Chapter 8: Back to When You Were Just a Fertilized Egg

Sapolsky explores the role of genetics in shaping human behavior, beginning with the fertilized egg and extending to modern genomic science. Sapolsky dismantles simplistic notions of “genes for behavior,” showing instead that genes operate within networks, are regulated by the environment, and produce effects that are small, context-dependent, and intertwined with culture. In Part I: Genes from the Bottom Up, Sapolsky reviews how genes encode proteins but are regulated by transcription factors and promoters, with 95% of DNA being noncoding but crucial for regulation. Genes don’t autonomously “decide” anything; they are turned on and off by environmental signals, from intracellular energy states to hormones and social stimuli. This leads to epigenetics, where chemical modifications switch genes on or off, sometimes for life and even across generations, echoing “neo-Lamarckian inheritance.” He introduces introns and exons, showing how alternative splicing allows one gene to produce multiple proteins, and describes transposons, or “jumping genes,” which can alter neuronal DNA, producing brains that are mosaics of slightly different genomes. Chance at the molecular level further complicates gene action, revealing the limits of genetic determinism. In Part II: Genes from the Top Down—Behavior Genetics, Sapolsky traces the history of studying genetics through family resemblance, twin studies, and adoption studies, while acknowledging their flaws. Identical twins share more similar environments than fraternal twins, prenatal environments confound adoption studies, and selective placement skews results. He explains the difference between inheritance (genes influencing average traits, like humans having five fingers) and heritability (genes explaining variation within a population). Heritability is often overstated, inflated by controlled environments, and collapses when environments broaden. For example, IQ heritability is high in wealthy environments but nearly disappears in poverty, where deprivation overwhelms genetic potential. Sapolsky emphasizes gene–environment interactions, such as phenylketonuria depending on diet, or depression risk depending on childhood trauma combined with serotonin transporter variants. In Part III: What Do Genes Actually Have to Do with Behavior?, Sapolsky reviews molecular genetics. Candidate gene studies focus on serotonin, dopamine, oxytocin, vasopressin, and steroid receptors. Variants in MAO-A and 5HTT (serotonin transporter) have been linked to impulsive aggression, but only in combination with childhood abuse or stress, undermining myths like the “warrior gene.” Dopamine receptor variants, especially DRD4-7R, are linked to novelty seeking, ADHD, and risk taking, but also heightened prosociality under secure attachment or religious priming. COMT and DAT gene variants influence stress regulation, impulsivity, and emotional reactivity, again contingent on environment. Oxytocin and vasopressin receptor variants affect bonding, empathy, and generosity, but can also predict xenophobia or aggression depending on context. Testosterone and estrogen receptor gene variants show small effects on aggression, anxiety, and sex differences, amplified only when interacting with hormonal environments. Sapolsky then explains how genome-wide association studies (GWAS) reveal that behaviors are massively polygenic: hundreds of genes each contribute tiny effects. Even large studies of height or educational attainment explain less than 10% of variation, demonstrating that no single gene dictates complex traits. Behavior emerges not from one “behavior gene” but from countless interacting genes and environments. The chapter concludes that genes matter, but not in deterministic ways. They provide propensities, vulnerabilities, and potentials, always shaped by environment, culture, and chance. Sapolsky cautions against both genetic denialism and genetic essentialism: the reality is that behavior arises from context-dependent networks of genes and environments, not immutable codes.