ongitudinal research in the social sciences has enriched our understanding of the antecedents and consequences of obesity (Crossman, Sullivan, and Benin 2006;Goodman and Whitaker 2002;Himes 2000;Mannino et al. 2006;Novak, Ahlgren, and Hammarstrom 2006;Sundquist and Johansson 1998). Unfortunately, for much of the twentieth century, social scientists did not routinely record the height and weight of study participants (but see, e.g., Ayres 1909). As a consequence, a number of important longitudinal studies that might otherwise provide excellent resources for obesity research-and, more generally, for research on the later consequences of early life characteristics and conditions-are limited by the lack of body mass indicators early in life.The Wisconsin Longitudinal Study (WLS) fi ts the previous description. The WLS is a cohort study of over 10,000 individuals that has been used to study a wide range of health issues, including cognitive function (Krahn et al. 2003;Seltzer et al. 2005