(1) the Pareto Principle – the rule of thumb is that 80% of your problems are caused by 20% of the causes.
(2) William S. Burroughs kind of wrote about this. I remember reading some bit of his that talked about how 10% of the population where what he termed the ‘incorrigible shits’. The other 90% of the population generally mind their own business and have a live and let live mentality, but it was the 10% of shits that made everyone else miserable by sticking their noses into everyone else’s business.