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What type of low-cost stock index exchange-traded fund (ETF) would give the best long-term total return?
Small cap and mid cap shares tend to outperform large cap shares in a bull market, but they tend to underperform large cap shares in a bear market. Since the stock markets tend to go up in the long term, this suggests that a low cost small and mid cap index ETF should offer the best long term returns.   Having said that, we are currently in a mature bull market having experienced over seven years without encountering a bear market.  If a bearish outlook is something you worry about, then perhaps a broad market index, which will be heavily weighted towards large cap shares, may be a better choice for you at this time, with an eye toward switching to small and mid cap indices during the next bear market.