Attention-deficit hyperactivity disorder (ADHD) affects an estimated 8 percent of children (12 percent of boys) and 4.4 percent of adults in the U.S. ADHD has a large heritable component (around 70 percent), suggesting that genes play a role in its etiology and that it can be modified by natural selection. Thus, ADHD’s high prevalence begs the question: Why hasn’t natural selection removed the genes that underlie ADHD from the human population? To begin to answer this question, and to better understand the phenomenon of ADHD, we must consider our current social environment, and the likely past environments that we have experienced over our evolutionary history, alongside genetic and molecular evidence.
We live in different social and ecological contexts than our ancestors. Widespread formal schooling and formal teaching are recent inventions of the past few hundred years. Before about 10,000 years ago, all humans were nomadic hunter-gatherers, without agriculture or domesticated animals. While our ancestors faced social pressures and needed to focus their attention to learn and practice complex foraging and hunting skills, the nature of the social and educational demands were qualitatively different from those we now face. While today we specialize in narrowly defined skills, hunter-gatherers were likely generalists, needing to acquire and practice a broad variety of subsistence and social skills.
From studies of modern hunter-gatherers, we can surmise that learning took place through play, observation, and informal instruction, rather than through the highly regimented classrooms almost all of us have experienced. It is no surprise that ADHD is usually diagnosed in children who have trouble focusing “properly” in school, and it continues to be a problem for adults when their work or lifestyle requires focusing in particular, regimented ways. There is good reason to believe that in our evolutionary past, ADHD was often not much of a problem and was perhaps even an asset.
Some intriguing evidence for this hypothesis comes from work on the genetics of ADHD. One gene associated with ADHD is called dopamine receptor D4 (DRD4), alleles of which change the sensitivity of a subtype of dopamine receptors that are expressed in the prefrontal cortex. ADHD is a complex trait (regulated by many genes), and the ADHD-associated allele in the DRD4 gene (called DRD4 7R) only accounts for a small portion of the cases of ADHD. Nonetheless, a variation of the DRD4 gene provides a window into the evolutionary forces that shaped our brain.
The 7R (ADHD-associated) allele of the DRD4 gene is peculiar in that it seems to have originated about 45,000 years ago and was then positively selected for. That is, the 7R allele conveyed some advantage to those who carried it