Intestinal helminths – including hookworm, roundworm, whipworm,
and schistosomiasis – infect more than one–quarter of the
world's population. Studies in which medical treatment is
randomized at the individual level potentially doubly
underestimate the benefits of treatment, missing externality
benefits to the comparison group from reduced disease
transmission, and therefore also underestimating benefits for
the treatment group. We evaluate a Kenyan project in which
school–based mass treatment with deworming drugs was randomly
phased into schools, rather than to individuals, allowing
estimation of overall program effects. The program reduced
school absenteeism in treatment schools by one–quarter, and was
cheaper than alternative ways of boosting school participation.
Deworming substantially improved health and school participation
among untreated children in both treatment schools and
neighboring schools, and these externalities are large enough to
justify fully subsidizing treatment. Yet we do not find eviden