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Issue 1, January 1998, pp.14-15.

Life course influences
Chris Power

What are life course influences?
Research into the causes of health inequalities is paying increasing attention to life course influences. These are factors occurring at different life stages - in childhood and early adulthood, for example - which influence later patterns of inequality.(1) A recent Department of Health report emphasised the importance of cumulative health-damaging and health-promoting physical and social environments in shaping socio-economic inequalities in health.(2)

It is likely that cumulative differential exposure to health damaging or health promoting environments is the main explanation for observed variations in health and life expectancy.
Variations in Health(2) p.1

Why are life course influences important?
The current emphasis on life course factors, and on the early life influences on adult health in particular, has been generated by evidence from two research areas.

First, there is evidence, primarily from the US, on the long-term benefits of pre-school interventions among children from deprived home backgrounds. Favourable effects of pre-school child development programmes were evident for factors known to be related to later health (such as educational achievement, adult income and home ownership, family formation), with positive effects occurring over several years following the initial intervention. We have some evidence from this work that circumstances in early life can have long-lasting influences on subsequent life trajectories that are in turn strongly linked to later health.

Second, there is increasing evidence on the long term effects of early life factors on adult disease, which has emanated particularly from a series of UK studies. Much of this work focuses on effects of low birthweight, with increasing evidence that this factor is associated with health outcomes 50 to 60 years later, in respect of coronary heart disease, stroke and respiratory disease mortality. Growth in childhood may have similar long- term effects. Other adversities in infancy and childhood - such as chest infection and overcrowding -appear to increase the risk of adult respiratory disease. In sum, recent research suggests that early life exerts a persisting influence on several adult diseases. Effort is now being directed at how early influences culminate or interact with adult influences that affect health. As a consequence, a broader research agenda is developing in relation to influences at different life stages.(3,4)

Research in the Health Variations Programme
A number of projects in the Health Variations Programme will contribute to the understanding of life course influences. Two of these projects are described below.

One project, by David Blane and his colleagues, is obtaining information from survivors of a health and nutrition survey conducted among children in the late 1930s. The participants, now well into their 60s and 70s, are being questioned about their lifetime occupational history, residential history, smoking and other health- related behaviours, and their health status is being assessed. Data from the original survey on nutritional status, health and growth in childhood are being combined with information about the rest of their lives to investigate how experiences in childhood and adulthood act together to produce socio-economic gradients in health in early old age.

A second project, based at the Institute of Child Health, is examining life course influences using the 1958 birth cohort study. This study has followed the same individuals from birth to age 33. Previous research on the cohort to age 23 has confirmed that health inequalities already exist by this stage of early adulthood, and has identified the differential accumulation of factors from birth as a key factor in the development of inequalities over this period.

In our project in the Health Variations Programme, we have been using the 1958 birth cohort study, which gives information on the same individuals as they grow older. We are examining this contemporary sample to speculate about future inequalities.(5) At age 33, the cohort is too young for many of the study participants to have developed serious chronic disease. But inequalities already exist for measures of health status that predict later morbidity and mortality (notably, self-rated health, respiratory symptoms, obesity and psychological distress). This descriptive work shows the patterns expected from cross-sectional evidence, with strong socio-economic gradients in risk factors at each life stage.

What is particularly disturbing is the strength and consistency of the social trend in multiple influences for adult disease accumulating over time. For example, when we examine life course factors related to adult respiratory disease, we find strong socio-economic gradients in factors believed to affect lung growth (low birthweight and maternal smoking in pregnancy, height, childhood socio-economic circumstances), and in factors believed to affect the rate of decline in lung functioning (pollution, passive smoking, adult smoking and low consumption of fresh fruit and vegetables). Similarly, for coronary heart disease and diabetes (non-insulin dependent), we find strong socio-economic gradients in relevant infant and childhood factors (notably low birthweight, height and childhood socio-economic circumstances) and adult life factors (such as gaining weight, low consumption of fresh fruit and vegetables, adult smoking, low social support, job strain and insecurity).

For each factor we have examined, exposure to risk is at its lowest among those born into professional and managerial home backgrounds and is highest among those with unskilled manual backgrounds. From these trends, we would expect strong inequalities in adult health in this study population as they grow older.

Future research and policy
This descriptive work provides preliminary evidence of the socio-economic patterning of risks for adult disease over the first three decades of life. As our research in the Health Variations Programme develops, we will examine the extent to which adult life experiences in the home or work environment (such as social support, and job strain and insecurity) and early life factors contribute towards the development of health inequalities, and whether these influences affect men and women in similar ways. We will be seeking to establish what the relative contribution of different factors and different life stages is to the development of health inequalities at age 33.

Research focusing on underlying causes, such as that on life course influences, is fundamental to policies designed to reduce health inequalities. There are many research challenges (conceptual and methodological) to be overcome, but the available evidence suggests that, for several health outcomes, it is important to identify the contribution of factors occurring at different life stages.

References:
1. Power, C., Manor, O. and Fox, A.. J. (1991) Health and Class: the Early Years, London : Chapman and Hall.
2. Department of Health (1996) Variations in Health: What can the Department of Health and the NHS Do? A Report Produced by the Variations Sub-group of the Chief Medical Officer's Health of the Nation Working Group, London : Department of Health.
3. Kuh, D. and Ben-Shlomo, Y. (eds.) (1997) A Life Course Approach to Adult Disease, Oxford : Oxford University Press.
4. Marmot, M. and Wadsworth, M. (eds.) (1997) 'Fetal and early child environment: long-term health implications.' British Medical Bulletin, 53.
5. Power, C. and Matthews, S. (1997) 'Origins of health inequalities in a national population sample' The Lancet vol. 350, pp.1584-89.