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This is Scientific American's 60-Second Science, I'm Cynthia Graber. This'll just take a minute. 

If evolution weeds out detrimental¹ traits, why do some seem to stick around? Well what's bad in our current environment may have been good in the conditions under which we evolved. New research indicates that even the tendency toward Attention Deficit² Hyperactivity Disorder³ may have been beneficial under the right circumstances. The study was published this week in BioMed Central. Dan Eisenberg is an anthropologist⁴ at Northwestern University. He investigated two groups of the same tribe of Kenyans. One group still lives a traditional nomadic⁵ life style, and one recently settled in villages. About 20% of individuals in both groups have a genetic⁷ mutation⁸ that's associated with food cravings and ADHD. Eisenberg collected body mass index and height data of adult males. Those with the ADHD associated gene⁶ who were still nomads⁹ were better nourished than those without the gene. But those with the gene who'd settled down were less fit. The ADHD related gene may encourage behaviors beneficial for nomads. A boy with this allele might more effectively defend livestock¹⁰ or locate food and water sources. But maybe he wouldn't do so well at farming or sitting in a classroom. 

Thanks for the minute for Scientific American's 60-Second Science, and I'm Cynthia Graber.