Environment vs. Innate Personality

My sister, Shakespeare, was talking the other day about motivation. I've discussed in terms of characters before a few times.

But my mind is a festering pool of associative logic and it got me thinking on another philosophical question that frequently comes up - what has the greatest influence on a character, environment or innate personality?

It's a philosophically big question and there's no "right" answer (in part, in my opinion, because both are factors but I get ahead of myself). But I have my own opinions.

In my own opinion, and in general, innate personality is the key element, not because environment isn't important but because personality determines how an individual responds to a particular environment. An environment that can break one person can make another stronger. An abused child can grow up to abuse other children or can grow up to become a social worker and protect other children.

You might think this question isn't important for a writing blog, but I think it is for a writer. When you have a character, he or she is at least partially developed already. You should understand both the environment and the innate traits that made him or her as they are now (and hopefully that combination makes sense) whether you pass that along to the reader or not. But you should understand why they are the way they are. I have several fairly abrasive characters. In each case, I understand why they are distrustful or caustic or argumentative (and usually provide enough background that the reader can understand even if it doesn't make them like a character per se).

If I don't understand them understand how they got how they are, how can I get them to grow as they work their way through the environment and plot I've devised for them?