Saturday, July 05, 2008

When ADHD is an adaptive advantage ...

First, the news article (excerpt from FuturePundit: Hyperactivity Gene Helps Nomadic Tribesmen?). Then the anecdote ...
.... Kenyan nomads do better with an ADHD gene whereas those who have converted to settled living do worse with this same version of the DRD4 gene.
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...  

...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...
There's a personal connection. 

Today we couldn't find child B at the pool. Since he has the classic Aspie tendency to wander, we became a bit anxious. 

We recruited child A, who, among other traits, has pretty severe ADHD. He also has a weird visual talent. He can pick things or people he's interested in out of a very large amount of visual input -- almost instantaneously. 

I told him we needed his "laser eyes" to find his brother, and I walked behind him. Zip. In 1-3 seconds he says "B is not in the big pool". He walks to where I can barely make out people in the diving pool. Zip. "B is by the climbing wall." A few minutes later I was able to spot Child B where Child A said he was.

In our society Child A is disabled. In a hunter gatherer society, at a slightly older age, he'd walk to high ground, spot game across the veldt, then head home to await delivery of the finest cuts of meat and play with his many children. I, in the same society, would be long dead. 

Adaptation is local.

Update: See also an earlier post on the same theme.

Update 7/5/08: The Economist has a more detailed review.

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