This paper focuses on the past and current effects of the impulsivity trait associated with ADHD, specifically the hyperactive/impulse subtype of ADHD, also referred to as ADHD-HI. One of the main questions in the class discussion was what is the difference between ADHD and ADHD-HI…according to various medical websites, I discovered that ADHD, or attention deficit hyperactivity disorder, has three basic forms: primarily inattentive, primarily hyperactive-impulsive, and a combination of the inattentive and hyperactive-impulsive. The combination ADHD is the most common, the primarily inattentive kind is often seen, especially in girls and adults, but is less common than the combination type, and the hyperactive-impulsive type, as discussed in this paper, is rare.
The paper looks at the possible influences of the persistence of ADHD in the population. Since ADHD is associated with an allele that has been shown to be positively selected for since the appearance of the anatomically modern human, the seven-repeat allele of dopamine receptor type D4 (DRD4-7R), perhaps ADHD is also selected for. The authors suggest that ADHD’s most characteristic feature is behavioral variability, which manifests itself as impulsivity, the willingness to take risks, and novelty-seeking. Since novelty-seeking has also been associated with DRD4-7R, the authors propose that perhaps this trait has been selected for and is beneficial to certain types of society, including female-dominated farming and migratory societies. This might explain why the DRD4-7R allele is prevalent in South America. Those who migrated to South America as an empty ecological niche would be more likely to explore if prone to novelty-seeking.
The authors seem to be examining the possibility of whether ADHD-HI have been selected for as a benefit to individual fitness. Besides being adaptive in the sense of behavioral variability, the authors suggest several other ways it might be adaptive to the individual: having the ability to gain maternal attention, to be more creative, to fight in an unpredictable manner, and to be more aggressive. However, with each possible adaptive behavior, they list a negative drawback, leading the reader to believe that perhaps they don’t believe in the idea of ADHD-HI as an benefit to individual fitness.
The authors hypothesize that the individual with ADHD-HI may use their novelty-seeking and risk-taking behaviors to gain exploratory knowledge, which can be advantageous for other members of his social group (ADHD-HI, as rare as it is, is most often seen in boys). The few individuals with ADHD-HI in the social group test social limits, perform risky experiments, and explore physical space, while the reliable and predictable non-ADHD members of the social group learn from this information risk-free.
The authors demonstrate this hypothesis with a computer simulation that loosely uses the optimal diet model of foraging in hunter-gatherers. A food-changing task is used to represent a changing environment and the accuracy with which each group obtains knowledge of food quality. Four groups with 40 members each are used in the simulation: 2 homogenous groups entirely composed of either “predictable” or “unpredictable” individuals and 2 mixed groups composed of either 5% or 25% unpredictable members. The “undpredictable” members simulate the risk-taking individuals with ADHD-HI. Of the 4 groups, the one containing the 5% “unpredictable” members, or 2 members out of the 40, gained the most accurate knowledge of food quality the fastest, thus the group members died more slowly over time. In the group with all predictable individuals, members were more likely to stick to only the foods that they knew were of good quality, so members died relatively quickly over time of malnutrition. In the group with all unpredictable individuals, the members were more likely to take risks and try foods of unknown quality but did not learn from the mistakes of others, thus dying relatively quickly over time from food poisoning.
Therefore, the authors conclude that perhaps ADHD-HI was selected for via group selection, rather than individual selection, early on in hominid evolution in certain types of societies like the hunter-gatherer society, where members had to forage for foods of both known and unknown quality. This disorder perhaps persists in humans today since the cost of including an unpredictable minority in society is small compared to the cost of lacking such individuals in a rapidly changing environment.
In class we discussed how the authors didn’t really go into a detailed account of how this group selection specifically occurred, in a genetic sense. They didn’t deal with the possible problems with their group selection theory, such as how group selection would even happen when individual selection will always occur at a faster rate when conflicting selection pressures are at work. In my opinion, even though the authors did ambitiously attempt to hypothesize about a disorder of relatively unknown cause, their group selection theory was not supported with facts and was unconvincing at best.