Thread: ADHD article from the economist

interesting shit. enjoy, my little tweakers.



THE MISFITS
Jun 12th 2008


The genetic legacy of nomadism may be an inability to settle

ABOUT one in 20 children (those under 18) have a group of symptoms that
has come to be known as attention-deficit hyperactivity disorder
(ADHD). About 60% of them carry those symptoms into adulthood. For what
is, at root, a genetic phenomenon, that is a lot--yet many studies have
shown that ADHD is indeed genetic and not, as was once suspected, the
result of poor parenting. It is associated with particular variants of
receptor molecules for neurotransmitters in the brain. A
neurotransmitter is a chemical that carries messages between nerve
cells and, in the case of ADHD, that chemical is often dopamine, which
controls feelings of reward and pleasure. The suggestion is that people
with ADHD are receiving positive neurological feedback for
inappropriate behaviour. The surprise is that the variant receptors are
still there. Natural selection might have been expected to purge them
from the population unless they have some compensating benefit.

Of course, this analysis turns on the definition of "inappropriate".
The main symptom of ADHD is impulsiveness. Sufferers have trouble
concentrating on any task unless they receive constant feedback,
stimulation and reward. They thus tend to flit from activity to
activity. Adults with ADHD tend to perform poorly in modern society and
are prone to addictive and compulsive behaviour. But might such people
do well in different circumstances?

One hypothesis is that the behaviour associated with ADHD helps people,
such as hunter-gatherers and pastoral nomads, who lead a peripatetic
life. Since today's sedentary city dwellers are recently descended from
such people, natural selection may not have had time to purge the genes
that cause it.

Dan Eisenberg, of Northwestern University in Illinois, and his
colleagues decided to test this by studying the Ariaal, a group of
pastoral nomads who live in Kenya. The receptor Mr Eisenberg looked at
was the 7R variant of a protein called DRD4. Previous work has shown
that this variant is associated with novelty-seeking, food- and
drug-cravings, and ADHD.

The team looked for 7R in two groups of Ariaal. One was still pastoral
and nomadic. The other had recently settled down. As they report in
this week's BMC EVOLUTIONARY BIOLOGY, they found that about a fifth of
the population of both groups had the 7R version of DRD4. However, the
consequences of this were very different. Among the nomads, who wander
around northern Kenya herding cattle, camels, sheep and goats, those
with 7R were better nourished than those without. The opposite was true
of their settled relations: those with 7R were worse nourished than
those without it.

How 7R causes this is not yet known. It may stem from behavioural
differences or it may be that different versions of DRD4 have different
effects on the way the body processes food. Nevertheless, this
discovery fits past findings that 7R and a set of similar variants of
DRD4, known collectively as "long alleles", are more common in
migratory populations.

One suggestion is that long-distance migration selects for long alleles
(see chart) because they reward exploratory behaviour. This might be an
advantage in migratory societies because it encourages people to hunt
down resources when they constantly move through unfamiliar
surroundings.

As for the Ariaal, there remains the question of why 7R--although it is
apparently beneficial to a nomadic way of life--is found in only a
fifth of the population. One possibility is that its effects are
beneficial only when they are not universal, and some sort of
equilibrium between variants emerges. A second is that the advantage is
gained when 7R exists along with another version of DRD4 (the genes for
the two variants having come from different parents). Unfortunately,
the way Mr Eisenberg collected the data does not allow these hypotheses
to be tested.

Either way, his research raises the question of whether people
suffering from ADHD and conditions related to it, such as addiction,
are misfits coping with a genetic legacy that was useful in the
evolutionary past, but is now damaging. As society continues to diverge
from that evolutionary past, the economic and social consequences of
being such a misfit may become increasingly important.



See this article with graphics and related items at http://www.economist.com/science/dis...ry_id=11529402