He is talking about the concept of nature versus nurture, and the fact that it has no scientific basis in describing human behavior. People like to talk about "twin studies," but there have been no proper twin studies of humans, particularly because they're unethical. The human twin studies we have that try and attribute behaviors to genes have all had major holes in them, have never involved controls, and have consistently been based on conjecture without direct empirical evidence.

There have also been numerous cases of identical twins (even quintuplets) in humans and other animals (namely rats) that have all demonstrated clearly different behavior whether or not they were raised together. In the case of the rats, an experiment was conducted by taking rats with identical genes and raising them separately in identical environments, and still their behaviors were very different. It's been well established that environmental factors (social and otherwise) are the primary contributors to behavior beyond the genes' creations of organs, appendages, and regulatory systems with specific capacities (i.e., we couldn't think without the brains our genes created).

What sort of controls are put into place and how in these twin studies? I've yet to read of any twin studies suggesting gene-linked behavior that weren't bunk.

The issue with the mice (not actually rats in this study; I was wrong about that) is that we can't actually perfectly replicate every attribute of an environment, and as other studies have demonstrated, humans and other animals are very sensitive to subtle environmental differences. (Study here.)

The womb, of course, is an environment and not a set of genes, so if fetal development were an issue, genes are again not the primary determiners of behavior. Hormones are also heavily impacted by environmental factors (including fetal development), and even still, they only have indirect influences on behavior — they don't control it directly. Ghrelin, orexin and PYY 3-36, for instance, can tell you that you ought to eat (whether or not this is the case, as people like myself have serious problems with these systems), but they cannot force you to eat.

Obviously natural selection has left behind animals who don't eat, and only those capable of recognizing the need survive, but this again falls in line with what I was saying earlier: genes only encode for organs and basic regulatory systems, but those systems don't directly control our behavior, only influence it to the extent of increasing our survivability. There is no such thing as a gene that tells you to "grow up gay" or "become a pianist." Much as some like to try and find correlations between these things, and even occasionally find minor statistical evidence, correlation does not equate causation, and no direct causal relationships have yet been found.

In the case of humans, in particular, the few thousand genes we have — most of them dormant, and the rest encoding for biological construction — would be in competition with the countless neuropathways formed between the trillions of neurons that allow for consciousness. Even if genes for behavior existed, they would have little to no effect against such competition, and they would likely be selected against as a result of their ineffectiveness. There is no way for genes to directly control the construction of those neuropathways, which are only built by interaction with the environment.

Edited at 2009-06-06 06:51 pm (UTC)

I wasn't saying that genes have no roles to play: behavior wouldn't be possible without them constructing the necessary components. The argument here was against determinism — the idea that there are genes for specific behaviors. That notion's been debunked by geneticists (including one of the world's foremost in the above quote). All genes can do is produce the bodies through which behaviors will be enacted, and they can produce regulatory systems which may in some cases effect tendencies. Intelligent creatures, of course, are fully capable of subverting those tendencies, perhaps the most powerful example being the cultural values which prevent sex, in direct opposition what genes "want" (in the sense of unconscious physical tendencies).

I posted that quote because determinism is one of my pet peeves. As far as I'm concerned, it's like a far more insidious "intelligent design" — a philosophy masquerading as science, even in an age where physicists have demonstrated that the universe is innately nondeterministic.

Edited at 2009-06-06 07:37 pm (UTC)