The solution is simple: Use room for error when estimating your future returns. This is more art than science. For my own investments, which I’ll describe more in chapter 20, I assume the future returns I’ll earn in my lifetime will be ⅓ lower than the historic average. So I save more than I would if I assumed the future will resemble the past. It’s my margin of safety. The future may be worse than ⅓ lower than the past, but no margin of safety offers a 100% guarantee. A one-third buffer is enough to allow me to sleep well at night. And if the future does resemble the past, I’ll be pleasantly
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