Integrating Behavioral Choice into Epidemiological Models of AIDS

Increased HIV risk creates incentives for people with low sexual activity to reduce their activity, but may make high-activity people fatalistic, leading them to reduce their activity only slightly, or actually increase it. If high-activity people reduce their activity by a smaller proportion than low-activity people, the compo-sition of the pool of available partners will worsen, creating positive feedbacks, and possibly multiple steady states. Early public health efforts may allow socie-ties to reach more favorable steady states. Nearly 18 million people have been infected by HIV WHO 1995, the majority through heterosexual transmission in devel-oping countries. Prevalence among 30- to 40-year-olds in some districts of Uganda is 40 percent Barnett and Blaikie 1992, and prevalence among prostitutes in Nairobi reached 80 percent by 1987 Over and Piot 1993. 1 Surveys of sexual activity and epidemiological models sug-gest that the behavior of a small group of highly sexually active people is critical to the spread of the epidemic Hethcote and Yorke 1984; Over and Piot 1993. However, most epidemiological models treat behavior as independent of prevalence. This may be in part because there are little data on how prevalence affects the rate of partner change, and in part because the available evi-dence suggests that people have reacted differently to the epi-demic. Although some people have adopted safer behavior in response to increased prevalence McKusick et al. 1985; Ahituv, Hotz, and Philipson 1993, there is anectodal evidence of fatalism among some TV drug users and homosexuals in developed coun-tries, and prostitution has continued at high levels in parts of Africa and Asia. 2 This paper integrates formal analyses of behavioral choice and epidemiological dynamics in heterogeneous populations. Un-surprisingly, increases in the probability of infection from an additional partner will create incentives for people with low sex-