Vilfredo Pareto (1848-1923) was a famous Italian economist who, amongst other things, derived some interesting relationships between income levels and the number of people who received more than that income.  Anecdotal evidence credits Pareto for the original recognition that 80% of the wealth (or it may have been income) is owned by 20% of the people—the original 80/20 rule.  The “rule” has now been extended into all sorts of areas that often have little supporting evidence behind them.  Next time you hear an out-of-context rule-of-thumb, remember this version of the 80/20 rule: that 80% of rules-of-thumb only apply 20% of the time!