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Does it make sense to buy an index ETF (e.g. S&P 500) when the index is at an all-time high?
In 1929 the Dow Jones Industrial Average peaked at roughly 390 just prior to the Great Depression. It did not return to that level again until 25 years later in 1954. 25 years is a long time to go without any returns, especially if you are a retiree. There is no easy answer with investing. Trying to time the tops and bottoms is widely regarded as a foolhardy endeavor, but whenever you invest you expose yourself to the possibility of this scenario. The only thing I highly recommend is not to base your decision on the historical returns from 1975 to 2000 that the other answers have presented. These returns can be explained by policy changes that many are coming to understand are unsustainable. The growth of our debt, income inequality, and monetary manipulation by central banks are all reasons to be skeptical of future returns.