Unobserved heterogeneity was introduced in 1920 as a modifier of individual hazards. The concept was termed frailty in demography to describe variation in individual longevity , and has been incorporated in methods for survival analysis. As the frailest individuals are removed earlier from a heterogeneous group, mean hazards appear to decrease over time – cohort selection – leading to some of the most elusive effects in population science. Despite the accumulation of documented fallacies induced by cohort selection, the issue remains largely overlooked. I will expose the ubiquity of the phenomenon and propose a general framework to infer and compare trait distributions, with examples of current interest in epidemiology and related disciplines:
(1) Vaccines appear less effective in high-incidence settings. Are they, really ?
(2) What is the real effect of Wolbachia on mosquito susceptibility to dengue viruses ?
(3) As populations of bacteria are exposed to antibiotics, their mortality rates decline due to selection for noninherited resistance4. How much does this phenomenon share with what has been described in human demography ?
(4) What does cohort selection add to the debate between neutral and niche theories of biodiversity?
 Vaupel JW, Manton KG, Stallard E (1979) Impact of heterogeneity in individual frailty on the dynamics of mortality. Demography 16: 439-454.
 Gomes MGM, Gordon SB, Lalloo DG (2016) Clinical trials: the mathematics of falling vaccine efficacy with rising disease incidence. Vaccine 34: 3007-3009.
 King JG, Souto-Maior C, Sartori L, Maciel-de-Freitas R, Gomes MGM (2018) Variation in Wolbachia effects on Aedes mosquitoes is a key determinant of invasiveness and vectorial capacity. Nat Commun (in press).
 Balaban NQ, Merrin J, Chait R, Kowalik L, Leibler S (2004) Bacterial persistence as a phenotypic switch. Science 305: 1622-1625.