ADHD an advantage for nomadic tribesmen?

A propensity for attention deficit hyperactivity disorder (ADHD) might be beneficial to a group of Kenyan nomads, according to new research published in the open access journal BMC Evolutionary Biology. Scientists have shown that an ADHD-associated version of the gene DRD4 is associated with better health in nomadic tribesmen, and yet may cause malnourishment in their settled cousins.

A study led by Dan Eisenberg, an anthropology graduate student from Northwestern University in the US, analyzed the correlates of body mass index (BMI) and height with two genetic polymorphisms in dopamine receptor genes, in particular the 48 base pair (bp) repeat polymorphism in the dopamine receptor D4 (DRD4) gene.

The DRD4 gene codes for a receptor for dopamine, one of the chemical messengers used in the brain. According to Eisenberg "this gene is likely to be involved in impulsivity, reward anticipation and addiction". One version of the DRD4 gene, the '7R allele', is believed to be associated with food craving as well as ADHD. By studying adult men of the Ariaal of Kenya, some of whom still live as nomads while others have recently settled, the research team investigated whether this association would have the same implications in different environments.

While those with the DRD4/7R allele were better nourished in the nomadic population, they were less well-nourished in the settled population. Although the effects of different versions of dopamine genes have already been studied in industrialized countries, very little research has been carried out in non-industrial, subsistence environments like the areas where the Ariaal live, despite the fact that such environments may be more similar to the environments where much of human genetic evolution took place.

Eisenberg explains, "The DRD4/7R allele has been linked to greater food and drug cravings, novelty-seeking, and ADHD symptoms. It is possible that in the nomadic setting, a boy with this allele might be able to more effectively defend livestock against raiders or locate food and water sources, but that the same tendencies might not be as beneficial in settled pursuits such as focusing in school, farming or selling goods".

These findings suggest that behavior differences previously associated with the DRD4 gene, such as ADHD, are more or less effective depending on the environment. Research into how this might occur in Ariaal children is planned in the near future.

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June 9, 2008