--- title: Does deworming pay off decades later? summary: One Kenyan program suggests that treating children for worms raised their earnings twenty years later, while systematic reviews of dozens of trials find little short-term benefit. Researchers and funders disagree about what to believe and what to do. status: open positions: - name: The long-run gains are real claim: A randomized program in Kenya found that children who got more years of deworming had higher consumption and hourly earnings as adults. Trials that treat children one by one miss the spillover benefits of treating whole schools, and short trials measure too little, too soon, to see effects that build over a lifetime. - name: The evidence is too thin to rely on claim: Dozens of trials, pooled in Cochrane and Campbell reviews, find that mass deworming does little for growth, cognition, school performance or survival. A large economic effect with no measurable health effect needs an explanation, and it rests mostly on one program whose reanalysis weakened some of its findings. - name: Act on expected value claim: Deworming may well do little, but it costs about 50 cents to a dollar per child treated. A modest chance of large lifelong gains can still make it a good bet, as long as the estimate is discounted heavily for the doubt. cruxes: - Whether large economic gains can exist when short-term health gains are too small to detect. - Whether results from heavily infected children in Busia in 1998 carry over to the lighter infections common today. - Whether new long-run studies in other places would find what the Kenya studies found. - How much weight a funder should give a small chance of a large benefit compared with a near-certain small one. conditions: [measurement-gap, silence-about-failure] sources: - title: Twenty-year economic impacts of deworming publisher: Hamory, Miguel, Walker, Kremer and Baird, PNAS 118(14) year: 2021 url: https://pmc.ncbi.nlm.nih.gov/articles/PMC8040658/ - title: 'Worms: Identifying Impacts on Education and Health in the Presence of Treatment Externalities' publisher: Edward Miguel and Michael Kremer, Econometrica 72(1) year: 2004 url: https://www.econometricsociety.org/publications/econometrica/2004/01/01/worms-identifying-impacts-education-and-health-presence - title: Public health deworming programmes for soil-transmitted helminths in children living in endemic areas (Cochrane systematic review) publisher: Taylor-Robinson, Maayan, Donegan, Chaplin and Garner, Cochrane Database of Systematic Reviews year: 2019 url: https://pmc.ncbi.nlm.nih.gov/articles/PMC6737502/ - title: 'Mass deworming to improve developmental health and wellbeing of children in low-income and middle-income countries: a systematic review and network meta-analysis' publisher: Welch and colleagues, The Lancet Global Health 5(1) year: 2017 url: https://doi.org/10.1016/S2214-109X(16)30242-X - title: 'Re-analysis of health and educational impacts of a school-based deworming programme in western Kenya: a pure replication' publisher: Aiken, Davey, Hargreaves and Hayes, International Journal of Epidemiology 44(5) year: 2015 url: https://pmc.ncbi.nlm.nih.gov/articles/PMC4681107/ - title: 'Re-analysis of health and educational impacts of a school-based deworming programme in western Kenya: a statistical replication of a cluster quasi-randomized stepped-wedge trial' publisher: Davey, Aiken, Hayes and Hargreaves, International Journal of Epidemiology 44(5) year: 2015 url: https://pmc.ncbi.nlm.nih.gov/articles/PMC4681108/ - title: 'Commentary: Deworming externalities and schooling impacts in Kenya: a comment on Aiken et al. (2015) and Davey et al. (2015)' publisher: Hicks, Kremer and Miguel, International Journal of Epidemiology 44(5) year: 2015 url: https://academic.oup.com/ije/article-lookup/doi/10.1093/ije/dyv129 - title: Deworming might have huge impact, but might have close to zero impact publisher: GiveWell Blog year: 2016 url: https://blog.givewell.org/2016/07/26/deworming-might-huge-impact-might-close-zero-impact/ - title: Why I mostly believe in Worms publisher: David Roodman, GiveWell Blog year: 2016 url: https://blog.givewell.org/2016/12/06/why-i-mostly-believe-in-worms/ --- ## The debate In 2004 Miguel and Kremer reported that a school deworming program in Busia, Kenya, cut absenteeism in treated schools by about a quarter and helped untreated children nearby, though it did not raise test scores. Later follow-ups estimated about 14 percent higher consumption, pooling the 15- and 20-year surveys, and 13 percent higher hourly earnings, pooling the 10-, 15- and 20-year surveys, among people assigned two to three additional years of treatment. Both estimates meet the 10 percent significance threshold, but not the conventional 5 percent threshold. Systematic reviews read the wider evidence differently. A 2019 Cochrane review of 51 trials concluded that mass deworming does not appear to improve height, hemoglobin, cognition, school performance or mortality, and it cautioned against basing programs on two older studies that found large weight gains. A 2017 review found little to no effect on weight, height, cognition, attendance or mortality, and judged the long-run evidence too weak to draw conclusions. In 2015 independent researchers published two reanalyses of the Kenya data, setting off what David Roodman, writing on GiveWell’s blog, later called the “great worm wars.” After coding errors were corrected, they found little evidence that attendance rose in neighboring schools, and evidence of better attendance in treated schools that depended on the method used and carried a high risk of bias. The original authors replied that the corrected data strongly support their findings on spillovers. GiveWell took the third position. In 2016 it wrote that most of its staff thought deworming more likely than not to have very little or no impact, but at $0.50 to $1 per person treated, it applied a 1 to 2 percent multiplier to the trial results and still found the expected value worth funding. Roodman judged the earnings result credible but was less sure it would hold in other times and places. ## Where Charitology stands Charitology grades deworming’s long-run evidence **mixed**: rigorous studies disagree, and the strongest positive evidence comes from one program. How much to stake on an uncertain chance of a large benefit is a value question, and Charitology doesn’t settle it. The evidence gap is clearer: long-run results from other times and places are what would move the debate. See the [deworming entry](/record/interventions/deworming/), [the replication rate](/open-problems/the-replication-rate/) and [expected value](/record/encyclopedia/expected-value/).