The most prominent finding of behavioral genetics has been summarized by the psychologist Eric Turkheimer: “The nature-nurture debate is over. . . . All human behavioral traits are heritable.” By this he meant that a substantial fraction of the variation among individuals within a culture can be linked to variation in their genes. Whether you measure intelligence or personality, religiosity or political orientation, television watching or cigarette smoking, the outcome is the same. Identical twins (who share all their genes) are more similar than fraternal twins (who share half their genes that vary among people). Biological siblings (who share half those genes too) are more similar than adopted siblings (who share no more genes than do strangers). And identical twins separated at birth and raised in different adoptive homes (who share their genes but not their environments) are uncannily similar.
I have several problems with this.
First, knowing a bit about addictive behavior, I know that predisposition to cigarette smoking or alcoholic drinking does NOT have “substantial fraction” due to heritability.
Second, on behaviors like that, people like Pinker have never even made an effort to sort out familiar social influence.
Third, the “uncannily similar” comes off as being nothing more than a door-slamming phrase, i.e., “How can you be scientific if you question it”?
Well, with twins, I question it on several grounds.
First, identical twins themselves differ on when after fertilization the twinning event occurred:
• Do they have separate amniotic sacs and placentas?
• Share a sac but with different placentas?
• Share even placentas?
All of the above are of course environmental and not genetic effects, but Pinker conveniently ignores that.
Second, are identical twins “uncannily similar”? Not necessarily. Again, Pinker refers us to no research; he just throws out a statement to us and demands we accept it as gospel truth.
’Tis true, Pinker does qualify both his own comments, and those of Turkheimer, with some more general versions of what I just noted, on the next webpage. But, to me, the carts was enough before the horse, and emphasized enough more, to tell me where Pinker falls.
And, on page 5, Pinker shows we still have far to go in our understanding of the how and of the specific genes of genetic heritability.
Height is widely acknowledged, due to the information from statistical correlation, as being the single most heritable human trait. But, as Pinker notes, in 2007 a genomewide scan of nearly 16,000 people turned up a dozen height-related genes. However, these genes collectively accounted for just 2 percent of the height variation; plus, a person who had most of the genes was barely an inch taller, on average, than the general population.
And, I haven’t even gotten to Pinker’s biggest flop, or deliberate oversight.
It’s becoming ever more clear that what has to this point been called “junk DNA” isn’t; rather, some of it may code for frequency of expression of a gene, or control what genes interact together and when, etc. And, though still looked askance by some geneticists precisely for what it hints at, Stanley Prusiner’s work on prions, along with other research, shows that the heritability pathway may not be a one-way street at the cellular level.
Pinker starts to wrap up his take on the science of personal genomics with this:
At the same time, there is nothing like perusing your genetic data to drive home its limitations as a source of insight into yourself.
Too bad, he doesn’t realize, or refuses to accept, the limitations of genetic data today go far beyond that.