There have been way too many studies showing the similarities between twins raised in totally different situations to lead me to believe in the nature side rather than the nurture side.
There have been numerous studies showing identical twins who were seperated and raised in different environments (posh suburb vs gritty inner-city). In nearly every single case, the twins turned out to exhibit the same behaviors.
The opposite studies have also been done where children from different parents have been adopted into the same home and raised the same way - yet they end up like night and day.
People are uncomfortable with this, because they think it means that we are all doomed by our genetic make-up and have no choice how we will end up. It's a rational fear - and of course anyone can change themselves and their behavior.
But - if we have no problem with genes determining our height, or eye color, our hair color and our looks - it's not much of a stretch to believe that those same genes determine our personality.
Most old sayings have a kernel of truth to them: "Blood will tell" is one of them.
But deciding what is part of "personality" is the trick here. That's not so easy as physical traits. We were discussing having the gumption to fight wars. I don't think you'll find a twin study to support genetics for that one...unless of course someone has finally found Patton's twin.