know about your status or your partner's, prudence dictates some caution. If you're tested, you know your status but don't necessarily know your partner's. If you're tested and wind up being negative, then the returns to prudence are higher, as you know with certainty that you aren't already infected so you can make things worse, but you might also think that the risks you've taken so far are safer than you'd thought. And how much weight people put on the risk they impose on partners is hard to tell: if you find out you're positive, you either reduce caution if partners' utility doesn't weigh heavily, or increase caution if others' utility counts.

And so what people do on getting a test result is an empirical question.

Enter Erick Gong. He finds that ..., well, scratch that. I'll just quote from his introduction as it's rare to see this kind of clarity in academic writing. Bottom line: if you're going to fund free testing, couple it with funding for anti-retro virals so that when people find out they're positive, they do less harm to others.

I use data from the Voluntary Counselling and Testing (VCT) Efficacy study conducted in Kenya and Tanzania, which randomly assigned people into HIV testing and followed up with them six months later (The Voluntary HIV-1 Counselling and Testing Efficacy Study Group,2000). I construct a measure of people's beliefs about their HIV status before getting tested using questions on the baseline survey. To measure risky sexual behaviour, I use biological markers that are not susceptible to self-reporting bias. Data are collected on newly contracted infections of gonorrhoea and chlamydia (henceforward known as ‘sexually transmitted infection’ or ‘STI’) that occur during the study.5 An STI only results from unprotected sex with someone who has an STI and serves as an objective measure of risky sexual behaviour. The random assignment of testing enables me to identify the effect that HIV tests have on sexual behaviour conditioned on prior beliefs of HIV infection.

My findings suggest that HIV tests have the largest effects on risky sexual behaviour when test results provide unexpected information to an individual. I find that people surprised by an HIV-positive test (i.e. those who believed they were at low risk for HIV before testing and learn they are HIV-positive) have a 10.5 percentage point increase in their likelihood of contracting an STI compared to an HIV-positive control group who had similar beliefs of HIV risk but were untested at baseline.6 I interpret this increase in contracting an STI as an indication that those surprised by an HIV-positive test increased their risky sexual behaviour – an unintended consequence of testing. I estimate that these types on average increased their number of new partners by about 2.4 over a six-month time frame. People surprised by an HIV-negative test (i.e. those who believed they were at high risk for HIV before testing and learn they are HIV-negative) have a 5 percentage point decrease in the likelihood of contracting an STI compared to an HIV-negative control group with similar beliefs of HIV risk but were untested at baseline.7 This decrease in the likelihood of contracting an STI suggests that those surprised by HIV-negative tests decrease their risky sexual behaviour. Both of these results indicate that when people make decisions about risky sexual behaviour, self-interests dominate altruistic preferences. People who discover they are HIV-positive no longer have any incentive to practice safe sex (i.e. ‘nothing to lose’), while those who learn they are HIV-negative face greater incentives to avoid risky behaviour. Finally, when HIV test results agree with a person's beliefs of HIV status, the effects of testing on STI likelihood are not statistically different from zero. This is consistent with an economic model where the behavioural responses to HIV tests are greatest when they provide unexpected information.

I use the empirical results described above and combine them with a simple epidemiological model to simulate the short-run effect of rolling out HIV testing in an urban setting. While this exercise inherently requires a set of strong assumptions, and hence the results should be interpreted with caution, it does address an important policy question. I use the distribution of beliefs of HIV risk and actual HIV status from the Demographic Health Surveys in Kenya, Mozambique and Zambia – all three countries faced with a generalised HIV epidemic. I find that in the cases of Kenya and Zambia, testing leads to declines in new infections, while testing leads to an increase in infections in Mozambique. However, when ARVs are provided at an earlier stage in the infection, testing leads to large reductions in HIV infections in all three countries. Since ARVs greatly reduce the infectivity of HIV-positive individuals, the aggressive provision of ARVs can mitigate the risks posed by HIV-positive individuals who increase their risky sexual behaviour after testing.8

The other particularly interesting bit: surprise negatives yield reductions in risk-taking.

I'd missed this when it came out in 2015. I thank Ole Rogeberg for the pointer.

Peltzman at the Ring?

The International Boxing Association last year banned the use of headgear by amateur boxers starting on the first of June, 2013. I'd missed it then but caught it via @SirWB. Why'd they do it? Peltzman effects, or at least perhaps:

Amateur boxers are to be banned from wearing headgear in a bid to reduce the number of head injuries.

While the move sounds counterintuitive, the theory is that opponents don't apply so much force if the head is unprotected.  

The new rules, from the International Boxing Association (AIBA), state that from June 1st, amateur, elite male boxers who compete internationally will be banned from wearing headgear, like their professional counterparts. Another reason for the move is that headgear can obscure peripheral vision, making it harder to see when a blow is being aimed at the side of the head. Indeed, research has shown that a lack of headgear actually reduces the risk of concussion.

While drivers take more risks around helmeted cyclists, drivers aren't really trying to knock out cyclists. If you're minimising expected harm to others subject to your own cost constraints, optimum abatement goes down with the other party's mitigating efforts because expected harm drops.

But I can imagine scenarios where optimal punch force either increases or decreases in the presence of helmets. If you need to apply more force to get any effect in the presence of helmets, then optimal force would be lower without helmets if exerting force is costly in terms of exhaustion. But it's also pretty plausible that it's much harder to knock out a helmeted opponent than an unhelmeted one; if a connected hit at maximal force is sufficiently unlikely to knock out a helmeted opponent relative to an unhelmeted one, then there could be a substantive change to the returns to strenuous force. Whether optimal punch force goes up or down then isn't obvious to me (though it likely would be to somebody who has boxed).

A Peltzman story on the defence side would require that helmeted boxers take less care to avoid hits to the padded areas than to the unpadded chin, and that cumulative hits to the padded parts cause longer-term damage. And, that isn't inconsistent with some of the commentary from trainers at the time of the ban:

• “I’m not for it at all. I like the protective gear. Without it, everyone’s going to have to be much more vigilant, that’s for sure.” — Dr. Sonny Arkangel, S.A. ringside physician

• “It’s going to change the way we train kids now. There was a comfort zone with headgear. Without it, your defense is going to have to be better.” — Joe Rodriguez, local trainer

A year's gone by, so that should be plenty of time for a diff-in-diff study: headgear continued to be worn for women, senior and youth divisions; it was only banned for elite males during advancing tournaments, according to the story above-linked. But whether any reduction in concussions were due to Peltzman effects or due to better defensive peripheral vision in the absence of helmets ... I can't see how that could be disentangled. Well, if they brought back blindfold boxing and and checked the magnitude of helmet effects there as compared to ... no that wouldn't work either as helmets could arguably also muffle sound.

Update: the Economics Department's pugilist, Steve Agnew, reports (via Paul Walker):

The second explanation from the trainers about boxers blocking with their headgear is right. The not wanting to hurt your opponent explanation is bollocks, as is the chance of knockout v strenuous punching argument.
 The biggest pain in the arse with headgear is the heat. The headgear prevents a reasonable amount of your body’s heat escaping through your head. It is much more tiring to fight with headgear than without. That’s why in most gyms, fighters don’t spar with headgear.The biggest pain in the arse with headgear is the heat. The headgear prevents a reasonable amount of your body’s heat escaping through your head. It is much more tiring to fight with headgear than without. That’s why in most gyms, fighters don’t spar with headgear.

CAFE kills?

American fuel economy standards have a bunch of offsetting effects. If you're going to be in an accident, you're safer in a bigger vehicle, but you're also increasing the risk for everyone else. Tightening the fuel economy regulations then have ambiguous effects: do they save more lives by reducing deaths caused by those in SUVs, or do they kill more people by ensuring that more who are involved in accidents are in smaller vehicles?

Mark Jacobsen runs the numbers.

I estimate the fleet-wide impact of historical CAFE rules to be 149 additional annual fatalities per mile-per-gallon (MPG) increment in stringency. In this case, the shift to smaller vehicles within the car and light truck categories prescribed under CAFE causes deterioration in safety that is only partially offset by reductions in poorly matched accidents. The safety effect translates to a welfare cost of approximately 33 cents per gallon of gasoline saved.⁵ In the context of related environmental externalities, damages of $25 per ton CO₂ amount to 22 cents per gallon of gasoline, and Parry and Small (2005) report costs from local air pollution of about 16 cents per gallon.

So tightening the regs by one mile-per-gallon costs $0.33 in lives lost for every gallon saved while providing $0.38 in reduced pollution for every gallon saved. Pretty close to a wash if we think that the regulations themselves are costly.

He estimates the effects of an alternative policy bringing all vehicles under a single unified standard and reckons that tightened fuel economy standards under that form of regulation come with approximately zero cost in terms of lives lost: drivers of light trucks substitute into safer cars.

Even better could be abolishing CAFE entirely in favour of a petrol tax. CAFE standards only make sense relative to a petrol tax where consumers are particularly myopic and ignore the future petrol costs when buying a thirsty car. They aren't.

Shirking air traffic controllers

It sure sounds like it would improve safety. Everybody knows that being an air traffic controller can be pretty stressful. All those planes up there, have to keep them from bumping into each other. So it just makes sense that you'd want to provide stress leave for those controllers who are obviously suffering, right? What could go wrong?

Doug Allen, currently visiting at Canterbury's Economics department as Erskine Fellow, points me to this classic 1982 piece in the American Economic Review.

The Federal Employees' Compensation Act (FECA), which provides income insurance for government workers injured or disabled on the job, changed in 1974. Employees with at least one dependent would receive 75% of their base salary as tax-free compensation if out on leave. Because air traffic controllers earned a lot, their net take-home pay was higher if they were out on disability. On-the-job stress is pretty hard to disprove.

FECA amendments in 1974 let employees submit evidence from their private clinical psychologists as evidence of disability; prior to that, the government selected the doctor. And, unlike things like "leg cut off" where most doctors would agree, it's harder to verify whether any doctor's diagnosis of stress is all that valid. So Staten and Umbeck predicted that we'd see the most action post-FECA change in claims for non-verifiable things like stress.

They also note that OWCP examiners, who check the veracity of claims, were told to check for specific indicators of on-the-job stress. Deterioration in job performance over time counts. And everybody's monitoring air traffic controller performance all the time, making sure that planes don't get too close together.

So what happens? After the rule change, a bunch of air traffic controllers figure out that they can earn more by demonstrating on-the-job stress, choose times of the day when it's safest to breach minimum separation between aircraft so there aren't actual collisions, and encourage planes to get just close enough to each other to trigger their near mid-air collision sensors. Controllers with more experience have much stronger incentive to do it, as the compensation is based on the employee's salary, and there's a reasonably steep salary progression. Before the law change, inexperienced staff and those with more than 10 years' service (actual burnout) reported the most 'system errors' (violation of separation rules). After the law change, those with 5-10 years' service started experiencing the most system errors.

They conclude:

The steady increase in controller compensation awards and retirements during the 1970's has been cited recently as evidence of stress in the occupation. However, it seems clear that this is in large part a product of making compensation and retirement less costly to obtain. Concern has also been expressed over the relation between job pressures placed on controllers and aviation safety. If one is willing to take reported System Errors as a measure of workforce efficiency, it is clear that measured efficiency deteriorated following the FECA amendments, but not because of traffic increases or equipment failures. We have argued that in attempting to formalize the criteria for adjudicating controller psychological claims, OWCP created an incentive for controllers to demonstrate deteriorating performance. 

We have attempted in this paper to constrain theory of shirking behavior to generate refutable propositions. The methods described in the paper can be employed in the investigation of any type of contract. However, the nature of the FECA provisions and the air traffic control profession provided the elements essential for demonstrating the effectiveness of this approach. That we chose air traffic controllers in particular is, of course, no reflection on the integrity of the profession. The maximization postulate upon which micro theory relies leaves no profession exempt from marginal responses to gains from shirking. We offer this study as a demonstration that economics as a behavioral science can provide useful insights into behavioral patterns likely to result from incentives to shirk.

In later work, they show that air traffic controllers became a lot more likely to do this kind of thing after finding a doctor who's willing to certify stuff as being stress-related.

Regulations should always pay some heed to that people respond to incentives.

Morning roundup

Your morning list of things worth knowing.

Item the First: climate sensitivity may be lower than we'd thought; this bends optimal policy towards adaptation and away from abatement. I still think it best to have a low budget-neutral carbon tax set to ramp up on a well-announced schedule, but with flexibility to twist the schedule up or down as we learn more about climate sensitivity.

If such estimates were right, they would require revisions to the science of climate change and, possibly, to public policies. If, as conventional wisdom has it, global temperatures could rise by 3°C or more in response to a doubling of emissions, then the correct response would be the one to which most of the world pays lip service: rein in the warming and the greenhouse gases causing it. This is called “mitigation”, in the jargon. Moreover, if there were an outside possibility of something catastrophic, such as a 6°C rise, that could justify drastic interventions. This would be similar to taking out disaster insurance. It may seem an unnecessary expense when you are forking out for the premiums, but when you need it, you really need it. Many economists, including William Nordhaus of Yale University, have made this case.

If, however, temperatures are likely to rise by only 2°C in response to a doubling of carbon emissions (and if the likelihood of a 6°C increase is trivial), the calculation might change. Perhaps the world should seek to adjust to (rather than stop) the greenhouse-gas splurge. There is no point buying earthquake insurance if you do not live in an earthquake zone. In this case more adaptation rather than more mitigation might be the right policy at the margin. But that would be good advice only if these new estimates really were more reliable than the old ones. And different results come from different models.

Item the Second: All that stuff about food deserts? Probably not worth worrying about. And fast food outlet concentration likely doesn't matter either.

The researchers, who reported their findings in last week's journal Preventing Chronic Disease, looked at the habits of 97,678 adults in the California Healthy Interview Survey, and their weight, body mass index, and other indicators such as income.

They overlayed a map to determine the number of fast-food places, full-service restaurants and various kinds of food stores within five ranges, from half a mile to three miles from home.

“Evidence is more tentative than often presented in the news media and in policy arguments” linking obesity with the food environment, the researchers said. That is, the idea that people who live close to lots of fast-food outlets and far from big, well-stocked supermarkets are more likely to be overweight or obese, or to show other health results of poor eating habits.

“The evidence is not clear on whether promoting or discouraging a particular type of food outlet is an effective approach to promoting healthful dietary behavior and weight status,” the researchers said. Los Angeles has tried legislating the types of food outlets in South L.A. to help bring down obesity rates.

Caveat: I've not had a chance to read the underlying paper. And there are all kinds of endogeneity problems in this general line of research: it's hard to find an instrument for fast-food outlets' opening, and you need one to sort out the endogeneity problem if there is any correlation: do fast-food outlets cause obesity, or do fast-food outlets open in places where a lot of obese people live? But, the LA Times reports they found only weak association between location and outcomes; they put it down to that most people are able to drive to get what they want.

Item the Third: Suppose your company's parked van gets hit by another driver, pushing it into a restricted parking space. And the other driver reports it to the police. Would you expect the police to call you to let you know your van had been smashed up? Or that they'd call Council who'd send out the tow-truck for parking violations? If you live in the UK....

Item the Fourth: Bicycle helmet laws still cost lives on net. Here's a website run by cycle campaigners pointing to the literature on it. Here's a town in Washington State that dropped its helmet law because Council worried about an interesting kind of legal liability.

For 15 years, until June 1, Milton, population 7,000, 45 minutes south of Seattle, required helmets for all bicyclists and skateboarders. But with its 12-officer police force stressed by an increase in domestic violence, alcohol abuse and property crime, all of which surged through the recession, law enforcement priorities now go way beyond hectoring people about their headgear.

And an inability to enforce a law on the books, the town’s insurance consultant argued, created administrative unevenness that — in the event of an accident by someone who was not nagged or cited about helmet use — posed a liability risk that could bankrupt the community with one swipe from a punitive-minded jury.

For once, silly jury verdicts have had a beneficial chilling effect. HT on Item 4 to Greg Dwyer.

Texting while driving

Texting while driving is dangerous and increases your chance of accidents. Bans on texting while driving seem to increase accident rates.

The Insurance Institute for Highway Safety says that 3 of every 4 states that have enacted a ban on texting while driving have seen crashes actually go up rather than down.

It's hard to pin down exactly why this is the case, but experts believe it is a result of people trying to avoid getting caught in states with stiff penalties. Folks trying to keep their phones out of view will often hold the phone much lower, below the wheel perhaps, in order to keep it out of view. That means the driver's eyes are looking down and away from the road.

So, here's a potential difference-in-difference study in this, the "thinking about honours projects for next year" time of year.

New Zealand banned texting while driving a few years ago. If we can get data on age-by-age participation in texting (presumably more prevalent among young people than among older people), we'd expect that the ban would have no effect on older cohorts' rates of single-vehicle accidents, that accident rates among youths will depend on the elasticity of texting risk-taking with respect to the law (could go either way - if the number of texting kids drops enough to offset the increased risk-taking by those still texting, accident rates drop; otherwise, they increase), and that multivehicle accident rates will change proportionately to the types of drivers involved (no change to multivehicle accidents where elderly drivers smash into each other, other cross-effects depend on the elasticity found in the single-vehicle accident rates).

Data we need:

• Age-by-age texting participation rates (don't know if any proxy exists)
• Age-by-age accident rates  (near certain this exists)
• Accidents broken down by ages of all drivers involved and single- or multi-vehicle status
• Checking whether other age-specific interventions coincided with the texting law change:
• The zero-alcohol limit for young drivers (which itself would make for a great diff-in-diff study not only on accident rates but also on crime rates)
• The 'Crusher Collins' crackdown on 'boy racers'

Hints on data warmly accepted.

Update: the original study is here.

Offsetting behaviour: vaccine edition

I'm more inclined to call stupidity over risk homeostasis on this one:

Some adolescent girls who get the HPV vaccine to prevent cervical cancer wrongly think they no longer need to practice safe sex, U.S. researchers said on Monday.

The study, published in the Archives of Pediatric & Adolescent Medicine, shows the need for better education about the vaccines and their limitations.

...The authors said the study was limited in that subjects came from a single urban clinic serving low-income clients so the findings may not apply to more general populations.

I wonder if anybody's running effects of vaccination on STD and condom use rates. It would be surprising if there weren't some drop-off in condom use with vaccination; the risk has indeed gone down. But I'd love to see numbers on it. Unfortunately, the kinds of things that would predict vaccination likely also predict sexual behaviour regardless of whether vaccination has taken place. Finding an exogenous instrument could be tough.

Offsetting behaviour and KPI measures

The first 5 minutes of this BBC video give wonderful examples of the perils of target based assessment in the public sector.

Set the target as being time on the hospital waiting list and they'll prioritize the easiest surgeries. Set the target as time to being seen in the Emergency Department and they'll assign a "Hello Nurse" (and not the good kind from Animaniacs either). Patients don't hurt targets about numbers left on gurneys if you take the wheels off the gurneys and call them beds. And so on.

Reminds me of the old stories of socialist planning bureaus. If the target were tons of widgets, you make one MASSIVE widget; if the target is number of widgets, you make all kinds of itty bitty ones. If it's number of shoes, why bother making both left and right?

The video goes on later about how this is all the fault of the market and of governments moving too much towards a market model, but I've never had my grocery store mess me about in this way. It only happens in the state sector where there is no performance measure OTHER than whatever the target is. In the competitive sector, profit and loss provide discipline. So if you hurt productivity trying to skew performance along one measure, the customers will make sure you pay for it. The video talks about problems in the UK's league tabling of schools where rich folks would move into good schools' zones, keeping poor kids from getting into good schools. But isn't the problem then less the league table and more the restrictions on zoning? If they'd have combined league tables with vouchers and zero zoning, wouldn't outcomes have been just a little different?

The lesson the BBC should have drawn was that giving customers vouchers and letting them choose their own providers, whenever possible (hospitals, schools yes; police, maybe not) makes an awful lot more sense than assessing bureaucratic performance against KPI measures when principal agent problems are rather severe.

Offsetting behaviour: Rush edition

Did a fiction article in Road and Track (1973) predict the Peltzman Effect before Peltzman?

Denis at ALD today points us to a brilliant piece by PJ O'Rourke.

America’s romantic foolishness with cars is finished, however, or nearly so. In the far boondocks a few good old boys haven’t got the memo and still tear up the back roads. Doubtless the Obama administration’s Department of Transportation is even now calculating a way to tap federal stimulus funds for mandatory OnStar installations to locate and subdue these reprobates.

Among certain youths—often first-generation Americans—there remains a vestigial fondness for Chevelle low-riders or Honda “tuners.” The pointy-headed busybodies have yet to enfold these youngsters in the iron-clad conformity of cultural diversity’s embrace. Soon the kids will be expressing their creative energy in a more constructive way, planting bok choy in community gardens and decorating homeless shelters with murals of Che.

I myself have something old-school under a tarp in the basement garage. I bet when my will has been probated, some child of mine will yank the dust cover and use the proceeds of the eBay sale to buy a mountain bike. Four things greater than all things are, and I’m pretty sure one of them isn’t bicycles. There are those of us who have had the good fortune to meet with strength and beauty, with majestic force in which we were willing to trust our lives. Then a day comes, that strength and beauty fails, and a man does what a man has to do. I’m going downstairs to put a bullet in a V-8.

O'Rourke nails it exactly. Regulation has forced the car to become utilitarian; Japan does utilitarian better than does Detroit. Read the whole thing.

The O'Rourke article, of course, drew to mind Rush's (to my mind) best song: Red Barchetta. I hadn't known that the song was based on an old fiction article from Road & Track in 1973. Wikipedia informs me:

The song was inspired by the futuristic short story "A Nice Morning Drive," written by Richard Foster and published in the November, 1973 issue of Road and Track magazine. The story describes a similar future in which increasingly-stringent safety regulations have forced cars to evolve into massive "Modern Safety Vehicles" (MSVs), capable of withstanding a 50-mile-per-hour impact without injury to the driver. Consequently, drivers of MSVs have become less safety-conscious and more aggressive, and "bouncing" (intentionally ramming) the older, smaller cars is a common sport among some.

Note the year: 1973. Peltzman's seminal article, The Effects of Automobile Safety Regulation, came out in the JPE in 1975. I wonder if Peltzman read Road and Track. The Road and Track article, "A nice morning drive", outlines the argument perfectly:

Despite the extent of the safety program, it was essentially a good idea. But unforeseen complications had arisen. People became accustomed to cars which went undamaged in 10-mph collisions. They gave even less thought than before to the possibility of being injured in a crash. As a result, they tended to worry less about clearances and rights-of-way, so that the accident rate went up a steady six percent every year. But the damages and injuries actually decreased, so the government was happy, the insurance industry was happy and most of the car owners were happy. Most of the car owners - the owners of the non-MSV cars - were kept busy dodging the less careful MSV drivers, and the result of this mismatch left very few of the older cars in existence. If they weren't crushed between two 6000-pound sleds on the highway they were quietly priced into the junkyard by the insurance peddlers. And worst of all, they became targets . . .

I'd always thought the gleaming alloy aircar in Red Barchetta was a police car trying to chase down a forbidden gas-powered car; Foster's article (above) suggests rather a post-regulation griefer car. I prefer the former; it's less depressing. In any case, Foster is picking up the basic Peltzman mechanism. More accidents as folks take less care, fewer deaths per accident, but external costs for folks outside of the safer vehicles. Should we really call it the Foster Effect?

Some days, I really miss my old 350-4bbl '69 Skylark Custom. It waits for me in Manitoba. Sigh.

And the offsetting effect will be...

Drivers will pay less heed to pedestrians, with an end-result of a small increase in the number of pedestrian-car collisions, but with a lower death rate per collision. Effect on pedestrian deaths overall then is ambiguous. That's my prediction for cars equipped with this new safety feature.

Researchers at Cranfield University in England have developed an external airbag they say will significantly reduce pedestrian fatalities and injuries in the event of a crash.

The system deploys a hood - or bonnet, as the British call it - airbag at the base of the windshield, which research shows is where a pedestrian’s head is most likely to hit. The system uses radar and infrared technology to “pre-detect” a collision and inflates quickly enough to cushion the impact, said Roger Hardy of the university’s Cranfield Impact Centre.

We do have evidence that drivers pay less heed of cyclists wearing more protective equipment; in this case, it's the car that's wearing the equipment.

Offsetting Behaviour: condom use edition

About a month ago, the Pope released a statement arguing against reliance on condoms in the fight against AIDS in Africa. He was excoriated for this view. David Friedman today wonders whether there might not be offsetting behaviour in condom use in Africa.

Just as with auto safety and auto accidents, making sex safer has two effects working in opposite directions. It makes the chance that a given act of sex will result in AIDS transmission lower. But, by lowering that risk, it reduces the incentive to avoid sex entirely, to avoid sexual acts such as anal intercourse that are particularly likely to transmit AIDS, to avoid sex with people likely to give you AIDS, such as prostitutes. On theoretical grounds we have no way of knowing whether the net effect will be more AIDS or less.

It turns out that there is evidence that, just as in the auto case, the two effects roughly cancel. That, at least, was the widely reported conclusion of a Harvard AIDS researcher who had actually looked at the data. “We have found no consistent associations between condom use and lower HIV-infection rates, which, 25 years into the pandemic, we should be seeing if this intervention was working.”

Todd Zywicki over at Volokh previously pointed to stronger confirmation of this offsetting effect. The Alligator, linked to by Zywicki, quotes Dr. Edward Green, Director of Harvard's AIDS Prevention Research project:

In epidemics that are population wide, where most HIV is found in the general population, for whatever reason we can't get people to use condoms consistently, and when they use them at all, that seems to have the effect of disinhibiting people's behaviours so they end up taking greater sexual risks and cancelling whatever risk reduction they have gotten from the technology they're using.

Condoms make you feel safer, so you take more risks. Always beware the offsetting behaviour.

Offsetting Behaviour

A nice example of offsetting behaviour, via Popular Mechanics. They even cite Peltzman!

Give people antilock brakes, airbags and other safety devices, and they “consume” the safety improvements by driving more aggressively. This phenomenon is called the Peltzman Effect, after economist Sam Peltzman, who first wrote about it in 1976. The decades-long effort to make highways straighter, wider and better-marked, with more guardrails and rumble strips, has eliminated one class of dangers only to foster another: the complacent driver with a cellphone in one hand and a cup of coffee in the other, steering the vehicle with a knee while occasionally glancing at what’s ahead.

Meanwhile, modifying roads and intersections so drivers are less comfortable—by making driving, in some ways, more dangerous—forces people to slow down and pay attention, producing a change in behavior that, paradoxically, results in more safety. This is also true for pedestrians, who Vanderbilt says are more cautious away from crosswalks than within them because they don’t know if cars will actually stop.

Solution? See my masthead...