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ADHD may have had evolutionary benefits

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ADHD may have had evolutionary benefits

While current diagnostic definitions of attention-deficit hyperactivity disorder (ADHD) are relatively new, the general condition has been identified by clinicians under a variety of names for centuries. Recent genetic studies have revealed the condition to be highly heritable, meaning the majority of those with the condition have genetically inherited it from their parents.

Depending on diagnostic criteria, anywhere from two to 16% of children can be classified as having ADHD. In fact, increasing rates of diagnosis over recent years have led to some clinicians arguing the condition is overdiagnosed.

What is relatively clear, however, is that the behavioural characteristics that underpin ADHD have been genetically present in human populations for potentially quite a long time. And that has led some researchers to wonder what the condition’s evolutionary benefits could be.

Imagine you are part of a wandering tribe of early humans. Your group comes across a field full of one kind of fruit and everyone is faced with a big question. Do you settle in the field and exploit the fruit stocks until they are all gone, or do you quickly take what you can and continue to explore for more varied foods?

This exploit or explore trade-off is fundamental to the survival of all animals. At what point is the risk of staying in one place greater than the risk of moving on to find out what is over the next hill?

In the early 2000s a team of scientists set out to study the genetics of a unique tribe of people in Northern Kenya. Known as the Ariaal, this population has traditionally been incredibly nomadic. Some members of the Ariaal settled down in one place over the 20th century and adopted modern methods of agriculture while other tribe members continued to live as nomadic pastoralists.

The scientists compared the genetic and health differences between these two cohorts of Ariaal and discovered something incredibly interesting. Generally, all of the Ariaal people carried a unique genetic mutation, dubbed DRD4/7R. This genetic trait had previously been identified commonly in people with ADHD.

In modern children diagnosed with ADHD, the genetic mutation generally correlates with restlessness and distractibility. And in those Ariaal children who had settled into sedentary Western behaviours, the gene was linked to poor health and distracted classroom behaviours. But in those Ariaal who still practised a traditional nomadic life, the gene mutation was linked to strength and better nutritional health.

“The DRD4/7R allele has been linked to greater food and drug cravings, novelty-seeking, and ADHD symptoms,” explained study leader Dan Eisenberg back in 2008. “It is possible that in the nomadic setting, a boy with this allele might be able to more effectively defend livestock against raiders or locate food and water sources, but that the same tendencies might not be as beneficial in settled pursuits such as focusing in school, farming or selling goods.”

So a fascinating hypothesis emerged. Could the genetic traits of ADHD be somewhat beneficial to a tribe by pushing some people to be ‘explorers’? What manifests in modern times as fidgety restlessness could actually have been useful to tribes foraging the countryside for food.

David Barack, from the University of Pennsylvania, along with a team of colleagues set out to empirically test that hypothesis. They produced a unique game where players were given eight minutes to collect as many berries as possible by hovering a mouse pointer over a bush. Each time they foraged from the same bush a player’s harvest would slightly decline but if they moved to a new bush they would suffer a time penalty.

So what would most players do? Stick to the same reliable berry-supplying bush? Or risk wasting time by trying another bush to see if it had more fruit? Explore or exploit?

Around 450 people participated in the experiment, and all were simultaneously screened for ADHD symptoms. Unsurprisingly, the researchers found those with higher ADHD scores moved on to new bushes sooner than others but more importantly, those with ADHD also tended to collect higher volumes of berries overall.

Writing in the newly published study, Barack and colleagues noted that participants without ADHD characteristics tended to over-harvest single patches. Looking at what would be an optimal harvest strategy for the game it was discovered that players with high ADHD scores were more successful overall.

“In addition, we discovered that participants that screened positive for ADHD more readily abandoned patches and achieved higher reward rates than did participants who screened negative,” the researchers concluded. “Given the over-staying displayed by participants overall, those with elevated ASRS scores made exploratory decisions that were more closely aligned with the predictions of optimal foraging theory, and, in this sense, behaved more optimally.”

The findings are in no way a final word on the possible evolutionary benefits of ADHD. But they do offer a compelling and plausible reason why a small percentage of humans continue to carry these traits. In the 21st century we may have pathologized ADHD as a negative disorder, but that could be just because these characteristics simply don’t easily suit the world we have constructed. In a different context, it is possible that someone with ADHD could be a saviour to a tribe by restlessly exploring new pastures.

The new study was published in the Proceedings of the Royal Society B.

Source: University of Pennsylvania via Scimex



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