ADHD & Pastoralism
About one in 20 children have a group of symptoms that has come to be known as attention-deficit hyperactivity disorder (ADHD). About 60% of them carry those symptoms into adulthood. ADHD is believed to be genetic, and is associated with particular variants of receptor molecules for neurotransmitters (chemicals that carry messages between nerve cells) in the brain. In the case of ADHD, the neurotransmitter is often dopamine, which controls feelings of reward and pleasure. People with ADHD apparently receive positive neurological feedback for “inappropriate” behaviour.
ADHD sufferers are impulsive. They have trouble concentrating on any task unless they receive constant feedback, stimulation and reward. They tend to perform poorly in modern society and are prone to addictive and compulsive behaviour.
Dan Eisenberg speculated that such behavior may be advantageous for people who lead a peripatetic life. Since today’s sedentary city dwellers are recently descended from such people, natural selection may not have had time to purge the genes that cause it.
Eisenberg tested this by studying the Ariaal, a group of pastoral nomads who live in Kenya. The receptor Mr Eisenberg looked at was the 7R variant of a protein called DRD4, a variant is associated with novelty-seeking, food- and drug-cravings, and ADHD. (See “Dopamine receptor genetic polymorphisms and body composition in undernourished pastoralists.”)
The researchers looked for 7R in two groups of Ariaal. One was still pastoral and nomadic. The other had recently settled down. They found that about a fifth of the population of both groups had the 7R version of DRD4. However, the consequences of this were very different. Among the nomads, who wander around northern Kenya herding cattle, camels, sheep and goats, those with 7R were better nourished than those without. Among their settled relations, those with 7R were worse nourished than those without it.
This discovery fits past findings that 7R and a set of similar variants of DRD4 (the “long alleles”) are more common in migratory populations.
There remains the question of why 7R is found in only a fifth of the Ariaal population. One possibility is that its effects are beneficial only when they are not universal, and some sort of equilibrium between variants emerges.