ADD is caused by a gene that is nicknamed the "migratory gene." The further you get from Africa the more common the gene is. It is theorized that this gene is responsible for human migration and that the people who paved the way had this gene.
“Stronger evidence that natural selection has continued to shape the brain in recent epochs comes from studies of DRD4, a mutation in a neurotransmitter receptor that Moyzis, Wang, and many others have linked to attention-deficit/hyperactivity disorder (ADHD). Children diagnosed with ADHD are twice as likely to carry the variant gene as those without the diagnosis. DRD4 makes a receptor in the brain less effective in bonding to dopamine, which might explain why Ritalin, which increases the amount of dopamine in the space between neurons, is often helpful in treating the problem.
Sequencing studies suggest that the DRD4 mutation arose 50,000 years ago, just as humans were spreading out of Africa. Its prevalence tends to increase the farther a population is from Africa, leading some investigators to dub it “the migratory gene.” At least one allele (or copy of the gene) is carried by 80 percent of some South American populations. In contrast, the allele is present in 40 percent of indigenous populations living farther north in the Americas and in just 20 percent of Europeans and Africans. Children with the mutation tend to be more restless than other youngsters and to score higher on tests of novelty-seeking and risk-taking, all traits that might have pushed those with the variant to explore new frontiers.
In the context of a modern classroom, it may be hard to understand why kids who appear distractible and disruptive might have a survival advantage. But research shows people with DRD4 do not differ in intelligence from national norms; if anything, they may on average be smarter. Moreover, behavior that may seem like a drawback today may not have been so in ancient environments. When broaching foreign terrain filled with unknown predators, “having the trait of focusing on multiple directions might have been a good thing,” Wang says. “People focused in one direction might get eaten.”