It's a fairly complicated subject, but opinions tend to fall more or less into two schools of thought: some people believe that behavior and personality are inherent and decided from birth, and some people believe that humans start as a blank slate and are taught behavior by those around them.

I personally fall into the latter camp. I am a twin. My brother and I are perfectly genetically identical, and yet our personalities could not be more different. If a single case can disprove your theory then it isn't incredibly sound.

[Sure, they have similarities -- they were raised in the same time period, by the same parents. But there are fundamental differences in Ford's opinion.]