along with those of various seminar and conference participants. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research. NBER working papers are circulated for discussion and comment purposes. They have not been peerreviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.

M any people state a desire to change health-related behaviors, yet struggle to do so. These behavioral problems in domains such as weight loss, smoking, and exercising have helped to motivate a rich literature in economics on time-inconsistent behavior (Strotz 1955(Strotz -1956Laibson 1997; Rabin 1999, 2001;Loewenstein, O'Donoghue, and Rabin 2003;DellaVigna and Malmendier 2006). This literature has shown that time inconsistency can generate patterns of behavior that lead to both "internalities," where one's short-run actions are perceived as suboptimal from one's long-run perspective, and traditional externalities coming through higher group-rated health insurance costs and spending on Medicare and Medicaid (Finkelstein et al. 2009).In the face of these problems, there is increasing interest from firms, insurance companies, policy makers, and health professionals in using financial incentives to motivate changes in health behaviors (Volpp et al. 2009a;Baicker, Cutler, and Song

CONTEXT: The relationship between individuals' perception of their risk for acquiring HIV and their use of condoms is poorly understood. Understanding this relationship is crucial to the development of effective strategies to fight HIV and AIDS. METHODS: Data from the Mozambique 2001 Adolescent and Young Adult Reproductive Health and Behavior Risk Survey are used to compare 15-24-year-olds' assessments of their HIV risk with assessments based on current and past sexual behavior. In bivariate and probit regression analyses, the relationship between correct risk assessment and the likelihood of condom use at last intercourse is examined. RESULTS: Twenty-seven percent of women and 80% of men who considered themselves to have no risk or a small risk of contracting HIV were actually at moderate or high risk. For both men and women, the prevalence of condom use at last sex was more than twice as high among those who assessed their risk correctly (30% and 16%, respectively) as among those who did not (14% and 6%). Multivariate analysis showed that correct assessment was positively associated with condom use; the association was driven by use among never-married individuals. Never-married males who assessed their risk correctly were 18% more likely than other males to report condom use; never-married females, 17% more likely than other females. CONCLUSIONS: Educational messages should aim at enabling individuals to correctly assess their own HIV risk and encouraging behavior change based on self-assessment of risk.International Family Planning Perspectives, 2006, 32(4):192-200 dividual's awareness of the consequences of HIV/AIDS and may lead to safer sexual practices.Other studies have reported a lack of association between HIV/AIDS knowledge and sexual behavior. In a survey of almost 1,500 Rwandan women, the vast majority had correct knowledge of HIV transmission, but only a small proportion had adopted any protective behavior in the last year. 10 Sexually active students in Dar-es-Salaam, Tanzania, who knew that condom use prevents HIV infection had a reduced likelihood of always using condoms. 11 In Ethiopia, a cohort study of factory workers with a high prevalence of HIV reported high-risk sexual behavior and low condom use, even though the majority mentioned condom use as the best way to prevent HIV. 12 A study using 1998 Kenyan DHS data reported that the odds of having risky sexual behavior were more than tripled among men and women who perceived their risk of HIV/AIDS as high, and found no association between knowledge of HIV transmission and sexual behavior. 13 The relationship between perception of risk and sexual behavior is complex and poorly understood. Studies conducted in different cultures have associated HIV risk perception with a wide range of variables: number of sexual partners, knowledge of sexual partners' past sexual behavior, fear of AIDS, shame associated with having AIDS, community perception of AIDS risk, knowing someone with AIDS, discussing AIDS at home, closeness of parent-child relation...

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We provide the first comprehensive assessment of the effects of mandatory seatbelt laws on self-reported seatbelt use, highway fatalities, and crash-related injuries among high school age youths using data from the Centers for Disease Control's (CDC) national, state, and local Youth Risk Behavior Surveys (YRBS) and the Fatality Analysis Reporting System (FARS) from 1991 to 2005, a period spanning over 20 changes in state seatbelt laws. Our quasi-experimental approaches isolate the independent effects of seatbelt laws net of demographic characteristics, area and year fixed effects, and smooth area-specific trends. Across all data sources, we find consistent evidence that state mandatory seatbelt laws - particularly those permitting primary enforcement - significantly increased seatbelt use among high school age youths by 45-80%, primarily at the extensive margin. Unlike previous research for adults, however, we find evidence against the selective recruitment hypothesis: seatbelt laws had consistently larger effects on those most likely to be involved in traffic accidents (drinkers, alcohol-involved drivers). We also find that mandatory seatbelt laws significantly reduced traffic fatalities and serious injuries resulting from fatal crashes by 8 and 9%, respectively. Our results suggest that if all states had primary enforcement seatbelt laws then regular youth seatbelt use would be nearly universal and youth fatalities would fall by about 120 per year.