Luck

I know it will raise some peoples hackles but:

An article in the New Scientist (it's in the re-cycling right now) reported on a meta analysis which put nurture at between 0 and 20% (but to appease the politically motivated make it 25%)
- it depends on the subject.

What was found was that luck plays a part in how we present as a personality. And just to put a number on the Nature contribution they had a figure around 1/2.

Which leaves maybe a third down to luck. And like the Nature v Nurture argument - you can argue the percentages till the politics become bloody obvious, but look at your own life before you do.