Disease, Disorder, or Neurodiversity: The Case of ADHD
The genes related to addiction and ADHD may have unique advantages.
Posted May 21, 2012
Many scientists believe that a certain amount of psychological diversity is built into the human race because it provides an evolutionary advantage for all of us. This built-in diversity in brain plans -- or neurodiversity -- has often been linked with genetic variation. So here's a true story about a genetic variation that causes problems in the classroom, but which may have provided real advantages for the original settlers of America.
When I was in Toronto recently, Jim Kennedy (a highly-renowned research psychiatrist) told me an amazing story over dinner. It concerned some research in which he’d participated, examining ADHD (attention deficit hyperactivity disorder), genetics, and migration.
ADHD is a disorder, if not a disease. Right? According to Dr. Kennedy, the study of native populations in the Americas suggests a different perspective. A genetic variant related to ADHD (the 7-repeat allele of the DRD4 gene) shows up at a very low rate in native groups living in northern Canada: 2-3%. The incidence of this variant increases, to something like 10-20% in native groups living in the southern U.S. In Central America, incidence of this variant increases up to 50%, and it exceeds 50% in parts of South America.
What could this possibly mean?
According to accepted theory, the Americas were first settled by migration waves from Asia, across a land bridge connecting Siberia with Alaska, at least 12,000 years ago and possibly much earlier. That bridge has since disappeared. So there was a gateway for migration, starting in northwestern North America and moving south, over many thousands of years. That's a perfect opportunity for genetic research.
But why would there be a greater hereditary risk for ADHD as the original settlers of America moved south? ADHD describes a syndrome in which people are more distractible, their attention wanders off target, and they are more attracted to novelty than to routine. In North American and European classrooms, that's bad news. You're supposed to be facing the board, listening to the teacher, and doing your assigned work. If a certain proportion of people don’t do this, if they have a “problem” with their attention, and if this problem is related to distinct neural mechanisms and perhaps to genetic predispositions, then the problem gets classed as a mental disorder or disease.
(Not too different from the "disease" model of addiction...)
But if this “problem” only arises in certain contexts – if it can be an advantage in other contexts – then the disease label starts to lose relevance. Imagine that you are a North American native, it’s 10-15,000 years ago, you live somewhere in northern Canada, and there is nowhere nearby to buy a Gortex jacket or even long underwear. It’s awfully cold for many months of the year. But you happen to have a predispositon to wander off into the woods, peak over the next hill, and lose attention to the normal duties of hunting and trapping. One fine day, while on your explorations, you look over the crest of a hill and find a long valley extending off to the south. The lands north of you are already settled, and they're cold as hell, so they’re not very appealing. But this valley to the south is completely uninhabited. You eventually move down there with your family, and maybe your cousins and their families too. Your children have a greater likelihood of having the same attraction to novelty, the same low tolerance for routine, because they carry many of your genes. So when they grow up, they are also more likely to discover greener pastures, and your descendants may continue to migrate southward.
By this process, the genetic makeup that confers an attraction to novelty and a disdain for routine will become correlated with geography. Tribes -- or groups within tribes -- who have a higher proportion of that genetic variant will be more likely to migrate, and will show up further and further south.
Today, in North America and Europe, we associate that genetic pattern with ADHD – a “disorder”. But for those aboriginal people, it may have been adaptive, moving them further and further away from the frozen North and opening up new possibilities for hunting, farming, and building civilizations.
Conclusion: the psychological qualities of a genetic distinction can’t be defined or labeled in a vacuum. The advantages or disadvantages of that distinction can only be described in context.
Research points to genetic patterns that are correlated with addiction, even though there is no addiction gene. I’ll get into that topic next post. For now, I want to leave you with the thought that addiction may arise partly from a predisposition that's not unhealthy or bad in itself. That disposition, and the way it cashes out in child development, may depend entirely on the nature of our society and the function it plays within that society.