There are reasons to expect the US economy to grow more slowly in the future than it has in the past. It doesn't seem to be growing at an exponential rate, which means its yearly % growth will go down over time.
Unfortunately government entities often perform estimates of future tax revenue, debt and spending ( e.g. of social security and medicare) based on unrealistically optimistic hopes for how fast our economy will grow. Private retirement plans are often made assuming future percentage returns on investment will match past returns, which may not be the case for those who invest only in the US.
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