Although the life course prospective study design has many benefits, and information from such studies is in increasing demand for scientific and policy purposes, it has potential inherent design problems associated with its longevity. These are in particular the fixed sample structure and the data collected in early life, which are each determined by the scientific principles of another time and the risk over time of increased sample loss and distortion through loss. The example of a national birth cohort in Britain, studied from birth so far to age 53 years is used to address these questions. Although the response rate is high, avoidable loss, which was low in childhood, increased in adulthood, and was highest in those in adverse socio-economic circumstances and those with low scores on childhood cognitive measures. Recent permanent refusal rate rises may be the result of better tracing and/or a response to increased requests for biological measurement. Nevertheless, the responding sample continues in most respects to be representative of the national population of a similar age. Consistency of response over the study's 20 data collections has been high. The size of the sample responding in adulthood is adequate for the study of the major costly diseases, and for the study of functional ageing and its precursors.
This study's continuation has depended not only on scientific value but also policy relevance. Although the problems inherent in the prospective design are unavoidable they are not, in the study described, a barrier to scientific and policy value. That seems also likely in Britain's two later born national birth cohort studies that have continued into adulthood.
First interpretations of the importance of early life socio-economic and developmental experience to adult physical and mental health tended to imply that the future pattern of life was fixed in those early years (Erikson, 1963; Reid, 1969; Barker, 1992). However, it also quickly became clear that later experience and exposure and their interaction with early life experience affect the processes of staying healthy and getting sick (Forsdahl, 1978; Mann, Wadsworth, & Colley, 1992; Barker, 1998; Bifulco & Moran, 1998; Kuh & Ben Shlomo, 1997). Thus, the importance of having data about all periods of life became evident, and sources of data to study the pathways from early life to adult outcomes are now sought (Susser, Terry, & Matte, 2000; Eaton, 2002).
In epidemiology and the social sciences ingenious discoveries of populations studied in early life but not later, have led investigators to find those populations in adult life in order to study health, behaviour and survival in relation to whatever early developmental measures and exposures were recorded. For example, in Britain Barker (1998) used register data on size at birth to study a range of adult health outcomes, Gunnell, Frankel, Nanchalal, Braddon, and Davey Smith (1996), Gunnell et al. (1998) and Blane, Montgomery, and Berney (1998) traced the population of the Boyd–Orr study of childhood nutrition and growth for studies of adult health and survival and Deary, Whalley, Lemmon, Crawford, and Starr (2000) traced participants in the Scottish Mental Survey of childhood mental ability in 1932. In the US the Berkeley Guidance study is also of this design (Caspi & Elder, 1988). These catch-up designs have the value of data from early life and/or childhood, but rely on recollection for data in the years between childhood and the first adult recontact.
Despite the difficulties of maintaining contact this study shows that is possible to carry out life course follow-up from birth to midlife with sample members as the primary source of data, and still maintain an acceptable response rate and national representativeness. That, and the similarity of response in the comparable national birth cohort studies begun in 1958 and 1970, suggests that the life course follow-up design is viable over a long-time period, and thus remains one of the most powerful ways to test life-course hypotheses. Britain continues to invest in the national life course studies now in adulthood and in new studies, the most recent begun in 2001 (Smith & Joshi, 2002). Each of these investigations was begun and continued because of policy as well as scientific relevance (Wadsworth, 1991; Wadsworth, Ferri, & Bynner, 2003).
The requirement by scientists and policy makers for life course data on health and ageing is likely to increase (Schroots, 1993; Magnusson, 1996; Kuh & Ben Shlomo, 1997; European Commission, 1999; Aboderin et al., 2002; Ben-Shlomo & Kuh, 2002). The long-term investment in the British National Birth cohorts now in adulthood is paying off in that they are proving, as intended, to be powerful resources for the study of life course influences on the processes of ageing (Wadsworth, 1991; Kuh & Ben Shlomo, 1997; Susser et al., 2000). Comparative studies of these national birth cohorts add a new dimension to ageing research that will show whether the considerable cohort differences in life course exposure to social, economic and environmental circumstances, health care, nutrition and educational opportunities, are also reflected in differences in adult health and ageing (Ferri et al., 2003; Kuh & Hardy, 2002).
