Monday, March 16, 2015

Four Laws of Behavior Genetics

SciAm writer John Horgan notes that the evidence for behavior genes has a history of being over-hyped:
In 1990 The New York Times published a front-page article by Lawrence Altman, a reporter with a medical degree, announcing that scientists had discovered “a link between alcoholism and a specific gene.”

That was merely one in a string of reports in which the Times and other major media hyped what turned out to be erroneous claims linking complex traits and disorders—from homosexuality and high intelligence to schizophrenia and bipolar disorder—to specific genes.

I thought those days were over, and that scientists and the media have learned to doubt extremely reductionist genetic accounts of complex traits and behaviors. I was wrong. Last Sunday, the “Opinion” section of the Times published an essay, “The Feel-Good Gene,” which states:
“For the first time, scientists have demonstrated that a genetic variation in the brain makes some people inherently less anxious, and more able to forget fearful and unpleasant experiences. This lucky genetic mutation produces higher levels of anandamide–the so-called bliss molecule and our natural marijuana–in our brains. In short, some people are prone to be less anxious simply because they won the genetic sweepstakes and randomly got a genetic mutation that has nothing at all to do with strength of character.”
The evidence for the "feel-good gene," which supposedly reduces anxiety, is flimsy, just like the evidence linking specific genes to high intelligence, violent aggression, homosexuality, bipolar disorder and countless other complex human traits and ailments. ...

Last fall, I quoted from a 2012 editorial in Behavior Genetics: “The literature on candidate gene associations is full of reports that have not stood up to rigorous replication. This is the case both for straightforward main effects and for candidate gene-by-environment interactions… As a result, the psychiatric and behavior genetics literature has become confusing and it now seems likely that many of the published findings of the last decade are wrong or misleading and have not contributed to real advances in knowledge.”
That is correct, but only part of the story as there have been real advances in the field. Razib Khan reports:
* First Law. All human behavioral traits are heritable.

* Second Law. The effect of being raised in the same family is smaller than the effect of genes.

* Third Law. A substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families.

* Fourth Law. A typical human behavioral trait is associated with very many genetic variants, each of which accounts for a very small percentage of the behavioral variability.
Researchers model behavior by assuming that it is determined by genes, family upbringing, and random factors. Twin studies have proved that the genetic influence is huge, and the family upbringing appears to be negligible. DNA studies have failed to find any individual genes with a large effect. Larger scale studies are starting to show that a combination of many small genetic effects can be significant. For more on what randomness means here, see my 2015 essay.

Parents are usually convinced that their child-rearing is the biggest influence on their kids, but this is an illusion, according to the research.

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