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How did ADHD evolve and survive?
Michelle Dawson (without endorsing it) directs my attention to this paper:
The evolutionary status of attention deficit/hyperactivity disorder (ADHD) is central to assessments of whether modern society has created it, either physically or socially; and is potentially useful in understanding its neurobiological basis and treatment. The high prevalence of ADHD (5–10%) and its association with the seven-repeat allele of DRD4, which is positively selected in evolution, raise the possibility that ADHD increases the reproductive fitness of the individual, and/or the group. However, previous suggestions of evolutionary roles for ADHD have not accounted for its confinement to a substantial minority. Because one of the key features of ADHD is its diversity, and many benefits of population diversity are well recognized (as in immunity), we study the impact of groups’ behavioural diversity on their fitness. Diversity occurs along many dimensions, and for simplicity we choose unpredictability (or variability), excess of which is a well-established characteristic of ADHD. Simulations of the Changing Food group task show that unpredictable behaviour by a minority optimizes results for the group. Characteristics of such group exploration tasks are risk-taking, in which costs are borne mainly by the individual; and information-sharing, in which benefits accrue to the entire group. Hence, this work is closely linked to previous studies of evolved altruism.
We conclude that even individually impairing combinations of genes, such as ADHD, can carry specific benefits for society, which can be selected for at that level, rather than being merely genetic coincidences with effects confined to the individual. The social benefits conferred by diversity occur both inside and outside the ‘normal’ range, and these may be distinct. This view has the additional merit of offering explanations for the prevalence, sex and age distribution, severity distribution and heterogeneity of ADHD.
Overall the argument is weak because it relies too much on group selection. An alternative tack is to admit that ADHD, and correlated traits, can have cognitive advantages and thus survival and mating advantages. One simple story is that many people with ADHD can use their "jumpiness" to propel themselves to sample and learn extra new pieces of information. The current distribution of identified cases from the ADHD population likely suffers from selection bias, namely that it identifies ADHD cases associated with greater life problems.
Addendum: Jerry Fodor has a recent paper challenging common applications of evolutionary psychology; Razib defends Darwin.
Posted by Tyler Cowen on July 30, 2009 at 06:39 AM in Science | Permalink
Comments
"The high prevalence of ADHD (5–10%) and its association with the seven-repeat allele of DRD4, which is positively selected"
Do we know what else that allele does? Do they consider that in the paper (my attention span for formal papers is not high, perhaps a little ironically)? That the allele is selected for doesn't necessarily have anything to do with ADHD, we have many, many imperfections that exist because they help do something else too.
Posted by: Andrew at Jul 30, 2009 8:10:20 AM
Vernon Smith in Discovery (which I recall you've read - in fact I think I heard about it from you) has some thoughts about this subject. He speculates that his capability for extreme mental focus may have arisen as a compensating mechanism for ADHD.
I haven't finished the book yet, so perhaps he has more to add about it later.
I'm not sure if he's suggesting that this increases fitness overall, but there is a hint of that. This feels a little bit wrong - like Scott Sumner's story that "Demand for champagne has increased, pushing up prices, but suppliers are worried that the high prices will reduce demand". Similarly there's a story on Economist's View this week proposing that Europe's high rates of death in the Middle Ages are responsible for its economic success.
In general it feels unlikely that mechanisms to compensate for a problem can outweigh the disadvantage of the problem itself. If they could, presumably people or societies without the original problem could (at least partly) apply the same compensation and presumably achieve some success.
Of course you have argued in CYOE that some of these cognitive traits are not "problems" at all but may inherently increase fitness, which is a valid point.
And it may be that in reality, people and societies don't apply certain "compensations", even if it would be in their interest to do so, without being prompted by having an identifiable problem to solve. So I'm not necessarily convinced by my own argument here. It's an interesting question, anyway.
Posted by: Leigh Caldwell at Jul 30, 2009 8:18:12 AM
Tyler,
Do you think there is a significant population of people who combine traits of autism/Aspergers and ADHD? Call them "jumpy" infovores or "Jeopardy champions"--people who must learn and order at least a little about a very wide range of subjects, always jumping from one to the next?
Posted by: NNM at Jul 30, 2009 9:02:49 AM
I tried to read this whole post, but I got distracted.
Posted by: AWhite at Jul 30, 2009 9:17:04 AM
The fact that the argument lies on the idea of group selection is not in itself a weakness. Group selection has been long ago rehabilited in evolutionary biology has well as in sociobiology, as David Wilson and Edward Wilson clearly demonstrate here :
http://evolution.binghamton.edu/dswilson/resources/publications_resources/Rethinking%20sociobiology.pdf
Group selection is a plausible mechanism but not a systematic one. Only case by case study can help to determine its strenght in evolutionary processes.
Posted by: C.H. at Jul 30, 2009 9:43:26 AM
NNM, I've been thinking a lot about exactly that. There are many overlaps between autistic and ADD or ADHD cognition. I'm severely ADD, and have Asperger's, so these questions have impacted my life on a regular basis. Your description fits me perfectly.
Posted by: Catana at Jul 30, 2009 9:44:59 AM
ADHD is particularly prevalent among high testosterone males with above average IQ's. I think it is a good idea to official register everyone in this group as having a "mental disability" and then making a law that people with a "mental disability" are nto allowed to own or possess a gun. This way will make the establishment much safer froma potential rebellion.
Posted by: Gabe at Jul 30, 2009 10:01:13 AM
"In general it feels unlikely that mechanisms to compensate for a problem can outweigh the disadvantage of the problem itself."
As I recall, the Industrial Revolution started that way. Steam power, used to make more efficient pumps to drain mines, very quickly led to benefits far greater than drier mines.
Posted by: jb at Jul 30, 2009 10:39:28 AM
Or the impulse control problems of ADHD people just leads then to reproduce in larger numbers. Sort of like rabbits, risk takers die off at faster rate but they reproduce more rapidly to compensate.
Posted by: DanC at Jul 30, 2009 10:43:08 AM
Gabe, how about low-testosterone ADD females with above-average IQs? Though females like me are believed by Simon Baron-Cohen to have "extreme-male" brains. Where is my testosterone when I need it?
Rebellion doesn't always involve guns, you know. My weapon is my keyboard.
Posted by: Catana at Jul 30, 2009 10:46:33 AM
Can you explain your reasoning that group selection is inherintly a weak argument? In evolutionary biology it plays a huge role from bacteria to the primates and has been major driver of traits considered "detrimental" to am individual.
Posted by: Tye at Jul 30, 2009 10:50:20 AM
"Rebellion doesn't always involve guns, you know. My weapon is my keyboard."
True Catana,
I guess that is why Jay Rockefeller said this:
Bush and Obama intelligence experts agree, Internet is the biggest problem>
Posted by: Gabe at Jul 30, 2009 11:00:36 AM
Catana, I think your supposed to be drinking more sodium fluoride so that you will be as docile a concentration camp inmate.
Posted by: Gabe at Jul 30, 2009 11:03:07 AM
Catana,
Thank you very much for your reply. I've never been diagnosed as autistic/Aspergers or ADD/ADHD, but from reading Tyler's book I recognize many traits of the former in myself, and my life's experience so far has convinced me I have certain traits of the latter. I hate to speak for Tyler, but I suspect he also falls somewhere in the intersection of the two (tenured conventional economist + world class blogger + world class foodist + world class music devotee, etc.), which I guess is why his work resonates with me so much.
Anyway, Catana (or Tyler or anyone else), I'm very interested to learn more about this. Please feel free to contact me at nmanham at eleraadvisorsllc dot com, or to direct me to any past or future blog posts on the subject.
Much appreciated.
Nadav
Posted by: NNM at Jul 30, 2009 11:07:23 AM
I'm convinced that, to a large degree, attention deficit disorders are bred into children behaviorally. We cater to the natural tendency for children to crave immediate gratification and thereby program them to anticipate and demand more of the same.
We've got a real problem when kids can't stomach a 10 minute drive to the grocery store without popping in a DVD or jacking their brain into some other electronic entertainment option. But this is a problem of our own creation - we somehow feel that if we don't cater to and foster the perpetual and instantaneous digital stimulation of our children, we're somehow failing them.
Unplug your children, stop feeding them rocket fuel at every meal (and 2 snacks in-between) and you may find that their behavior is far more like that of a normal human being.
Posted by: Rob O. at Jul 30, 2009 11:22:03 AM
It seems that aspergers/ADD/ADHD people are less dependent on social norms and more dependent on self-constructed belief system...this is what is known as "anti-social"...therefore we as a society need to make every effort to fix these people...whether that means drugging them up or just railroading them into "special" schools we cannont let these anti-socials contaminate the healthy population with their sick minds.
Posted by: Professional Educator at Jul 30, 2009 11:45:01 AM
It seems that aspergers/ADD/ADHD people are less dependent on social norms and more dependent on self-constructed belief system...this is what is known as "anti-social"...therefore we as a society need to make every effort to fix these people...whether that means drugging them up or just railroading them into "special" schools we cannont let these anti-socials contaminate the healthy population with their sick minds.
Posted by: Professional Educator at Jul 30, 2009 11:46:55 AM
@Professional Educator: I hope that was tongue in cheek.
Actually, people with ADHD can hyper-focus on a small number of tasks (varies by individual).
Also the fact that ADHD is selected positively suggests that there is a benefit to it (in terms of natural selection), the real question is what is the benefit.
Posted by: Doc Merlin at Jul 30, 2009 11:54:28 AM
the literature on dopamine receptor genes and behavior variation is huge. this paper is representative of a large class.
Posted by: razib at Jul 30, 2009 12:21:19 PM
I'll chime in with the other requests for clarification of Prof. Cowen's views on group selection theory. Falsification problem? Something else? Why weak?
I'll also note that bipolar tends to be misdiagnosed as ADHD in adolescents. There seems to be a genetic component to it contra Rob O.
My own conjecture is that every brain can get into a positive feedback loop under high stress circumstances, a loop that results in reduced need for sleep, increased aggressiveness, and reduced impulse control or high confidence levels. ADHD/Bipolar brains probably have a lower threshold for establishing a self-sustaining positive feedback loop.
As far as evolutionary explanations, I think the advantages that such a brain would confer on individuals in battle are substantial enough to explain their prevalence. Like Tyler, I'm not sure one needs to resort to group selection theory to explain their prevalence, although perhaps it adds some light at the margins. For example, how many charismatic leaders fit this neurological profile?
Posted by: Michael F. Martin at Jul 30, 2009 1:45:03 PM
I'm not sure how important a role group selection plays in evolution, but I can offer two observations of ADHD behavior that may be helpful.
Any school administrator who moves quietly in and out of classrooms several times a day will tell you that the ADHD kids are almost always the first to notice you at the door or in the back of the room. I suspect that a hunter/gatherer group with some members who are constantly aware of changes in the local environment would have a survival advantage.
Rocket fuel food, high stimulation toys, television, lack of exercise, and parental indulgence all play a role in how genetic ADHD is manifested in a given child. That said, keep in mind that the modern world school environment(specifically requiring that children sit and concentrate for several hours every day)is brand new in evolutionary history and not always healthy or even a good preparation for adult life. How many rapid-fire-multi-tasking business execs have found their skill set to be better matched to business than it was to school?
Posted by: jpora at Jul 30, 2009 2:38:57 PM
I don't understand why the hypothesis is dismissed for favoring group selection. It is possible that a certain number of genes for ADHD are kept around in the population, simply because a "low dose" of such genes are favored by other genes. Even the gene itself might favor a small but stable prevalence of its own gene, because the alternative may be complete absence of the gene. Think of Maynard's Smith's "hawks" and "doves," which refer, not to different species, but to different attitudes within a populations. It may not be advantageous, for even the hawk gene, to avoid extending its reach, because it can only lead to too many hawks killing each other (thus allowing doves to win overall), and prevents hawks from relying on other genes that are ultimately necessary for overall "hawk" stability.
Posted by: adina at Jul 30, 2009 2:48:26 PM
When I was a kid, some friends of mine and I were playing with firecrackers. A friend of mine threw a firecracker into a large steel drum to see whether it would make an interesting noise. He said "Huh, it didn't go off." I said "mine did." See, I'd forgotten that I'd just lit a firecracker, and was still holding it. No serious injury, because they were pretty small firecrackers.
But you know what? The ability to forget an explosive device that you are holding, which you yourself just lit, in less than three seconds? That's actually pretty much a disability.
There are upsides and downsides. I think, though, it's important to recognize that even though there are certainly upsides, there are also certainly downsides. The net outcomes may be decent enough, and even beneficial for the group, but recognizing the limitations is important too. I think what's really broken is probably our notion of how we define a "disability". I think that, if I can't do something fairly basic that other people can, it's in society's best interests to try to accommodate me at least a little. That shouldn't be tied to whether or not I'm overall disadvantaged by the trait -- just to whether I have specific weak spots that can be mitigated by a bit of support.
Posted by: seebs at Jul 30, 2009 3:10:18 PM
Haven't read this paper, but I'm pretty sure DRD4 is involved with pre-frontal dopamine - i.e. reward for behavior.
Posted by: MonkeyMan at Jul 31, 2009 12:46:04 AM
I remember reading a book by Tom Hartmann in which he suggested that ADHD made a person a particularly good hunter. The ability to hyperfocus (stalk prey, make good weapons), coupled with easy distractibility (the hunter is also the hunted, so you need to stay on your toes) seems like a pretty good combination of traits to have when you're the hunting part of a hunter-gatherer tribe. In contrast, the gatherers need to be detail oriented, able to keep lots of tasks going at the same time, etc.
His other suggestion was that we've always had about the same percentage of people who were ADD, but our modern lifestyle has put the hunters at a disadvantage. If you find a career as a fireman, or a cop, or something else that allows you the kind of adrenaline and mental stimulation that ADHD people seek, you'll do fine. But put the same person behind a desk and ask him to shuffle papers all day, and he'll be a mess. As society demands more and more of the latter sorts of people, more and more are diagnosed as ADHD.
I don't know how well founded the theory is, and it's been at least a decade since I read it, so I can't remember the details. But it's certainly a theory that makes a lot of superficial sense.
Posted by: Dan H. at Jul 31, 2009 9:22:45 PM
