1. The Ultimate Causes of ADHD
Austin Pittsley
Purdue University
Word Count: 2,104

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Research within the past decade of the psychological condition ADHD has come to the
conclusion that genetic influences are a very important part of its overall etiology (Faraone and
Mick 2010). Although no single gene seems to cause ADHD, and numerous environmental
factors play significantly influential roles, this still begs the important question of ADHD’s
evolutionary status. The significance of this question is summed up by Williams and Taylor
(2006:399), “The evolutionary status of [ADHD] is central to assessments of whether modern
society has created it… and is potentially useful in understanding its neurobiological basis and
treatment.” For a trait to have arisen from evolutionary processes, three conditions must be met.
First, the trait must be heritable; able to be passed down generations. Second, the trait must be
variable; multiple phenotypes must exist. Finally, differential selection must occur such that
certain phenotypes are more likely to survive and reproduce than other phenotypes, thus
increasing the frequency for such a trait. The first two conditions have so far been met. The
heritability of ADHD is undisputed; in fact it is one of the most heritable psychological
conditions (Faraone and Mick 2010). Similarly, the presence of variation is also undisputed.
ADHD, by current definitions, is a variant of typical human cognition, characterized by
increased hyperactivity, impulsivity, and inattention (APA 2013:97-99).
The final condition, that the trait confers differential reproductive success, has yet to be
confirmed and is the subject of this paper. At first glance it would seem that ADHD is a huge
detriment to the individual, and thus could not arise evolutionarily because it would be selected
against. Research has consistently shown that individuals with ADHD are at greater risk for
numerous problems, such as injury, substance abuse, and poor educational attainment, just to
name a few (Murphy and Barkley 1996). How then could ADHD possibly have evolved and
achieved such high frequencies we see today? In this paper, I will present some of the most

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recent research attempting to answer this question. The following hypotheses can be roughly
split into two categories. The first inquiries into whether ADHD-like qualities may have been
evolutionarily beneficial to our hominin ancestors and today are simply leftovers of a bygone
era. The second deals with a specific gene coding for the dopamine receptor D4, the DRD4 gene,
which has frequently been associated with ADHD-like behaviors, and thus received a great
amount of research.
PAST ADVANTAGES OF ADHD
Two of the first comprehensive explanations for ADHD’s past reproductive benefits are
that it is conducive to those involved in hunting and warfare-related tasks (Shelley-Tremblay and
Rosen 1996). According to the hunter theory, first posed by Thomas Hartmann in his book
Attention deficit disorder: a different perception (1993), successful pre-agricultural hunters
possessed many of the same qualities as those with ADHD. The ability to rapidly switch their
attention, constantly monitor their surroundings, and hyper-focus on the task at hand would have
been extremely beneficial to the hunter. However, these traits are maladaptive to settled
agricultural lifestyles, as well as modern lifestyles, and those with ADHD today simply possess a
vestigial mindset from our pre-agricultural ancestors. Shelley-Tremblay and Rosen (1996) cast
doubt on Hartmann’s hunter theory, pointing out that successful hunting also requires stealth,
concentration, and patience; qualities often lacking in ADHD. They likewise bring up the fact
that pre-agricultural humans obtained most of their diet from gathering (with a few exceptions),
and claim hunting would have exerted little selective pressure. Instead, Shelley-Tremblay and
Rosen argue that warfare and the struggle for resources with other organisms may have provided
a necessary advantage. Citing the hyperactivity and aggressiveness associated with ADHD, they
claim that these qualities, especially if they were encouraged and cultivated, would have

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benefitted people in competition for food resources. Unfortunately, Shelley-Tremblay and Rosen
bring up scant evidence for this environment of constant conflict and warfare being pervasive
conditions. Whether warfare, competition, and hunting could have provided the necessary
selective pressure is yet uncertain and has received little more research.
Impulsivity and unpredictability are two other qualities that have received attention from
researchers and may possibly have provided selective advantages. Fairbanks et al. (2004) studied
social relations of male vervet monkeys and found that social impulsivity among migrating
adolescent males strongly predicted attainment of alpha male status. While there are many
limitations when comparing human and non-human social practices, the study provides a real-
world instance of ADHD-like behavior greatly increasing reproductive success in a quasi-human
social situation. In addition, the impulsivity found among the males who obtained alpha-status
subsided as they aged. ADHD among humans shows a somewhat similar pattern; childhood
ADHD does not always lead to ADHD in adulthood. Unpredictability was shown by Williams
and Taylor (2006) to be useful among foraging groups. The researchers ran simulations of
foraging parties where 0%, 5%, 25%, and 100% of the group foraged unpredictably; choosing
food items at random. There results showed that groups where 5% of the members foraged
unpredictably maximized their foraging activities. The unpredictable members took extra risks,
consuming food of dubious toxicity for example, but gained more food knowledge in the
process. This food knowledge was then spread to the group, and the group benefited overall. If
too many members were predictable (0% unpredictability) then food knowledge was gained
slowly, and if too many members were unpredictable then too many risks were taken, eventually
outweighing any benefits. Further tests by the pair demonstrated how such unpredictability could
be selected for at the group level and maintained in low levels by a population. Williams and

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Taylor’s theory is appealing for a few reasons. First, foraging contributed greatly to the diets of
hunter-gatherer groups, and a trait that aided such a task would have been positively selected for.
Second, their theory explains why ADHD is present at such low frequencies by being selected
positively for a group level. However, it must be remembered that for a trait to undergo natural
selection, it must confer some reproductive advantage on the individual, because that is the level
at which traits are passed along. The theory is also contingent upon the assumption that the
foraging context they modeled actually happened, and happened often enough for selection to
act.
Another proposed benefit of ADHD and ADHD-like behaviors incorporates evidence of
prenatal stress and later outcomes and ADHD’s apparent polygenetic nature. Glover (2011)
theorizes that psychopathologies associated with prenatal stress, namely increased anxiety,
ADHD, and conduct disorders may be adaptive responses to the stressful environments causing
the prenatal stress. Anxiety and distracted attention may make one more vigilant and perceptive
of constantly shifting environmental dangers. Impulsivity may increase willingness to explore
new environments, and aggression and conduct disorders may be adaptive in a very competitive
and predatory environment. Genes may alter one’s plasticity to the prenatal stress response,
producing stronger affects in some but not others. Therefore, genetic variations linked to ADHD
and ADHD-like behaviors may have been selected for among populations experiencing greater
amounts of stress. This theory, unlike those before, combines multiple etiological factors to
explain how they work in tandem to increase fitness in a certain environment. Unfortunately, this
theory is also contingent upon certain assumptions about the environments ancestral humans
lived in.

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THE DRD4 GENE: AN EVOLUTIONARY LINK?
` Although molecular genetic research of ADHD has implicated numerous genes as causal
factors, the DRD4 gene specifically has received the vast amount of research attention to date.
The DRD4 gene codes for part of dopamine receptor D4, and is one of the most variable genes in
the human genome. Variants of this gene, specifically the 2-repeat and 7-repeat alleles, have
been associated with ADHD and ADHD-like behaviors such a novelty-seeking (Ding et al.
2002). Ding et al. found in their analysis of various DRD4 alleles that the 7R allele showed
evidence of having been naturally selected for. The 7R allele, they argue, required such a great
number of alterations to produce that its current frequency is not compatible with it being simply
a result of random mutation. Rather, the high frequency can be best explained as due to having
been natural selected for. Wang et al. (2004) built upon this conclusion in their analysis of the
DRD4 gene, and concluded that the 7R allele probably arose as a variant of the 2R allele about
40,000-50,000 years ago. Interestingly, the 2R allele is associated with similar novelty-seeking
behaviors as the 7R allele, albeit much more moderately. Thus, it seems likely that the 7R allele
arose as a mutation from the 2R allele, which may have already undergone selective pressure and
achieved higher frequency among populations in stressful, competitive environments. The 7R
allele would then have conferred an even greater advantage than the 2R allele.
Further research of the 7R allele has focused on its prevalence and effects among
nomadic populations of people. Chen et al. (1999) collected data on the frequency of the 7R
allele from multiple populations worldwide. Their results showed a strong association between
higher frequencies of the 7R allele and migration histories. They also tested the frequency of
other alleles that have been well-documented and found no such correlation with migration
history as they did with the 7R allele. Chen et al. suggest that the 7R allele indirectly encouraged

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migration and was advantageous to such populations. Testing this theory, Eisenberg et al. (2008)
studied the relationship between the 7R allele and nutrition among Ariaal men in northern
Kenya. The Ariaal are primarily nomadic pastoralists, but many also recently began living in
settled, semi-agricultural communities. Eisenberg et al., using numerous metrics to assess
nourishment, found that 7R allele carriers were better nourished among the nomadic
communities than non-carriers, but were more poorly nourished among settled communities.
Similar to some of the other studies presented here, Eisenberg et al.’s results suggest that the 7R
allele, and presumably the novelty-seeking and ADHD-like behavior associated with it, is
beneficial traits in specific contexts, such as nomadism.
More recent research of the 7R allele has studied its connection to qualities like altruism,
acculturation, and social behavior. Sasaki et al. (2013) examined the interaction between the 7R
and 2R alleles, and religious prosocial priming. Their results found that religious prosocial
priming led to more prosocial behavior versus a neutral prime in those carrying the 7R and 2R
alleles, but no difference in prosocial behavior was found between the primes among non-
carriers. Similarly, Kitayama et al. (2014) found the 2R and 7R carriers were more norm-
congruous with respect to independence versus interdependence. They compared independence
and interdependence among European-American and Asian-born Asian university students, and
then examined whether the 7R and 2R alleles had any effect. The results were striking. Although
on average, the European-Americans were more independent and the Asian-born Asians were
more interdependent, this difference disappeared among non-carriers and increased drastically
among carriers of the 2R and 7R alleles. This line of research into the social effects of DRD4
alleles is relatively new, but so far the results have been positive. It seems that the 2R and 7R

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alleles greatly influence susceptibility to social and cultural pressures, making carriers more
prosocial, independent, or interdependent when there is social pressure to do so.
CONCLUSION
The ultimate cause of ADHD and ADHD-like symptoms is still uncertain to this date.
However, a testable hypothesis can be constructed from the research I’ve presented here.
Executive cognitive functions are plastic traits, and ADHD is a variant of such functions that is
particularly beneficial to stressful environmental conditions where resources are scarce and
competition is high. Genetic factors like the 2R and 7R genes mutated and were selected for in
these environments, and thus increased in frequency and prevalence. Other etiological factors
have similar effects to those genes; maternal stress brought on by environmental factors would
cause fetal changes that made offspring more suited to dealing with those stressors. The
particular qualities found in ADHD that would have been potentially beneficial can be grouped
into three basic advantages. First, ADHD individuals have selective attention and heightened
impulsivity that enhances their skill at certain tasks, many possibly related to the important
activity of foraging. Second, impulsivity may have encouraged migration from unfavorable to
more favorable environments, encouraging humans to seek out and utilize new untapped
resources. Third, ADHD individuals and those with the risk factors presented here show an
increased susceptibility to social and cultural pressures, which may have increased altruistic and
social tendencies when beneficial for the individual, as well as enhancing one’s ability to gain
social capital and resources. While these conclusions of mine are far from proven, they are the
most promising avenues for future research seeking to find what benefits ADHD could have
provided, and thus its ultimate cause.

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BIBLIOGRAPHY
American Psychological Association. 2013. Diagnostic and Statistical Manual of Mental
Disorder, Fifth Edition. 97-103.
Chen C., et al. 1999. Population Migration and the Variation of Dopamine D4 Receptor (DRD4)
Allele Frequencies Around the Globe. Evolution and Human Behavior 20:309-24.
Ding Y., et al. 2002. Evidence of positive selection acting at the human dopamine receptor D4
gene locus. Proceedings of the National Academy of Science 99:309-14.
Eisenberg D., et al. 2008. Dopamine receptor genetic polymorphisms and body composition in
undernourished pastoralists: An exploration of nutrition indices among nomadic and
recently settled Ariaal men of northern Kenya. BMC Evolutionary Biology 8.
Fairbanks L., et al. 2004. Adolescent Impulsivity Predicts Adult Dominance Attainment in Male
Vervet Monkeys. American Journal of Primatology 64:1-17.
Faraone S. and Mick E. 2010. Molecular Genetics of Attention Deficit Hyperactivity Disorder.
Psychiatric Clinics of North America 33:159-180.
Glover V. 2011. Annual Research Review: Prenatal stress and the origins of psychopathology:
an evolutionary perspective. Journal of Child Psychology and Psychiatry 52:356-67.
Hartmann T. 1993. Attention deficit disorder: a different perception. Lancaster, UK:
Underwood-Miller.

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Kitayama S., et al. 2014. The Dopamine D4 Receptor Gene (DRD4) Moderates Cultural
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Murphy K. and Barkley R. 1996. Attention deficit hyperactivity disorder adults: Comorbidities
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Wang E., et al. 2004. The Genetic Architecture of Selection at the Human Dopamine Receptor
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