Brain Rules for Baby (John Medina) / quote 64 of 91
Temperament is not just in the genes
So you can see temperament at birth, and it remains stable over time. Does that mean temperament is completely controlled by genes? Hardly. As we saw with the ice-storm children in the Pregnancy chapter, it is possible to create a stressed baby simply by increasing the mother’s stress hormones—not a double helix in sight. The involvement of genes is a scientific question, not a scientific fact. Happily, it is being researched. Studies of twins so far show that there is no one gene responsible for temperament. (Gene work almost always begins with twins, the gold standard being twins separated at birth and raised in different households.) When you look at the temperaments of identical twins, the degree of resemblance, the correlation, is about 0.4. This means there is probably some genetic contribution, but it is not a slam dunk. For fraternal twins and non-twin siblings, the correlation is between 0.15 and 0.18, even less of a slam dunk.