New World Disorder
Was ADHD an evolutionary asset?
Is ADHD a disease?
The U.S. government says it is. So does the professional Diagnostic & Statistical Manual for Mental Disorders. The condition's very name incorporates this assumption: attention-deficit/hyperactivity disorder. Lots of kids with ADHD have trouble functioning in modern society.
But what if society were different? What if it were structured so that having ADHD was actually an advantage?
This isn't some futuristic thought experiment. A new study suggests that this ADHD-friendly world may actually be part of our past.
The study, led by Dan Eisenberg of Northwestern University and published in BMC Evolutionary Biology, examined a Kenyan tribe called the Ariaal. Part of the tribe has recently settled into an agricultural community. Another part remains nomadic. The tribesmen were tested for DRD4 7R, a genetic variant that, Eisenberg notes, "has been linked to greater food and drug cravings, novelty-seeking, and ADHD symptoms." He and his colleagues report:
DRD4 7R+ genotypes were associated with indices of better nutritional status among nomads, particularly higher fat free mass, but worse indices in the settled individuals. This suggests that the 7R allele confers additional adaptive benefits in the nomadic compared to sedentary context.
This difference, the authors report, is "consistent with past findings of higher 7R allele frequencies in nomadic populations around the world."
But how would the gene help nomads? The authors speculate:
Increased impulsivity, ADHD-like traits, novelty-seeking like traits, aggression, violence and/or activity levels may help nomads obtain food resources, or exhibit a degree of behavioral unpredictability that is protective against interpersonal violence or robberies. … It might be that the attention spans conferred by the DRD4/7R+ genotype allow nomadic children to more readily learn effectively in a dynamic environment (without schools), while the same attention span interferes with classroom learning in Songa, the settled community. 7R+ boys might develop into warriors (the life-stage of an Ariaal male that lies between childhood and manhood) and men who can more effectively defend against livestock raiders, perhaps through a reputation of unpredictable behavior that inspires fear. Among 7R+ men in the settled community of Songa, such tendencies might be less well suited to practicing agriculture and selling goods at market. It might also be that higher activity levels in 7R+ nomads are translated into increased food production, while such activity levels in settled men are a less efficient use of calories in food production.
Remember, this is not a study of genetic differences between populations. The two Ariaal groups are genetically identical—the agricultural group became settled only 35 years ago—and the groups intermarry. The difference lies in their lifestyles. The point of the study is that the same gene has different effects in different settings.
Will Saletan covers science, technology, and politics for Slate and says a lot of things that get him in trouble.