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Is there data and proof that a diversified portfolio can generate higher returns than the S&P 500 Index?
While it's definitely possible (and likely?) that a diversified portfolio generates higher returns than the S&P 500, that's not the main reason why you diversify. Diversification reduces risk.  Modern portfolio theory suggests that you should maximize return while reducing risk, instead of blindly chasing the highest returns. Think about it this way--say the average return is 11% for large cap US stocks (the S&P 500), and it's 10% for a diversified portfolio (say, 6-8 asset classes). The large cap only portfolio has a 10% chance of losing 30% in a given year, while the diversified portfolio has a 1% chance of losing 30% in a year. For the vast majority of investors, it's worth the 1% annual gap in expected return to greatly reduce their risk exposure. Of course, I just made those numbers up. Read what finance professors have written for the "data and proof". But modern portfolio theory is believed by a lot of investors and other finance experts. There are a ton of studies (and therefore data) on MPT--including many that contradict it.