ABOUT one in 20 children (those under 18) 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. For what is, at root, a genetic phenomenon, that is a lot—yet many studies have shown that ADHD is indeed genetic and not, as was once suspected, the result of poor parenting. It is associated with particular variants of receptor molecules for neurotransmitters in the brain. A neurotransmitter is a chemical that carries messages between nerve cells and, in the case of ADHD, that chemical is often dopamine, which controls feelings of reward and pleasure. The suggestion is that people with ADHD are receiving positive neurological feedback for inappropriate behaviour. The surprise is that the variant receptors are still there. Natural selection might have been expected to purge them from the population unless they have some compensating benefit.

Of course, this analysis turns on the definition of “inappropriate”. The main symptom of ADHD is impulsiveness. Sufferers have trouble concentrating on any task unless they receive constant feedback, stimulation and reward. They thus tend to flit from activity to activity. Adults with ADHD tend to perform poorly in modern society and are prone to addictive and compulsive behaviour. But might such people do well in different circumstances?

One hypothesis is that the behaviour associated with ADHD helps people, such as hunter-gatherers and pastoral nomads, 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.

Dan Eisenberg, of Northwestern University in Illinois, and his colleagues decided to test 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. Previous work has shown that this variant is associated with novelty-seeking, food- and drug-cravings, and ADHD.

The team looked for 7R in two groups of Ariaal. One was still pastoral and nomadic. The other had recently settled down. As they report in this week’s BMC Evolutionary Biology, 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. The opposite was true of their settled relations: those with 7R were worse nourished than those without it.

How 7R causes this is not yet known. It may stem from behavioural differences or it may be that different versions of DRD4 have different effects on the way the body processes food. Nevertheless, this discovery fits past findings that 7R and a set of similar variants of DRD4, known collectively as “long alleles”, are more common in migratory populations.

One suggestion is that long-distance migration selects for long alleles (see chart) because they reward exploratory behaviour. This might be an advantage in migratory societies because it encourages people to hunt down resources when they constantly move through unfamiliar surroundings.

As for the Ariaal, there remains the question of why 7R—although it is apparently beneficial to a nomadic way of life—is found in only a fifth of the population. One possibility is that its effects are beneficial only when they are not universal, and some sort of equilibrium between variants emerges. A second is that the advantage is gained when 7R exists along with another version of DRD4 (the genes for the two variants having come from different parents). Unfortunately, the way Mr Eisenberg collected the data does not allow these hypotheses to be tested. http://louis-j-sheehaN.NET

Either way, his research raises the question of whether people suffering from ADHD and conditions related to it, such as addiction, are misfits coping with a genetic legacy that was useful in the evolutionary past, but is now damaging. As society continues to diverge from that evolutionary past, the economic and social consequences of being such a misfit may become increasingly important.



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