good chat between philosopher Patricia Churchland and Ed Wilson where Wilson suggests we shouldn't even look at it that way as it is obsolete. Wilson was the guy who picked a fight with the blank-slate orthodox back in the 70's when he published the book Sociobiology and was unprepared for the political backlash which was pretty brutal at the time.
I know that they avoid explicitly stating there is an innate factor when it comes to children because that implies they can never improve and people go berserk at the suggestion. My European friends tell me that American tourists are loud and obnoxious. The extroverted thing does appear to be heavily influenced by culture. Psychologist Laurie Helgoe contests the notion that most people are extroverted in a book she wrote showing studies where the majority actually identified themselves as introverted. I watched a documentary about psychopathy awhile back and a researcher from Glasgow was saying that comparisons show that psychopaths tend to be more reserved and quiet in the UK, whereas in North America they are very outgoing and charismatic.
Neuroscience is really adding to our understanding of these things, it seems like everyday something exciting is coming out of that field and I am tempted to go back and study it. Don't get me started!