The aims of the study were to answer the questions: are there universal values for birth incidence and population prevalence for the rare neurodevelopmental condition Prader-Willi syndrome (PWS)? If so, why are published estimates so widely different? A review of the 11 published estimates revealed methodological flaws resulting in higher estimates. Consideration of characteristics of PWS and the use of appropriate methods showed that any accurate estimate would depend on the demographics of the population at that point in time. In the absence of studies of the effects of characteristics, such as ethnicity, on prevalence rates, it is not known whether other factors also influence incidence and prevalence rates. (https://jmg.bmj.com/content/63/9/543)