From what I recall, Pinker makes no such assertion. Rather, he argues that we underestimate genetic effects and that while this is understandable from an ethical/political perspective, it is ultimately harmful.

In fact, I vaguely recall that on the issue of parenting he argued that genetics play a big role, but that so does parenting.

Basically, he's arguing that we factor in genetics more, not that we disregard parenting and social environment as a significant factor.

(but be sure to let me know if I'm wrong. It's been quite a while since I read The Blank Slate)