The term "genetic load" first emerged in a paper written in 1950 by the geneticist H. Muller. It is a mathematical model based on biological, social, political and ethical arguments describing the dramatic accumulation of disadvantageous mutations in human populations that will occur in modern societies if eugenic measures are not taken. The model describes how the combined actions of medical and social progress will supposedly impede natural selection and make genes of inferior quality likely to spread across populations-a process which in fine loads their progress. Genetic load is based on optimal fitness and emerges from a "typo-logical view" of evolution. This model of evolution had previously, however, been invalidated by Robert Wright and Theodosius Dobzhansky who, as early as 1946, showed that polymor-phism was the rule in natural populations. The blooming and persistence of the concept of genetic load, after its theoretical basis had already expired, are a historical puzzle. This persistence reveals the intricacy of science and policy-making in eugenic matters. The Canguil-hemian concept of 'scientific ideology' (1988) is used along with the concept of 'immutable mobile' (Latour 1986) and compared with the concept of 'co-production' (Jasanoff 1998), to provide complementary perspectives on this complex phenomenon.