ment. Phenotype emerges as a function of this constant dialogue, and any effort to ascribe percentage values to isolated variables is likely to be biologically meaningless.

The critical questions in this arena concern gene expression. Studies of gene expression focus on the regulation of gene activity and the mechanisms of gene-environment interactions. The human genome project and the recent development of microarray chip technologies offer researchers remarkable tools for examining the development of vulnerability or resistance to disease. Microarray chips are thin wafers, approximately the size of a dime, containing a densely packed, orderly arrangement of thousands of different probes, utilized for matching known and unknown DNA samples. The technology monitors an entire genome on a single chip, thereby facilitating the detection of gene expression with unprecedented resolution (Ekins and Chu, 1999; Schena, 2000). Realizing the full potential of whole genome analyses will require multidisciplinary research projects that integrate molecular biology with physiology and the behavioral and social sciences. As modern genetics identifies individual or multiple genes associated with many human diseases, such advances will only underscore the importance of understanding the environmental factors that regulate expression of these and other genes.

Scientifically, the key task is to define the pathways that lead to disease. This requires understanding first how genes and related factors might be associated with the onset of particular disease outcomes and, second, tracking relevant mediating conditions. The behavioral and social sciences are essential to advancing knowledge of these environmental conditions. Full explication of health pathways thus hinges on integrative multidisciplinary research.

In the subsections that follow we present examples illustrative of the current evidential base that connects behavior and the social environment to gene expression (direct and indirect) and subsequent pathways to health outcomes. As a collectivity, these examples represent a coarse-grained description of what is, in fact, a dynamic process of interrelationships between social and physical environments and complex patterns of gene expression that operate over the entire lifetime of different organisms. The examples are each suggestive of new research directions that, if pursued, would notably enhance understanding of pathways to health outcomes.

GENE EXPRESSION AND PRENATAL DEVELOPMENT

Prenatal life represents a period of cell division and differentiation, the result of which is tissue formation and function. Development is costly in metabolic terms, requiring massive amounts of energy reserves to fuel the growth of bone, muscle, brain cells, and so on. The mother is the sole



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