Commentary: Missed opportunities.

the course of economic development, populations have become increasingly urban and geographically mobile. Cities created a new ecology for disease, and colonization brought unfamiliar diseases to colonizers and colonized alike. Urban death rates were usually very much higher than rural ones, and mortality rates from infections were often catastrophically high when new diseases were first introduced to populations. That economic development led to increased rates of urbanization may explain why economic growth rates were sometimes positively correlated with mortality. 11 However, there have also been processes of biological adaptation to the new risks. With new (as distinct from endemic) diseases, people lack the benefit of immunity acquired in early childhood , and selective process have not yet taken their toll of the most genetically susceptible sections of the population. After citing evidence that there were important genetic differences in susceptibility to tuberculosis, Burnett and White 12 gave examples of populations such as American Indians and Mauritians amongst whom it took—largely unaided by economic growth—around a 100 years for mortality rates from TB to fall from initially very high rates to rates as low as Europeans living in similar circumstances. These changes reflect some combination of the benefits of immunity acquired in early life to disease which have become endemic, and a process of genetic selection. Diseases in which there are both genetic differences in resistance, and high death rates before people reach reproductive ages, will tend to remove the most vulnerable sections of the population from the gene pool. Although McKeown and Lowe 3 thought economic growth was the most likely cause of the decline in infectious disease mortality, these process may account for why they also thought some diseases had become less severe. The four explanations I have suggested—increased assets, qualitative improvement, psychosocial liberalization and biological adaptation—may have worked singly or together. Surprisingly, 30 years after Preston's article we know very little of the balance between them. References 1 Preston S. The changing relation between mortality and economic development. If only we could sometimes go back and redirect the path of scholarship. Of course we cannot. Even when lives are at stake, as they are in finding the most effective policies to enhance survival, all we can do is to make the best judgments we can at each moment. Consider some alternative paths suggested by Samuel Preston's 1975 essay, 'The Changing Relationship between Mortality and Level of Economic Development' …