We are embedded within an ever-changing social world; however, genetic influences on behavior were thought to be largely independent of this social world within any given lifespan. This notion was called into question by our work suggesting the social regulation of gene expression in human leukocytes (white blood cells) in a broad, representative sample of middle-aged and older adults in the Chicago Health, Aging, and Social Relations Study (CHASRS). As George Slavich and Steven Cole noted: "Contrary to the notion that our 'molecular selves' are fixed across time and situations … is increasing evidence that changes in the expression of literally hundreds of genes can occur as a function of the physical and social environments we inhabit." The important role of the human brain in modulating gene expression is suggested by the fact that people's perceived social isolation rather than objective social isolation is more closely related to the observed differences in gene expression. The implication is that certain genes can be "turned on" and "turned off" by different perceived social environments. Given the focus on older adults, we have studied these effects in leukocytes, but the effects are theoretically meaningful—the perception of being on the social perimeter is related to changes in gene transcripts in the leukocytes that prepare the organism for assaults.