Here We Go Again: Genetics and the Environment

Hellow everyone (or anyone. Who knows who is out there?) Today I'd like to revisit an old, old topic..whether intelligence is most influenced by your genes or your environment. Notice that I said "most influenced." EVERYBODY who is knowledgable in the field agrees that both genetics and environment have effects. The issue is "which is largest." I want to add, "which and where," because, as I will try to show, any answer to this question has to be specific to time and place. Here is the problem.

The usual measure of "influence" is the heredity coefficient, h. It wouldn't be appropriate to go into an essay on statistics, here's the gist of the argument. The heredity coefficient is a measure of the relative amount of variation due to genetic variation, compared to the total variation in intelligene. The "relative" is impotant, and show is the word "variation." Conceptually, h = (Amount of variation in intelligence associated with genetic variation) / (Total amount of variation in intelligence.) That second term can be broken down, so that we have h = (amount of variation in intelligence associated with genetic variation) /(Amount of variation associated with genetic variation + Amount of variation associated with environmental variation). It is easy to find statements saying that h is in the range .5 to .8, which means that from 50% to 80% of the variation in intelligence is associated with genetic variation. But wait a moment!

The .5 to .8 figure has been supported by many studies, BUT they are studies that are restricted in an important way. The majority of these studies of Europeans or European derived North Americans, and the socioeconomic status of the participants tends strongly to be middle class or above. These people have a surprisingly uniform environment...they go to reasonably good schools, have reasonably good nutrition (especially in infancy and early child), and so forth. In other words, insofar as effects on intelligence are concerned, there is relatively little environmental variation, and when environmental variation goes down, the RELATIVE genetic variation has to go up. Recent work by Prof. Eric Turkheimer and his colleagues at the University of Virginia, studying lower SES groups, found h values well below the .5 to .8 range. Why? I am pretty sure that one reason is that the range of variation of environmental variables in lower SES groups can be much greater than the range of environmental variation in middle and upper SES groups. To be informal, the effects of the home environment between a family with a mother who is a stay at home mom and a family where mother works but arranges for 'quality time' and good day care are probably pretty small. If we compare a situation where mother has a serious drug/drinking problem to a situation where mother is working, but she still arranges for day care and spends time with her child, the effects are probably huge. Environmental variation goes up, so the RELATIVE genetic effect, h, has to go down.

Things can go the other way around. Here is a hypothetical example. A little over 2000 people live on Norfolk Island, in the Pacific between New Zealand, Australia, and New Caledonia. Roughly half of these people are descended from a small group of 18th century HMS Bounty mutineers, who fled to nearby Pitcairn Island to escape capture by the British Navy after the mutiny. (Many of the descendents of the mutineers left Pitcairn for Norfolk because Norfolk is much larger, and could support a growing number of their descendents. Now, I do not know of any studies of the genetic intelligence on Norfolk island. What I strongly suspect, though, is that the h ratio on Norfolk Island is pretty small, because half the population are the descendents of the original Pitcairn Island group, just six men and eleven women. The genetic variation goes down, so the influence of genetics on variation in intelligence has to go down.

So, my point is that there is no "size" for genetic effects on intelligence. It depends upon the population being studied.

Any comments?