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Why would I want a diversified portfolio, versus throwing my investments into an index fund?
Index funds are well-known to give the best long-term investment. Are they? Maybe not all the time! If you had invested in an index fund tracking the S&P500 at the start of 2000 you would still be behind in terms of capital appreciation when taking inflation into considerations. Your only returns in 13.5 years would have been any dividends you may have received. See the monthly chart of the S&P500 below.  Diversification can be good for your overall returns, but diversification simply for diversification sake is as you said, a way of reducing your overall returns in order of smoothing out your equity curve. After looking up indexes for various countries the only one that had made decent returns over a 13.5 year period was the Indian BSE 30 index, almost 400% over 13.5 years, although it also has gone nowhere since the end of 2007 (5.5 years). See monthly chart below.  So investing internationally (especially in developing countries when developed nations are stagnating) can improve your returns, but I would learn about the various international markets first before plunging straight in. Regarding investing in an Index fund vs direct investment in a select group of shares, I did a search on the US markets with the following criteria on the 3rd January 2000:  If the resulting top 10 from the search were bought on 3rd January 2000 and held up until the close of the market on the 19th June 2013, the results would be as per the table below:  The result, almost 250% return in 13.5 years compared to almost no return if you had invested into the whole S&P 500 Index. Note, this table lists only the top ten from the search without screening through the charts, and no risk management was applied (if risk management was applied the 4 losses of 40%+ would have been limited to a maximum of 20%, but possibly much smaller losses or even for gains, as they might have gone into positive territory before coming back down - as I have not looked at any of the charts I cannot confirm this). This is one simple example how selecting good shares can result in much better returns than investing into a whole Index, as you are not pulled down by the bad stocks.