Book Review

The Society and Population Health Reader: Income inequality and health

N Engl J Med 2000; 342:221January 20, 2000

Article

The Society and Population Health Reader: Income inequality and health
Edited by Ichiro Kawachi, Bruce P. Kennedy, and Richard G. Wilkinson. 501 pp., illustrated. New York, New Press, 1999. $40 (cloth); $19.95 (paper). ISBN: 1-56584-526-9 (cloth); 1-56584-571-4 (paper).

According to the United Nations Human Development Report published in 1996, the 358 richest persons on earth control assets equivalent to the annual income of 45 percent of the world's population. In the United States, the top 1 percent of income earners receives 12 percent of the entire country's pretax income and holds 37 percent of the wealth. At the same time, evidence is accumulating that disparities in income have profound social consequences and that the social and economic structure of a society may fundamentally determine the health of its members. The Society and Population Health Reader provides the reader with the original reports on income inequality and its effects on public health.

That being poor is associated with ill health would seem to go without saying. Many studies in the past three decades have established that absolute levels of income or education or class are related to morbidity and mortality. The Whitehall studies of the British civil service, begun in 1967, showed that mortality rates were three times as high among people in the lowest employment grades (porters) than among those in the highest grades (administrators) in a relatively homogeneous population of office-based civil servants who had access to comprehensive health care. The Whitehall studies and the Multiple Risk Factor Intervention Trial in the United States show that conventional risk factors (smoking, obesity, inactivity, and high blood pressure) explain only 25 to 35 percent of the differences in mortality rates among persons of different incomes. In fact, socioeconomic status is now understood to be the single most powerful determinant of health. This in itself is a stunning notion — one that is overlooked by physicians who are not oriented to population health.

Physicians are accustomed to think of the socioeconomic determinants of disease in terms of an individual person's risk factors. The story, revealed in these collected papers, is more complex. It now seems clear that absolute wealth or income is a less important determinant of health than the relative disparity in income or the income gap between the rich and the poor. Wilkinson is a leading proponent of the relative-income hypothesis. He finds no clear relation between income and health when comparisons are made between countries. For example, there is no relation between the per capita gross domestic product and life expectancy at birth in comparisons among developed countries with similar levels of industrialization.

Wilkinson and now a number of other scientists have, however, found a strong relation between income inequality and health. Countries with the longest life expectancy — Japan, Iceland, and France — are not necessarily the wealthiest, but they are the countries with the least income inequality. These relations are not limited to nations but can also be seen at regional and local levels. In the United States, for example, states with the narrowest, most equitable income distributions, such as New Hampshire, have longer life expectancy than less egalitarian states, such as Mississippi and Louisiana.

What connects income inequality to the health of individual persons? Biologic scientists have explored the connections between social rank and biologic determinants of disease, such as immune function and endocrine responses to stress. Several such studies are presented in this book. Social scientists have long asked why some communities have effective institutions, respect for the law, and healthy citizens, whereas others do not. Social capital, defined as civic engagement and mutual trust among community members, may be an important intervening variable between income inequality and health. Communities with high social capital, or cohesiveness, are associated with higher levels of political participation, lower levels of crime and delinquency, and better health status.

The conclusions reached by several of the book's authors, that socially engineered reductions in income inequality might produce substantial health benefits for the population, are certain to provoke controversy. The debate about an assured national income — dormant for 20 years — may be revived. The relative-income hypothesis has implications for matters of public policy, including education, poverty, and crime, and the authors suggest specific additional research that is needed to support the development of policy in these areas. Some policy makers are designing programs on the basis of socioeconomic research. Dr. David Satcher, the surgeon general of the United States, has, for example, made the reduction of inequalities in health based on race and class the priority of his office.

The Society and Population Health Reader is organized in four sections, each with an introductory essay that includes study questions and suggestions for further reading, followed by reprints of recent research articles — 40 in all — as they appeared in the medical literature. About half the articles are by the editors. Although some criticism of the relative-income hypothesis is presented, for the most part, the book reflects the editors' positions. For the general reader who is willing to stay the course and plow through the 40 articles, I cannot think of a better introduction to this fascinating field. This book is uniquely useful for the professional student of these topics or for research scientists developing an interest in the field.

Michael McCally, M.D., Ph.D.
Mount Sinai School of Medicine, New York, NY 10029