Risk compensation

Booth's rule#2: "The safer skydiving gear becomes, the more chances skydivers will take, in order to keep the fatality rate constant"[1]

Risk compensation is a theory which suggests that people typically adjust their behavior in response to the perceived level of risk, becoming more careful where they sense greater risk and less careful if they feel more protected. Although usually small in comparison to the fundamental benefits of safety interventions, it may result in a lower net benefit than expected.[n 1]

By way of example, it has been observed that motorists drove faster when wearing seatbelts and closer to the vehicle in front when the vehicles were fitted with anti-lock brakes. There is also evidence that the risk compensation phenomenon could explain the failure of condom distribution programs to reverse HIV prevalence and that condoms may foster disinhibition, with people engaging in risky sex both with and without condoms.

By contrast, shared space is a highway design method which consciously aims to increase the level of perceived risk and uncertainty, thereby slowing traffic and reducing the number of and seriousness of injuries.

Overview

Risk compensation is related to the broader term behavioral adaptation which includes all behavior changes in response to safety measures, whether compensatory or not. However, since researchers are primarily interested in the compensatory or negative adaptive behavior the terms are sometimes used interchangeably.[n 2]

The theory emerged from road safety research after it was observed that many interventions failed to achieve the expected level of benefits but has since found application in many other fields.[n 3][n 4]

Peltzman effect

The reduction of predicted benefit from regulations that intend to increase safety is sometimes referred to as the Peltzman effect in recognition of Sam Peltzman, a professor of economics at the University of Chicago Booth School of Business, who published "The Effects of Automobile Safety Regulation" in the Journal of Political Economy in 1975 in which he controversially suggested that "offsets (due to risk compensation) are virtually complete, so that regulation has not decreased highway deaths".[2] According to Peltzman, regulation was at best useless, at worst counterproductive.[3][4][n 5] Peltzman found that the level of risk compensation in response to highway safety regulations was complete in original study. But "Peltzman’s theory does not predict the magnitude of risk compensatory behaviour." Substantial further empirical work has found that the effect exists in many contexts but generally offsets less than half of the direct effect.[5]


The Peltzman effect can also result in a redistributing effect where the consequences of risky behaviour are increasingly felt by innocent parties (see moral hazard). By way of example, if a risk-tolerant driver responds to driver-safety interventions, such as compulsory seat belts, crumple zones, ABS etc. by driving faster with less attention, then this can result in increases in injuries and deaths to pedestrians.[6]

Risk homeostasis

Risk homeostasis is a controversial hypothesis, initially proposed in 1982 by Gerald J. S. Wilde, a professor at Queen's University at Kingston, which suggests that people maximise their benefit by comparing the expected costs and benefits of safer and riskier behaviour and which introduced the idea of the target level of risk.[n 6] He proposed four constituents to a person's calculations relating to risk:[n 7]

Wilde noted that when Sweden changed from driving on the left to driving on the right in 1967, this was followed by a marked reduction in the traffic fatality rate for 18 months after which the trend returned to its previous values. He suggested that drivers had responded to increased perceived danger by taking more care, only to revert to previous habits as they became accustomed to the new regime.[n 8]

In a Munich study, part of a fleet of taxicabs were equipped with anti-lock brakes (ABS), while the remainder had conventional brake systems. In other respects, the two types of cars were identical. The crash rates, studied over three years, were a little higher for the cabs with ABS,[7] Wilde concluded that drivers of ABS-equipped cabs took more risks, assuming that ABS would take care of them; non-ABS drivers were said to drive more carefully since they could not rely on ABS in a dangerous situation.

The idea of risk homeostasis is disputed. One author claimed that it received "little support"[n 9] another suggested that it "commands about as much credence as the flat earth hypothesis",[n 10] a third noted that the proposal did create considerable media attention: "What set the debate alight, rather like petrol on flames, was the proposition in 1982 that road users did not just adapt to perceptions of changing risk through compensatory behaviors, but that the process was a homeostatic one, producing overall equilibrium in safety-related outcomes".[n 11] Others claimed that road fatality statistics, which have fallen considerably since the introduction of safety measures, do not support the theory.[8][9][10][11][12]

Examples

Road transport

Anti-lock brakes

Anti-lock braking systems are designed to increase vehicle safety by reducing skidding.

A number of studies show that drivers of vehicles with ABS tend to drive faster, follow closer and brake later, accounting for the failure of ABS to result in any measurable improvement in road safety. The studies were performed in Canada, Denmark, and Germany.[13][14][15] A study led by Fred Mannering, a professor of civil engineering at Purdue University supports risk compensation, terming it the "offset hypothesis".[16] A study of crashes involving taxicabs in Munich of which half had been equipped with anti-lock brakes noted that crash rate was substantially the same for both types of cab, and concluded this was due to drivers of ABS-equipped cabs taking more risks.[17]

However, the Insurance Institute for Highway Safety released a study in 2010 that found motorcycles with ABS 37% less likely to be involved in a fatal crash than models without ABS.[18] A 2004 study found that ABS reduced the risk of multiple vehicle crashes by 18 percent, but had increased the risk of run-off-road crashes by 35 percent.[19]

Bicycle helmets

A Spanish study of traffic accidents between 1990 and 1999 found that cyclists wearing bicycle helmets who were involved in accidents were less likely to have committed a traffic law violation than unhelmeted cyclists, and that helmeted cyclists were no more likely to have committed a speeding violation in association with the accident than unhelmeted cyclists. The authors concluded that "although the findings do not support the existence of a strong risk compensation mechanism among helmeted cyclists, this possibility cannot be ruled out".[20] In one experimental study, adults accustomed to wearing helmets cycled more slowly without a helmet, but no difference in helmeted and unhelmeted cycling speed was found for cyclists who do not usually wear helmets.[21][22]

Motorists may also alter their behavior toward helmeted cyclists. One study by Walker in England found that 2,500 vehicles passed a helmeted cyclist with measurably less clearance (8.5 cm) than that given to the same cyclist unhelmeted (out of an average total passing distance of 1.2 to 1.3 metres).[23] The significance of these differences has been re-analysed by Olivier.[24] See also slides 40-45 in for further analysis by Olivier of the data from the Walker study.[25]

In 1988 Rodgers re-analysed data which supposedly showed helmets to be effective and found both data errors and methodological weaknesses. He concluded that in fact the data showed "bicycle-related fatalities are positively and significantly associated with increased helmet use" and mentioned risk compensation as one possible explanation of this association.[26]

Seat belts

A 1994 research study of people both wore and habitually did not wear seatbelts concluded that drivers were found to drive faster and less carefully when belted.[27]

In Britain in 1981 at a time when the government was considering the introduction of seat belt legislation, John Adams of University College London, suggested that there was no convincing evidence of a correlation between the seat-belt legislation and reductions injuries and fatalities based on a comparison between states with and without seat belt laws. He also suggested that some injuries were displaced from car drivers to pedestrians and other road users.[28] The "Isles Report" echoed these concerns.[29] Adams subsequently argued that the reduction in fatalities that followed the introduction of legislation could not be attributed with confidence to seat-belt use due to the simultaneous introduction of breath testing for driving under the influence of alcohol.[30]

However, a 2007 study based on data from the Fatality Analysis Reporting System (FARS) of the National Highway Traffic Safety Administration concluded that between 1985 and 2002 there were "significant reductions in fatality rates for occupants and motorcyclists after the implementation of belt use laws", and that "seatbelt use rate is significantly related to lower fatality rates for the total, pedestrian, and all non-occupant models even when controlling for the presence of other state traffic safety policies and a variety of demographic factors".[31]

Swedish change to driving on the right

In Sweden, following the change from driving on the left to driving on the right there was a drop in crashes and fatalities, which was linked to the increased apparent risk. The number of motor insurance claims went down by 40%, returning to normal over the next six weeks.[32][33] Fatality levels took two years to return to normal.[34][n 12]

Shared space

Main article: Shared space

Shared space is an approach to the design of roads, where risk compensation is consciously used to increase the level of uncertainty for drivers and other road users by removing traditional demarcations between vehicle traffic by removing curbs, road surface markings, traffic signs. The approach has been found to result in lower vehicle speeds and fewer road casualties.[35]

Speed limits

The control of traffic speeds using effectively enforced speed limits and other traffic calming methods plays an important role in the reduction of road traffic casualties;[36][37] speed limit changes alone without accompanying enforcement or traffic calming measures will not.[38]

A 1994 study conducted to test the risk homeostasis theory, using a driving simulator, found that increasing posted speed limits and a reduction of speeding fines had significantly increased driving speed but resulted in no change in the accident frequency. It also showed that increased accident cost caused large and significant reductions in accident frequency but no change in speed choice. The results suggest that regulation of specific risky behaviors such as speed choice may have little influence on accident rates.[39]

Sport

Ski helmets

Recent studies indicate that skiers wearing helmets go faster on average than non-helmeted skiers,[40] and that overall risk index is higher in helmeted skiers than non-helmeted skiers.[41] Moreover, while helmets may help prevent minor head injuries, increased usage of helmets has not reduced the overall fatality rate.[42]

Other recent studies have concluded that helmet use is not associated with riskier behavior among skiers and snowboarders, and that helmet usage reduces the risk and severity of head injuries.[n 13][n 14][43]

Skydiving

'Booth's rule #2', often attributed to skydiving pioneer Bill Booth, states, "the safer skydiving gear becomes, the more chances skydivers will take, in order to keep the fatality rate constant".[1][44] Even though skydiving equipment has made huge leaps forward in terms of reliability, including the introduction of safety devices such as AADs, the fatality rate has stayed roughly constant when adjusted for the increasing number of participants.[45][46] This can largely be attributed to an increase in the popularity of high performance canopies, which fly much faster than traditional parachutes. A greater number landing fatalities in recent years has been attributed to high speed manoeuvres close to the ground.

Safety equipment in children

Experimental studies have suggested that children who wear protective equipment are likely to take more risks.[47]

Health

Risky sexual behavior and HIV/AIDS

Evidence on risk compensation associated with HIV prevention interventions is mixed. Harvard researcher Edward C. Green argued that the risk compensation phenomenon could explain the failure of condom distribution programs to reverse HIV prevalence, providing a detailed explanations of his views in an op-ed article for The Washington Post[48] and an extended interview with the BBC.[49] A 2007 article in the Lancet suggested that "condoms seem to foster disinhibition, in which people engage in risky sex either with condoms or with the intention of using condoms".[50][51] Another report compared risk behaviour of men based on whether they were circumcised.[52]

Notes

  1. ↑ Vrolix (2006) "Behavioural adaptation generally does not eliminate the safety gains from programmes, but tends to reduce the size of the expected effects"
  2. ↑ Vrolix (2006) "A term, closely related to risk compensation, is ‘behavioural adaptation’. Behavioural adaptation is a wider term referring to all behavioural changes triggered by a safety measure (OECD, 1997). Strictly spoken, this includes all positive and negative behavioural changes induced by road safety measures. Nevertheless, the emphasis is primarily put on the negative aspects of this phenomenon"
  3. ↑ Vrolix (2006) "Risk compensation is the term given to a theory which tries to understand the behaviour of people in potentially hazardous activities. In the context of the road user, risk compensation refers to the tendency of road users to compensate for changes in the road system that are perceived as improving safety by adapting behaviour (Elvik and Vaa,2004). So measures, designed to improve traffic safety, may bring along negative consequences in a way that individuals increase the riskiness of their driving behaviour because they feel safer (Dulisse, 1997)"
  4. ↑ Hedlund (2000) "The early risk compensation literature deals with road safety... Several recent studies examine risk compensation in response to both aggregate and specific consumer product and workplace safety regulations"
  5. ↑ Hedlund (2000) "This conclusion startled the road safety community and challenged the role of government in attempting to improve safety through regulation. In Peltzman's view, government regulation was useless and perhaps even counterproductive."
  6. ↑ Wilde (1998) "The level of risk at which the net benefit is expected to maximize is called the target level of risk in recognition of the realization that people do not try to minimize risk (which would be zero at zero mobility), but instead attempt to optimize it"
  7. ↑ Wilde (1998) "Besides macroeconomic influences, there are other factors that influence the level of accepted risk; these are of a cultural, social, or psychological kind. In general, the amount of risk that people are willing (in fact, prefer) to take can be said to depend on four utility factors and will be greater to the extent that factors..."
  8. ↑ Wilde (1998) "In the fall of 1967 Sweden changed over from left hand to right hand traffic. This was followed by a marked reduction in the traffic fatality rate. About a year and a half later, the accident rate returned to the trend before the changeover."
  9. ↑ Hedlund (2000) "The extreme views of risk homeostasis have attracted little support"
  10. ↑ Williams (1998) "Risk homeostasis is not a theory. It is a hypothesis that repeatedly has been refuted by empirical studies. As Evans has noted, it commands about as much credence as the flat earth hypothesis"
  11. ↑ Rudin-Brown and Jamson (2013) Page 28, 'Early Theories of Behavioural Adaptations' by Oliver Carsten: "What set the debate alight, rather like petrol on flames, was the proposition in 1982 that road users did not just adapt to perceptions of changing risk through compensatory behaviors, but that the process was a homeostatic one, producing overall equilibrium in safety-related outcomes"
  12. ↑ Rudin-Brown and Jamson (2013)"An example of risk overestimation in the short run is offered by the experience in Sweden when that country changed from left- to right-hand driving in the fall of 1967. This intervention led to a marked surge in perceived risk that exceeded the target level and thus was followed by a very cautious behavior that caused a major decrease in road fatalities. ...the accident rate returned to 'normal' within 2 years."
  13. ↑ Ruedl etc (2010) "Helmet use is not associated with riskier behaviour on slopes. In addition, helmet use has to be recommended because helmet use reduces the risk of head injuries among skiers and snowboarders"
  14. ↑ Ruedl etc (2010) "Safety helmets clearly decrease the risk and severity of head injuries in skiing and snowboarding and do not seem to increase the risk of neck injury, cervical spine injury, or risk compensation behavior"

Citations

Other references

  1. ↑ 1.0 1.1 Resilience: Why Things Bounce Back. ISBN 1451683804.
  2. ↑ Sam Peltzman. "The Effects of Automobile Safety Regulation". JSTOR 1830396.
  3. ↑ Pope, Adam T.; Tollison, Robert D. (2009). ""Rubbin' is racin": evidence of the Peltzman effect from NASCAR". Public Choice March 2010 142 (3–4). pp. 507–513. doi:10.1007/s11127-009-9548-2. Retrieved 2014-04-15. "The Peltzman (1975) effect predicts that when automobile safety regulations are made mandatory, at least some of their benefits will be offset by changes in the behavior of drivers
  4. ↑ "The Peltzman Effect: Do Safety Regulations Increase Unsafe Behavior?" (PDF). Journal of Safety, Health and Environmental Research.
  5. ↑ Vrolix (2006). "Behavioural Adaptation, Risk Compensation, Risk Homeostasis and Moral Hazard in Traffic Safety" (PDF).
  6. ↑ David R. Martinelli; Maria-Paulina Diosdado-De-La-Pena. "Safety Externalities of SUVs on Passenger Cars: An Analysis Of the Peltzman Effect Using FARS Data" (PDF). West Virginia University 2008. "In general, safety regulation did decrease the probability of death for drivers, but this is offset by involving themselves in a riskier behavior, which reassigns the change of deaths from vehicle occupants to pedestrians
  7. ↑ R.M. Trimpop (1994). The Psychology of Risk Taking Behavior. p. 219. ISBN 0444899618. Over a period of 36 months they observed part of a taxi fleet in Munich, Germany. Half of the observed vehicles were equipped with an anti-lock braking system (ABS)... The overall accident rate showed a slight increase for ABS taxis, but no significant differences between cars with the superior brake-system (ABS) versus cars without the system
  8. ↑ Leonard Evans (March 1986). "Risk Homeostasis Theory and Traffic Accident Data". Risk Analysis 6 (1). pp. 81–94. doi:10.1111/j.1539-6924.1986.tb00196.x. Retrieved 2014-04-15. risk homeostasis theory should be rejected because there is no convincing evidence supporting it and much evidence refuting it
  9. ↑ D. C. Andreassen (1985). "Linking deaths with vehicles and population". Traffic Engineering and Control 26 (11): 547–549.
  10. ↑ J. Broughton (1988). "Predictive models of road accident fatalities". Traffic Engineering and Control 29 (5): 296–300.
  11. ↑ S. Oppe (1991). "The development of traffic and traffic safety in six developed countries". Accident Analysis and Prevention 23 (5): 401–412. doi:10.1016/0001-4575(91)90059-E. PMID 1741895.
  12. ↑ J. R. M. Ameen and J. A. Naji (2001). "Causal models for road accident fatalities in Yemen". Accident Analysis and Prevention 33 (4): 547–561. doi:10.1016/S0001-4575(00)00069-5. PMID 11426685.
  13. ↑ Grant and Smiley, "Driver response to antilock brakes: a demonstration on behavioural adaptation" from Proceedings, Canadian Multidisciplinary Road Safety Conference VIII, June 14–16, Saskatchewan 1993.
  14. ↑ Sagberg, Fosser, and Saetermo, "An investigation of behavioural adaptation to airbags and antilock brakes among taxi drivers" Accident Analysis and Prevention #29 pp 293–302 1997.
  15. ↑ Aschenbrenner and Biehl, "Improved safety through improved technical measures? empirical studies regarding risk compensation processes in relation to anti-lock braking systems". In Trimpop and Wilde, Challenges to Accident Prevention: The issue of risk compensation behaviour (Groningen, NL, Styx Publications, 1994).
  16. ↑ Venere, Emil (2006). "Study: Airbags, antilock brakes not likely to reduce accidents, injuries". Purdue University News Service.
  17. ↑ Gerald J. S. Wilde (1994). "Remedy by engineering?". Psyc.queensu.ca. Retrieved 2010-12-07.
  18. ↑ "Motorcycle ABS: Skepticism Debunked". Ultimate Motorcycling. 2012-05-16. Retrieved 2012-08-18.
  19. ↑ "Effectiveness of ABS and Vehicle Stability Control Systems" (PDF). Royal Automobile Club of Victoria. April 2004. Retrieved 2010-12-07.
  20. ↑ Lardelli-Claret, P; De Dios Luna-Del-Castillo, J; Jiménez-Moleón, JJ; García-Martín, M; Bueno-Cavanillas, A; Gálvez-Vargas, R (2003). "Risk compensation theory and voluntary helmet use by cyclists in Spain". Injury Prevention 9 (2): 128–32. doi:10.1136/ip.9.2.128. PMC 1730952. PMID 12810738.
  21. ↑ Phillips, Ross Owen; Fyhri, Aslak; Sagberg, Fridulv (2011). "Risk Compensation and Bicycle Helmets". Risk Analysis 31 (8): 1187–95. doi:10.1111/j.1539-6924.2011.01589.x. PMID 21418079.
  22. ↑ Goverde, Marcel (September–October 2009). "Helmets Make You Bicycle Faster". Annals of Improbable Research 15 (5): 6–9.
  23. ↑ Walker, Ian (2007). "Drivers overtaking bicyclists: Objective data on the effects of riding position, helmet use, vehicle type and apparent gender". Accident Analysis & Prevention 39 (2): 417–25. doi:10.1016/j.aap.2006.08.010. PMID 17064655.
  24. ↑ Olivier, Jake; Bell, Melanie L. (2013). Rapallo, Fabio, ed. "Effect Sizes for 2×2 Contingency Tables". PLoS ONE 8 (3): e58777. doi:10.1371/journal.pone.0058777. PMC 3591379. PMID 23505560.
  25. ↑ Olivier, Jake (15 May 2012). "Mandatory helmet legislation in New South Wales: A statistical perspective" (PDF). Statistical Society of Australia (NSW branch). Retrieved 2013-04-04.
  26. ↑ Rodgers GB (1988). "Reducing Bicycle Accidents: A Re-evaluation of the Impacts of the CPSC Bicycle Standard and Helmet Use". Journal of Products Liability 11: 307–17.
  27. ↑ Janssen, W. (1994). "Seat belt wearing and driving behaviour: An instrumented-vehicle study Apr; Vol 26(2)". Accident Analysis and Prevention. pp. 249–2.
  28. ↑ "The efficacy of seatbelt legislation: A comparative study of road accident fatality statistics from 18 countries" (PDF). Dept of Geography, University College, London. 1981.
  29. ↑ "Isles Report" (PDF).
  30. ↑ Adams (1995). "The Failure of Seat Belt Legislation" (PDF).
  31. ↑ Houston, David J., and Lilliard E. Richardson (9 June 2011). "Risk Compensation or Risk Reduction? Seatbelts, State Laws, and Traffic Fatalities". Social Science Quarterly (Blackwell Publishing Limited): 913–936. Check date values in: |year= / |date= mismatch (help)
  32. ↑ John Adams (1985). Risk and Freedom: Record of Road Safety Regulation. Brefi Press. ISBN 9780948537059.
  33. ↑ Flock, Elizabeth (2012-02-17). "Dagen H: The day Sweden switched sides of the road". Washington Post. On the day of the change, only 150 minor accidents were reported. Traffic accidents over the next few months went down. ... By 1969, however, accidents were back at normal levels
  34. ↑ "On September 4 there were 125 reported traffic accidents as opposed to 130-196 from the previous Mondays. No traffic fatalities were linked to the switch. In fact, fatalities dropped for two years, possibly because drivers were more vigilant after the switch." Sweden finally began driving on the right side of the road in 1967 The Examiner Sept 2, 2009
  35. ↑ Ben Hamilton-Baillie (2008). "Towards shared space" (PDF). A recognition of ‘risk compensation effect’ prompts a fresh understanding of the adverse effects of measures such as traffic signals, signs, pedestrian guard rails and barriers on safety, and of their tendency to discourage informal physical activity. It may seem perverse to argue that well being can be improved through making spaces feel riskier, but that is the firm conclusion from both research, and from empirical studies
  36. ↑ World Health Organisation (2004). "World report on road traffic injury prevention". Retrieved 2010-04-13. In high-income countries, an established set of interventions have contributed to significant reductions in the incidence and impact of road traffic injuries. These include the enforcement of legislation to control speed and alcohol consumption, mandating the use of seat-belts and crash helmets, and the safer design and use of roads and vehicles.
  37. ↑ British Columbia Ministry of Transportation (2003). "Review and Analysis of Posted Speed Limits and Speed Limit Setting Practices in British Columbia" (PDF). p. 26 (tables 10 and 11). Retrieved 2009-09-17.
  38. ↑ "Speed limits". Contrary to popular belief, local speed limits should only be used if 85 out of 100 vehicles are already travelling at the speed it is wished to impose. Experience shows the speeds of these 85 vehicles are likely to influence the speeds of the other 15. This makes the speed limit largely self-enforcing and consequently makes for a manageable enforcement task
  39. ↑ Jackson JSH, Blackman R (1994). "A driving-simulator test of Wilde's risk homeostasis theory" (PDF). Journal of Applied Psychology.
  40. ↑ Shealy, JE; Ettlinger, CF; Johnson, RJ (2005). "How Fast Do Winter Sports Participants Travel on Alpine Slopes?". Journal of ASTM International 2 (7): 12092. doi:10.1520/JAI12092. The average speed for helmet users of 45.8 km/h (28.4 mph) was significantly higher than those not using a helmet at 41.0 km/h (25.4 mph)
  41. ↑ Lana Ružić & Anton Tudor (2011). "Risk-taking Behavior in Skiing Among Helmet Wearers and Nonwearers". Wilderness & Environmental Medicine 22 (4). pp. 291–296. Retrieved 2014-04-15. The main findings of this study indicate that the overall Risk Index is higher in helmeted skiers than non-helmeted skiers. The population that contributes the most to the overall Risk Index value is male helmet wearers, signifying that male helmet wearers take more risks while skiing than others
  42. ↑ Shealy, Jasper E. et al (2008). "Do Helmets Reduce Fatalities or Merely Alter the Patterns of Death?". Journal of ASTM International 5 (10). This paper presents results that suggest that while helmets may be effective at preventing minor injuries, they have not been shown to reduce the overall incidence of fatality in skiing and snowboarding even though as many as 40 % of the population at risk are currently using helmets
  43. ↑ Scott, M. D; Buller, D. B; Andersen, P. A; Walkosz, B. J; Voeks, J. H; Dignan, M. B; Cutter, G. R (June 2007). "Testing the risk compensation hypothesis for safety helmets in alpine skiing and snowboarding". Inj. Prev. 13 (3): 173–7. doi:10.1136/ip.2006.014142. PMC 2598370. PMID 17567972. No evidence of risk compensation among helmet wearers was found.
  44. ↑ The Click Moment: Seizing Opportunity in an Unpredictable World. ISBN 9780241963517.
  45. ↑ Andrew Stewart. "On risk: perception and direction". pp. 364–365. his finding is in alignment with risk compensation theory because it predicts that, essentially, skydivers will compensate for any new safety mechanism and consequently perform more dangerous types of jumping
  46. ↑ "US Skydiving Fatalities History".
  47. ↑ Morrongiello BA, Walpole B, and Lasenby J. (2007). "Understanding children's injury-risk behavior: Wearing safety gear can lead to increased risk taking". Accident Analysis & Prevention 39 (3): 618–623.
  48. ↑ Green, Edward C. (2009-03-29). "The Pope May Be Right". The Washington Post.
  49. ↑ "The pope was right about condoms, says Harvard HIV expert". Sunday Sequence. BBC Radio Ulster. 2009-03-29.
  50. ↑ Shelton, James D (2007-12-01). "Ten myths and one truth about generalised HIV epidemics". The Lancet 370 (9602): 1809–1811. doi:10.1016/S0140-6736(07)61755-3.
  51. ↑ Gray, Ronald; Kigozi, Godfrey; Serwadda, David; Makumbi, Frederick; Watya, Stephen; Nalugoda, Fred; Kiwanuka, Noah; Moulton, Lawrence H et al. (2007-02-01). "Male circumcision for HIV prevention in men in Rakai, Uganda: a randomised trial". The Lancet 369 (9562): 657–666. doi:10.1016/S0140-6736(07)60313-4. PMID 17321311.
  52. ↑ Nicholas, Wentao Xiong, Christine Mattson and Wilson (2011). "Is Sex Like Driving? Risk Compensation Associated with Randomized Male Circumcision in Kisumu, Kenya" (PDF). Williams College Economics Department Working Paper Series.

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