Behavioral Genetics

There are two major kinds of research that go under this name:

• Attempts to determine to what degree various behavioral traits such as intelligence and personality are inherited (presumably genetically), determined by the family environment, or by other unknown factors. This was investigated by means such as comparing the similarity of twins with that of ordinary siblings and with unrelated people, and attempting to determine to what degree the family environment is responsible by comparing the similarity of twins raised together and raised apart and the similarity of unrelated adopted children to other children in the adopting family. This is an overt attempt to scientifically resolve the Nature Versus Nurture controversy.
• More recently, there has been a great deal of work that attempts to determine specific genetic markers or genes that are associated with specific behaviors such as novelty-seeking or mental illness. Numerous correlations have been found, but they are generally not strong, supporting the idea that genetic basis of behavior is complex, involving many genes, and that behaviors that seem similar may actually associated with several distinct genetic patterns.

The first body of results definitely qualifies as puzzling evidence about the human condition, and we will discuss it in some detail. Although the second research program could ultimately yield specific knowledge about what genes promote certain behavior traits, leading to interesting ethical issues with genetic testing and human genetic engineering, at this time we find the results rather unsurprising. The continuing popular interest in this topic seems to be largely related to the X proved real fallacy and a lack of appreciation that all behavior must ultimately have a genetic explanation, just as all behavior must have an electrochemical explanation. This does not mean that we must abandon psychological and sociocultural explanations for behavior.

The major findings are that for a wide range of traits examined:

1. The inherited (presumably genetic) component of individual variation is 50% to 70%.
2. The component of variation due to family is 0% to 20%.
3. The rest is other unknown causes or measurement error. Though this is more or less by definition the environmental contribution, it may be the prenatal environment rather than the social environment.

This is true for a wide range of traits. Intelligence (as defined by IQ) and personality (as defined by personality tests) have gotten the most attention because they most directly relate to the sociopolitical heart of the nature-nurture controversy, but basically the same results have been seen for many other traits such as religiosity, hours of TV watched, and even whether when you cross your arms you put the left or right on top.

This body of research seems quite solid, but was intellectually marginalized and demonized for decades due to its direct connection with the controversies over IQ testing. The researcher who dominated early work in this area was Cyril Burt, who was successfully discredited for research fraud shortly after his death in the 70's. Whatever the truth of this matter is (many of the accusations didn't hold up), it seems clear that his early results were real because they have been reproduced with close agreement.

These results were correctly seen by both sides as being incompatible with extreme pro-nurture positions popular in intellectual circles during the 60's and 70's — that all humans have equal potential, and that any difference is due to environment. Though some may still hold these views, the science is not on their side. However, there are subtleties even in this interpretation, see Nature Versus Nurture.

The other interesting conclusion is that the family environment doesn't seem to have much effect on these things, so perhaps it hardly matters what (if anything) parents do beyond providing food, clothing and shelter. Judith Rich Harris attracted considerable attention for this interpretation in The Nurture Assumption. We don't buy this extreme interpretation, but these results do call into question ideas such as “Buying the right baby toys will increase my kids IQ”.

So it's been shown that some traits that can be measured numerically are more influenced by our biological inheritance than by any other cause, but how should we understand those numbers? A major degree of interpretive freedom comes from the meanings we assign to the things being measured. Perhaps IQ is not the same as “intelligence”, so it could still be that intelligence is strongly influenced by parenting. Clearly personality tests don't measure everything that we would colloquially call personality, so perhaps important aspects of personality are strongly influenced by parenting. We feel this verges on a semantic quibble, and is not very productive. For one thing, many of the heritable traits such as the self-reported importance of religion in one's life seem to have obvious meaning and importance. They don't depend on the semantically troublesome procedure of assigning commonly understood words to the anonymous results of factor analysis.

One interesting point is that, by their very design, intelligence and personality tests measure something that is relatively stable over a person's life, so will end up measuring the completely stable genetic contribution. The designers set out to measure something that was stable because it is the common understanding of these words that they describe something stable. They chose questions that gave consistent results over a lifetime, and therefore measure something that is not much affected by social experience. So it is not at all surprising that these tests measure tendencies present at birth, and hardly surprising that they measure the genetic contribution so precisely. In other words, if we accept that these tests define intelligence and personality, then it is almost true by definition that intelligence and personality are highly inheritable.

We're willing to more-or-less accept this because it is in line with common-sense popular understandings of these terms. By definition, personality and intelligence are things that are innate (stable over a lifetime) and little influenced by social experience (I'm just a glass-half-empty kind of guy.) It was a plausible idea that personality and intelligence are largely determined by early social experience (in the family) that takes place before verbal tests can be given. If this were true, we would replace the common belief of stability with the more refined one of early plasticity followed by stability, but the evidence has gone the other way. Clever pre-verbal tests in infants (see Descarte's Baby) have found stability of traits such as anxiety from a very early age. The behavioral genetics results are another nail in the coffin. Parenting has little effect on these things.

This does not mean that parenting has no effect, just that it affects other things. One thing that the family environment and parenting style clearly do affect is the kind of family environment the children will create and the parenting style that the children will use when they grow up. This has important implications for children's future life choices and happiness, and evolutionarily significant effects on the number of grandchildren.