A Darwinian train of thought in danger of derailment

It is not clear why this book has been saddled with its elliptic and railway-centred main title, when the subtitle would have done the job much better. The connection is unfortunate because mention of railways nowadays conjures up memories of aborted journeys and disarray, feelings similar to those produced by reading the book's text. As its concluding statement is unquestionably sound, the journey does at least end well, but the argument along the way is at times distinctly shaky.

It would be hard to disagree with Richard Wilkinson's closing sentence: "If we are to improve health and social capital, if we are to free ourselves of anti-social prejudices and create a more inclusive society, the reduction of inequalities must surely be a key political objective." But this is self-evident. What is less obvious is the connection to Darwin's principle of evolution by natural selection, and yet this is meant to be the work's raison d'être .

The book, one of a series entitled "Darwinism Today", seeks to link the psycho-social dimension of health inequality, via the health-diminishing effects of chronic stress, to an evolutionary hypothesis about the significance of social inequality in human society. There can be no argument about the reality of the inequalities that exist, even in the most affluent nations, in health and life expectancy. These are real, long-standing and a severe indictment of societies that aspire to a just apportionment of the means to live lives free from preventable disease. When life span began to lengthen a few centuries ago, it became clear that the advances occurred first among aristocrats. The general populace lagged behind by a good century and a half. Even today, when most of us expect to live to the limit of our biological potential rather than be cut down in our prime, it is still the wealthy who tend to live longer.

This continuing inequality in longevity is but one of several lines of evidence that tell us that the biological ageing process is a great deal more malleable than previously thought. In earlier times, the rich might have led safer lives, thus having greater statistical expectation of life in a world full of extrinsic dangers. But the idea that the circumstances of our existence might affect the intrinsic rate of ageing has been a more recent realisation - one that fits with the emerging view that what makes us age is not some inexorable ageing clock, but the lifelong accumulation of subtle faults and damage within the cells of our bodies.

A central claim in the book is that the difference in material factors such as nutrition, housing and heating is less important in explaining the difference in health and longevity between the haves and have-nots than is the difference in social status and perception of self-worth - in other words, the social pecking order. Social status, Wilkinson asserts, has important effects on biological stress, which in turn affects our vulnerability to a wide range of diseases. Here, the locomotive of the book's argument runs on safe-enough rails, although the evidence that material factors are as unimportant as Wilkinson would have us believe is far from convincing.

The aroused state of stress presumably evolved to give us an edge when fighting or fleeing the perils of our ancestral environment, but our bodies are not designed to live in such an aroused state all of the time. Not only is chronic stress in adult life potentially bad for us, but much recent evidence shows that very young animals that are deprived of secure early emotional attachment to their mothers manifest lifelong perturbations of their normal stress-response mechanisms. This alternative programming of the stress-response reactions is revealed in the form of different levels of circulating stress hormones and altered emotional reactivity.

It is at this juncture that Wilkinson advances his evolutionary hypothesis, and it is here that the train of his argument advances on to shaky tracks. Transmission of stress from mother to offspring is suggested to be important in preparing the child for the kind of society and social relationships that it will have to deal with. Early social experience is seen as a taster for the kind of society in which the child will live its life. Therefore, Wilkinson asserts, early programming of stress responses might make evolutionary sense if it prepares individuals for responses appropriate to their social rank. In other words, we are offered a Darwinian take on an old idea: that it is important to know one's station in life.

The history of Darwinian thinking is littered with what have come to be called "just-so stories", ideas that make an entertaining tale but lack the harder rationale required by modern evolutionary life-history theory. To be tenable as an evolutionary hypothesis, rather than a just-so story, the hypothesis in question would need to be worked up into a convincing quantitative model that could explain why it would serve the individual's genes to follow such a course, when it is not at all obvious why this should be the case. The book lacks any such argument and the plausibility of the hypothesis rests on nothing more than unsubstantiated assertion.

The gap that needs bridging the most is the gap between the psycho-social perspective from which the author writes and the more biological side of his story. Yes, the effects of chronic stress are important, but the evidence for material factors also being important too is dismissed on scant evidence. For a book in a series that seeks to present the latest Darwinian thinking and to explore its application to humans, the result is disappointing.

Tom Kirkwood is professor of medicine, University of Newcastle upon Tyne. He gave the BBC's 2001 Reith Lectures, "The End of Age".