My wife maintains that identical twins are more likely to share personality traits than same-sex fraternal twins. I am skeptical. Is there any evidence that identical genetics leads to similar personalities, once similar environmental factors are accounted for?
There are some amazing stories in Steve Pinker’s latest book. Clearly, to the extent that personality is genticially determined, there will be great similarities. We are discovering that there are large and small elements of personality that do seem to be genetic.
Pinker cites examples where identicial twins separated since birth both compulsively wind a string around one particular finger, spend all their weekends hanggliding, share a fear of rodents, and are both accountants. (Example is not real, but the level of specificity was that precise.) Hard to explain those traits any other way.
A strict answer to your question would be “Yes, there is evidence from numerous studies that identical twins are more likely to have similar personalities than fraternal twins or other types of siblings, who are in turn more likely to have similar personalities than unrelated individuals”. Most of the studies I’ve seen references to seem to suggest than personality is roughly 50% heritable, 50% environmental. Almost depressingly predictable results, perhaps, but no less valid for that.
Matt Ridley’s Nature Via Nurture : Genes, Experience, and What Makes Us Human admirably deals with the evidence on both sides of the nature/nurture question, as does Steven Pinker’s somewhat more polemical The Blank Slate: The Modern Denial of Human Nature. The strength of Ridley’s argument is that he focuses on the complex, inextricable interdependence of genetics and enviroment in determining personality.