The personality metrics they're using are the Big-5. The Big 5 has good test-retest scores (if you take the test a month apart, you're likely to get the same scores). Big 5 is used in a ton of psychological studies now.

If I had to bet on nature vs nurture, I'd place bets on nature. Separated twin studies seem to suggest genes are a strong determiner. And if you've ever met a kid who stubbornly prefers to be introverted or extroverted-- it often has nothing to do with how adults are treating them.