Friday, March 02, 2012

Social Determinants of Health: is Psychosocial Stress the Primary Determinant and what are the Implications for the Field of Epidemiology?

The evidence linking social factors to health outcomes has been researched since the 1940s (Krieger, 2001). The so-called social determinants of health has generated its own field of research called social epidemiology. The whole field hinges on social constructs or structural hierarchies embedded in human societies that heavily affect health. Psychosocial stress is one such risk factor that has been identified as an explanation for how social factors influence health outcomes. One of the big debates in public health is centered on competing theories of causation for the social determinants of health. This paper is an analysis of what the argument between psychosocial theorists and the neo-materialists says about the future of epidemiology.

One of the largest studies to garner support for the psychosocial risk factor is a case-control study by Rosengren et al. (2004). This large, multicentre study found an association between psychosocial stress and acute myocardial infarction (AMI). The researchers operationalized psychosocial stress by asking questions about eight variables: stress at work, stress at home, general stress, financial stress, stressful life events, locus of control, feeling depressed, and clinical depression. They concluded that the effects of psychosocial stress were independent of lifestyle factors such as socioeconomic status, smoking, and education and that “the size of the effect was less than that for smoking but comparable with hypertension and abdominal obesity”(p. 961).

The three dominant models of causation for the social determinants of health are neo-materialism, psychosocial theories, and the effect modification hypothesis (Jayasinghe, 2011; Marmot and Wilkinson 2001)
The neo-materialists posit that the root cause is the structure of society and the political system, and that the key to solving this problem is through development—providing material needs to people. The psychosocial theory is closer to the biomedical model in that it argues that there is a direct effect on the body’s immune system through the impact of stress from the environment, and that leads to higher rates of mortality. The effect modification hypothesis positions stress as a “risk regulator” (Marmot and Wilkinson 2001). This means that people in low-income groups have higher levels of stress, which in turn causes those people to make poor lifestyle choices, such as smoking tobacco or consuming excessive amounts of alcohol, and thus have poor health outcomes (Lynch et al., 1997).

The heated debate between these differing camps of causal models is both political and methodological in nature. The neo-materialists posit that the major factor is one’s materials or assets and that having a home, car, and computer are proxies for a higher quality of life. Research has confirmed that once one’s basic needs are met, there are significant health improvements(Macleod, et al., 2001; Lynch et al., 1997). Neo-materialists derive a large part of their evidence from studies on the ecological level, which makes it difficult to generalize to individuals. They also argue that the material level is more amenable to change than psychosocial stress, saying that reducing stress throughout the population is difficult if not impossible (Marmot and Wilkinson 2001).

The psychosocial theorists contend that people’s health is more than their material circumstances and, moreover, that understanding the exact physiological mechanisms of action provides a clearer picture of the disease process(Marmot and Wilkinson 2001). The psychosocial variable has proven to be a highly stable measure, albeit sometimes a subjective one. One of the major points of contention is that humans cannot be randomized to treatment groups; therefore we cannot truly ascertain causation because we have to resort to animal models to experiment on. This type of research brings us closer to understanding causal mechanisms, but because humans and primates are vastly different—despite being 98% genetically similar—we cannot fully accept evidence from those studies. However, there has been compelling evidence in primate models that shows conclusively that social hierarchies lead directly to higher levels of stress in those species. Many psychosocial theorists have measured stress objectively with biomarkers such as cortisol and glucocorticoids, which lead directly to atherosclerotic plaque build up, a major risk factor for cardiovascular disease (Sapolsky, 2005). Another argument made by the psychosocial theorists is that social hierarchies embedded within a country mask the neo-materialist’s hypothesis and demonstrate that key components of the psychosocial variable, autonomy and social capital, are the important building blocks to good psychosocial health(Marmot and Wilkinson 2001).

The effect modification hypothesis has strong evidence linking low socioeconomic status (SES) with poor behavioural choices, and some studies have shown that people of low SES are less likely to change their behaviour (Lynch et al,1997). This means that people in low SES groups have higher levels of stress and, due to their low income, cannot afford higher quality foods and health care, which in turn negatively affects their health. Many argue that this blames the individual and is the least productive of all theories(Lynch et al,1997). The criticism here is not so much about methodology as it is about the politically unsavory way it analyzes people.

From a reductionist point of view, the psychosocial theorists have a stronger argument than the neo-materialists because newer technological advances create the possibility that as time advances our abilities to measure will get better and better. According to the hierarchy of evidence, ecological studies are lower ranking, and psychosocial studies command a higher level of evidence. But because it is so difficult to reduce the social determinants of health into neat categories such as education or SES, there is a call from some contemporary epidemiologists for a paradigm shift away from such reductionist searches for causation.(Jayasinghe, 2011; Krieger 1994; Glass and McAtee 2006) Although reductionism works well in identifying strong infectious agents, it is ill-equipped to handle many complex systems like the social determinants of health(Jayasinghe, 2011; Glass and McAtee 2006)

This debate about causation is a digression from the real issue, which is to find workable solutions to issues raised by the social determinants of health. Epidemiology is in a state of transition or stagnation, depending on whether you view the glass as half empty or full. Many epidemiologists seem content to push an outdated model that can’t address many of today’s problems(Jayasinghe, 2011; Krieger 1994; Glass and McAtee 2006) To refocus on problem solving means moving epidemiology from a field distracted by concern with causation and controlling for bias and shifting it instead towards following Kreiger’s assertion that whatever the means of causation, be it direct or indirect, intervening on social determinants such as education and early childhood development should be a high priority in addressing health outcomes (1996).

References:

1.Krieger, N. Theories for social epidemiology in the 21 century: an ecosocial perspective. International Journal of Epidemiology. (2001);30:668-677.
2.Rosengren, A., Hawken, S., Yusuf, S. et al. Association of Psychosocial Risk Factors with Risk of Acute Myocardial Infarction in 11,119 Cases and 13,648 Controls from 52 Countries (the INTERHEART study): Case-control study. Lancet. (2004);364:953-962.
3.Jayasinghe, S. Conceptualising population health: from mechanistic thinking to complexity science [electronic article]. Emerging themes in Epidemiology. (2011). 8:2 http://www.ete-online.com/content/8/1/2.
4.Marmot, M., Wilkinson, R.G. Psychosocial and material pathways in the relation between income and health: a response to Lynch et al. British Medical Journal. (2001);322:1233-1236.
5.Lynch, J.W., Kaplan, G.A., Salonen, J.T. Why poor people behave poorly? Variation in adult health behaviours and psychosocial characteristics by stages of the socioeconomic life course. Social Science and Medicine. (1997); 44(6):809-819.
6.Macleod, J., Smith, S.D., Heslop, P., Metcalfe, C., Carroll, D., Hart, C.Are the effects of psychosocial exposures attributable to confounding? Evidence from a prospective observational study on psychological stress and mortality. J Epidemiol Community Health. (2001);55:878-884.
7.Sapolsky, R.M.The influence of social hierarchy on primate health. Science. (2005); 308(648); (doi: 10.1126/science.1106477).
8.Krieger, N. Epidemiology and the web of causation: has anyone seen the spider? Social Science Medicine. (1994);39(7):887-903.
9.Glass, T.A., McAtee, M.J. Behavioural science at the cross roads in public health: extending horizons, envisioning the future. Social Science and Medicine. (2006); 62:1657-1671.
10.Krieger, N., Zierler, S. What explains the public’s health?: A call for epidemiologic theory. Epidemiology.(1996);7(1):107-109.

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