After seeing how different an adopted child acted compared to the non adopted ones in a close family member's family, I believe a lot more in nature causing big differences in behavior vs. nurture. Even when you have your own kids, children born 1 or 2 years apart in the same family can have very different personalities.

People don't like it, because it's used to justify fatalistic write offs of people. I agree it's wrong to do that because people are often wrong in writing off people, especially the aggressively independent :)