over a long period of time regardless of whether they were in a gathering or farming society. K. H. Ko further states that the decreased need for 'hyperfocus' was building the conditions favourable for human language. Glickman & Dodd (1998) found that adults with self-reported ADHD scored higher than other adults on self-reported ability to hyper-focus on "urgent tasks", such as last-minute projects or preparations. Adults in the ADHD group were uniquely able to postpone eating, sleeping and other personal needs and stay absorbed in the "urgent task" for an extended time. Positive/negative gene selection Genetic variants (such as DRD4 7R allele) conferring susceptibility to ADHD are very frequent — implying that the trait had provided selective advantage in the past. However, "it is unlikely that the observed bias towards nonsynonymous amino-acid changes has been achieved only by positive selection favoring the DRD4 7R allele because the bias can be found not only in VNTR motifs of the DRD4 7R allele but also in those of the other alleles", observed by more recent research — the DRD4 7R allele had no evidence for recent positive selection. Furthermore, a recent 2020 study found that ADHD allele frequency has been decreasing for up to 35,000 years, indicating negative selection. These conclusions however, are still compatible with the theory: "Overall, our results are compatible with the mismatch theory for ADHD but suggest a much older time frame for the evolution of ADHD-associated alleles compared to previous hypotheses." Frequency of ADHD in nomadic tribes A 2008 New Scientist article by Tim Callaway reports that research of ADHD and related traits in different cultures offers some support for the hunter versus farmer hypothesis. According to evolutionary anthropologist Ben Campbell of the University of Wisconsin–Milwaukee, studies of the Ariaal, an isolated nomadic group in Kenya, suggest that hyperactivity and impulsivity—key traits of ADHD—have distinct advantages to nomadic peoples. Additionally, nomadic Ariaal have high rates of a genetic mutation linked to ADHD, while more settled Ariaal populations have lower rates of this mutation. Henry Harpending of the University of Utah reports that with this genetic mutation, "You probably do better in a context of aggressive competition." A genetic variant associated with ADHD has been found at higher frequency in more nomadic populations and those with more of a history of migration. Consistent with this the health status of nomadic Ariaal men was higher if they had the