I.D.

But, as I.D. stresses, acknowledging personality's genetic roots makes it all the more imperative that we understand the role played by environment - not only because it's powerful but also because, unlike our genes, we can do something about it. Indeed, science's ancient definition of temperament as inborn is currently being rewritten as new research shows that our life experience can be literally as well as figuratively engraved in our nervous systems, often as emphatically as the marks of genes.