In 1947, the Scottish Council for Research in Education and the Population Investigation Committee conducted a survey of Scottish schoolchildren, exploring the relations between tested intelligence and fertility. The survey was not only significant for its size, measuring the IQ of all 11-year-olds at school on the day of testing, some 80,805 children, but also because it was a repeat survey. Its purpose was to establish whether the intelligence of the population had declined because of the negative correlation between IQ and family size. The paper will explore how the impetus for the 1947 survey came from attempts to revive the fortunes of the eugenics movement, based upon the interdisciplinary study of population. While most expected the study to provide evidence of a decline in intelligence, it revealed an increase. This was in spite of a continuing process of differential fertility. This paper will explore the influence of these results, described as a “paradox,” upon the future development of the eugenics movement and the sciences of population. While for many, the results were seen to have completely, and thankfully, undermined eugenic fears of degeneration, the supposed “resolution” of the paradox in 1962 provided the basis of a meritocratic and optimistic “new eugenics” that sought to reunite social and biological scientists concerned with human betterment in Britain and the United States. © 2007 Wiley Periodicals, Inc.