The Hertfordshire Ageing Study (HAS)—From 1911 until 1948 the birth weight of each baby born in Hertfordshire was recorded. Those born between 1920 and 1930 and who were still living in the county were invited to participate in a study on influences on ageing (Syddall et al. 2009). In 2003–2005, 359 people (60 % of those surviving) took part in a follow-up study ; 357 completed the HADS.