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Is a “total stock market” index fund diverse enough alone?
and seems to do better than the S&P 500 too. No, that's not true. In fact, this fund is somewhere between S&P500 and the NASDAQ Composite indexes wrt to performance. From my experience (I have it too), it seems to fall almost in the middle between SPY and QQQ in daily moves. So it does provide diversification, but you're basically diversifying between various indexes. The cost is the higher expense ratios (compare VTI to VOO).