Or maybe the NT's through less socialisation have less offspring... the meek are supposed to inherit the earth are they not?
I think that would of eliminated NTs from the population, although we could say we are moving towards that.
More accurately, I think that it should be said that the tail ends are eliminated from the gene pool, so there is a pruning off both sides (the mentally unstable ones). As such, the core personality traits remain the same... I'd say that the distribution is dependent on the series of gene combination that create express these traits.
Another reason for one particular set to be dominant over the other is historical survival rates, which could be associated to Ss (philosophers and such don't tend to get much respect in rough times!)... but currently that isn't much of an issue. However, IQ and children are anti-correlated, so that could be an issue of reproducability... smart people not being all that horn doggy. That might off been true before now too.
So, I think the three core factors are;
1) Gene configuration is balanced around the presence in society (dominance, etc)
2) Survival traits historically have weeded out certain traits (namely S:N)
3) Certain traits lead to less offspring