[Social inequalities in health, early child development and biological embedding].

This paper describes evidence that led to the concept of biological embedding and introduces approaches designed to elucidate its mechanisms and role. Biological embedding occurs when: experience gets under the skin and alters human biological and developmental processes; systematic differences in experience in different social environments in society lead to systematically different biological and developmental states; these differences are stable and long term; and, finally, they have the capacity to influence health, well-being, learning, or behavior over the life course. The concept of biological embedding emerged from insights in population health on the unique characteristics of socioeconomic gradients: ubiquity in poor and post-scarcity societies alike; gradient seen regardless of whether SES is measured by income, education or occupation; cutting widely across health, well-being, learning and behavior outcomes; replicating itself on new conditions entering society; and, often, showing that flatter gradients mean better overall societal outcomes. Most important, the gradient begins the life course as a gradient in developmental health, suggesting that the emergence of a multifaceted resilience/vulnerability early in life is the best place to look for evidence of biological embedding. In order to understand its character, the metaphor of the "archeology of biological embedding" has been used, wherein the surficial stratum of the "dig" is experience and behavior; the shallow stratum is organ system and cellular function; and the deep stratum is gene function. We are now ready to address the fundamental question of biological embedding: how do early childhood environments work together with genetic variation and epigenetic regulation to generate gradients in health and human development across the life course?