The researchers, led by Dan Eisenberg of Northwestern University, said the same variation was associated with malnourishment in Kenyans having the same genetic background, but who had been living in settled communities for 35 years.

The study that included scientists from the University of Wisconsin-Milwaukee, the University of Nevada, and Boston University analyzed the body mass index and height of 65 nomadic men, and 87 settled men of the Ariaal -- a community in northern Kenya. Researchers also determined whether the men had variations in their DRD4 gene, one variation of which is associated with features of ADHD.

The scientists said men with that variation were better nourished in the nomadic population, but less well-nourished in the settled population. Eisenberg explained the variation "has been linked to greater food and drug cravings, novelty-seeking and ADHD symptoms," and that those traits might help nomads locate food and defend livestock, but the variation was found to be maladaptive among members of a settled population.

The study appears in the open access journal Evolutionary Biology.