Is ADHD an Advantage For Nomadic Tribesmen In Kenya? (6/13/2008)

A propensity for attention deficit hyperactivity disorder (ADHD) might be beneficial to a group of Kenyan nomads, according to a new study led by a Northwestern University graduate student.

The research, published today (June 10) in the open access journal BMC Evolutionary Biology, shows that an ADHD-associated version of the gene DRD4 is associated with better health in nomadic tribesmen, yet may cause malnourishment in their settled cousins.

"Our findings suggest that some of the variety of personalities we see in people is evolutionarily helpful or detrimental, depending on the context," said Dan Eisenberg, a first-year anthropology graduate student in Northwestern's Weinberg College of Arts and Sciences and lead author of the study. "This insight might allow us to begin to view ADHD as not just a disease but something with adaptive components."

The study looked at a group of traditionally cattle-herding people called the Ariaal who live in northern Kenya. Eisenberg collaborated with Benjamin Campbell, a visiting assistant professor of anthropology at the University of Wisconsin - Milwaukee, who ran the fieldwork.

Eisenberg and his research team analyzed the correlates of body mass index (BMI) and height with two dopamine receptor genes, DRD4 and DRD2. The DRD4 gene codes for a receptor for dopamine, one of the chemical messengers used in the brain. "This gene is likely to be involved in impulsivity, reward anticipation and addiction," said Eisenberg.

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 tribe of Kenya, some of whom still live as nomads while others have recently settled and begun to grow crops, the researchers investigated whether the different versions of DRD4 have the same manifestations in different environments.

The researchers found a striking difference in the two populations. Those with the DRD4/7R allele were better nourished in the nomadic population but less well-nourished in the settled population.

The effects of different versions of dopamine genes have been studied in industrialized countries, but very little research has been carried out in non-industrial, subsistence environments like the areas where the Ariaal live. This is despite the fact that such environments may be more similar to the environments where much of human genetic evolution took place.

"The DRD4/7R allele has been linked to greater food and drug cravings, novelty-seeking and ADHD symptoms," said Eisenberg. "It is possible that in a 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, might vary in effectiveness depending on the environment. Research into how this might occur in Ariaal children is planned in the near future.

Note: This story has been adapted from a news release issued by the Northwestern Unversity

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