Is there any data that shows how diversifying results in better returns than just sticking to an all-stock portfolio?
"This paper by a Columbia business school professor says: The standard 60%/40% strategy outperforms a 100% bond or 100% stock strategy over the 1926-1940 period (Figure 5) and over the 1990-2011 period (Figure 6). This is based on actual market data from those periods.  You can see the figures in the PDF.  These are periods of 14 and 21 years, which is perhaps shorter than the amount of time money would sit in your IRA, but still a fairly long time.  The author goes on with a lot of additional discussion and claims that ""under certain conditions, rebalancing will always outperform a buy-and-hold portfolio given sufficient time"". Of course, there are also many periods over which a given asset mix would underperform, so there are no guarantees here.  I read your question as asking ""is there any data suggesting that rebalancing a diversified portfolio can outperform an all-in-one-asset-class portfolio"".  There is some such data.  However, if you're asking which investing strategy you should actually choose, you'd want to look at a lot of data on both sides.  You're unlikely to find data that ""proves"" anything conclusively either way. It should also be noted that the rebalancing advantage described here (and in your question) is not specific to bonds.  For instance, in theory, rebalancing between US and international stocks could show a similar advantage over an all-US or all-non-US portfolio. The paper contains a lot of additional discussion about rebalancing.  It seems that your question is really about whether rebalancing a diverse portfolio is better than going all-in with one asset class, and this question is touched on throughout the paper.  The author mentions that diversification and rebalancing strategies should be chosen not solely for their effect on mathematically-calculated returns, but for their match with your psychological makeup and tolerance for risk."
diversifying; but isn't that what mutual funds already do? They diversify and reduce stock-specific risk by moving from individual stocks to many stocks, but you can diversify even further by selecting different fund types (e.g. large-cal, small-cap, fixed- income (bond) funds, international, etc.).  Your target-date fund probably includes a few different types already, and will automatically reallocate to less risky investments as you get close to the target date. I would look at the fees of different types of funds, and compare them to the historical returns of those funds.  You can also use things like morningstar and other ratings as guides, but they are generally very large buckets and may not be much help distinguishing between individual funds. So to answer the question, yes you can diversify further - and probably get better returns (and lower fees) that a target-date fund.  The question is - is it worth your time and effort to do so?  You're obviously comfortable investing for the long-term, so you might get some benefit by spending a little time looking for different funds to increase your diversification. Note that ETFs don't really diversify any differently than mutual funds, they are just a different mechanism to invest in funds, and allow different trading strategies (trading during the day, derivatives, selling short, etc.). "Rebalancing has been studied empirically quite a bit, but not particularly carefully and actually turns out to be very hard to study well.  The main problem is that you don't know until afterward if your target weights were optimal so a bad rebalancing program might give better performance if it strayed closer to optimal weights even if it didn't do an efficient job of keeping near the target weights. In your particular case either method might be preferred depending on a number of things: You can see why there isn't a generally correct answer to your question and the results of empirical studies might very wildly depending on the mix of assets and risk tolerance.  Still if your portfolio is not too complicated you can estimate the costs of the two methods without too much trouble and figure out if it is worthwhile to you. EDIT In Response to Comment Below: Your example gets at what makes rebalancing so hard empirically but also generally pretty easy in practice.  If you were to target 75% Equity (25% bonds?) and look at returns only for 30 years the ""best"" rebalancing method would be to never rebalance and just let 75% equity go to near 100% as equity has better long term returns.  This happens when you look only at returns as the final number and don't take into account the change in risk in your portfolio. In practice, most people that are still adding (or subtracting in retirement) to a retirement portfolio are adding (removing) a significant amount compared to the total amount in their portfolio.  In the case you discribe, it is cheaper (massively cheaper in the presence of load fees) just to use new capital to trade toward your target, keeping your risk profile.  New money should be large enough to keep you near enough your target.  If you just estimate the trading costs/fees in both cases I think you'll see just how large the difference is between the two methods this will dwarf any small differences in return over the long run even if you can't trade back all the way to your target." Diversifying your portfolio between asset A and asset B only reduces the portfolio risk if asset A and asset B are not correlated. If they have either a low correlation or a negative correlation to each other, then you benefit from combining them in a portfolio in terms of risk reduction. The standard deviation of returns will be lower in a portfolio of low or uncorrelated assets. If on the other hand you combine two correlated assets into a portfolio you are doubling down on the same assumption, which means you are not reducing your risk. You are also wasting capital because now you have allocated capital to 2 separate trades / investments yet they have shown a high tendency of moving together. Here is an article that discusses this further: Why Diversify your Stock Portfolio Some years your portfolio may perform better than the benchmark, and some years it may be the other way around.  Without a benchmark you will never know. And by the way if you choose poorly, you will never beat the benchmark. If the benchmark goes up 20% but your fund/investment only went up 3% you did make money, but you might want to reevaluate your strategy. Stock portfolios have diversifiable risk and undiversifiable risk.  The market rewards investors for taking undiversifiable risk (e.g. owning an index of oil producing companies) and does not reward investors for assuming diversifiable risk (e.g. owning a single oil producing company).  The market will not provide investors with any extra return for owning a single oil company when they can buy an oil index fund at no additional cost. Similarly, the market will not reward you for owning a small-cap index fund when you can purchase a globally diversified / capitalization diversified index fund at no additional cost.  This article provides a more detailed description. The Vanguard Total World Stock Index Fund is a much better staring point for an equity portfolio. You will need to make sure that the asset allocation of your overall portfolio (e.g. stocks, bonds, P2P lending, cash) is consistent with your time horizon (5-10 years). 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. If you have 100% of your money in one security that is inherently more risky than splitting your money 50/50 between two securities, regardless of the purported riskiness of the two securities. The calculations people use to justify their particular breed of diversification may carry some assumptions related risk/reward calculations.  But these particular justifications don't change the fact that spreading your money across different assets protects your money from value variances of the individual assets.  Splitting your $100 between Apple and Microsoft stock is probably less valuable (less well diversified) than splitting your money between Apple and Whole Foods stock but either way you're carrying less risk than putting all $100 in to Apple stock regardless of the assumed rates of return for any of these companies stock specifically. Edit: I'm sure the downvotes are because I didn't make a big deal about correlation and measuring correlation and standard deviations of returns and detailed portfolio theory.  Measuring efficacy and justifying your particular allocations (that generally uses data from the past to project the future) is all well and good.  Fact of the matter is, if you have 100% of your money in stock that's more stock risk than 25% in cash, 25% in bonds and 50% in stock would be because now you're in different asset classes.  You can measure to your hearts delight the effects of splitting your money between different specific companies, or different industries, or different market capitalizations, or different countries or different fund managers or different whatever-metrics and doing any of those things will reduce your exposure to those specific allocations.   It may be worth pointing out that currently the hot recommendation is a plain vanilla market tracking S&P 500 index fund (that just buys some of each of the 500 largest US companies without any consideration given to risk correlation) over standard deviation calculating actively managed funds.  If you ask me that speaks volumes of the true efficacy of hyper analyzing the purported correlations of various securities. There is no research or scientific data to arrive at how much in Single Fund is good, how much is more risky. Its a question of risk appetite, how much money if you loose you are fine. This is similar to making a decision as to how much diversified should a portfolio be ... i.e. what mix of FD/Mutual Funds/Shares/Gold/Real Estate one should have. 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). The reason diversification in general is a benefit is easily seen in your first graph.  While the purple line (Betterment 100% Stock) is always below the blue line (S&P), and the blue line is the superior return over the entire period, it's a bit different if you retired in 2009, isn't it?  In that case the orange line is superior: because its risk is much lower, so it didn't drop much during the major crash.  Lowering risk (and lowering return) is a benefit the closer you get to retirement as you won't see as big a cumulative return from the large percentage, but you could see a big temporary drop, and need your income to be relatively stable (if you're living off it or soon going to).   Now, you can certainly invest on your own in a diverse way, and if you're reasonably smart about it and have enough funds to avoid any fees, you can almost certainly do better than a managed solution - even a relatively lightly managed solution like Betterment.  They take .15% off the top, so if you just did exactly the same as them, you would end up .15% (per year) better off. However, not everyone is reasonably smart, and not everyone has much in the way of funds. Betterment's target audience are people who aren't terribly smart about investing and/or have very small amounts of funds to invest.  Plenty of people aren't able to work out how to do diversification on their own; while they probably mostly aren't asking questions on this site, they're a large percentage of the population.  It's also work to diversify your portfolio: you have to make minor changes every year at a minimum to ensure you have a nicely balanced portfolio.  This is why target retirement date portfolios are very popular; a bit higher cost (similar to Betterment, roughly) but no work required to diversify correctly and maintain that diversification. Diversification is a risk-mitigation strategy. When you invest in equities, you generally get a higher rate of return than a fixed income investment. But you have risks... a single company's market value can decline for all sorts of reasons, including factors outside of the control of management. Diversification lets you spread risk and concentrate on sectors that you feel offer the best value. Investing outside of your currency zone allows you to diversify more, but also introduces currency risks, which require a whole other level of understanding. Today, investing in emerging markets is very popular for US investors because these economies are booming and US monetary policy has been weakening the dollar for some time. A major bank failure in China or a flip to a strong dollar policy could literally implode those investments overnight. At the end of the day, invest in what you understand. Know the factors that can lower your investment value. "Do not reinvent the wheel! Historical data about stock market returns and standard deviations suffer from number of issues such as past-filling and mostly survivorship bias -- that the current answers do not consider at all. I suggest to read the paper ""A Century of Global Stock Markets"" by Philippe Jorion (UC Irvine) and William Goetzmann (Yale), here. William Bernstein comments the results here, notice that rebalancing is sometimes a good option but not always, his non-obvious finding where the low SD did not favour from rebalancing:  Look at the final page of the paper,  ""geometric returns -- represent returns to a buy-and-hold strategy"" and the ""arithmetic averages -- give equal weight to each observation interval."", where you can find your asked ""historical effect of Rebalancing on Return and Standard Deviation"". The paper nicely summarizes the results to this table:  The results in the table are from the interval 1921-1996, it is not that long-time but even longer term data has its own drawbacks. The starting year 1921 is interesting choice because it is around the times of social-economical changes and depressing moments, historical context can be realized from books such as Grapes Of Wrath (short summary here, although fiction to some extent, it has some resonance to the history). The authors have had to ignore some years because of different reasons such as political unrest and wars.  Instead of delving into marketed spam as suggested by one reply, I would look into this search here. Look at the number of references and the related papers to judge their value.  P.s. I encourage people to attack my open question here, hope we can solve it!" "Index funds are well-known to give the best long-term investment. Not exactly. Indexes give the best long term performance when compared to actively managing investments directly in the underlying stocks. That is, if you compare an S&P500 index to trying to pick stocks that are part of it, you're more likely to succeed with blindly following the index than trying to actively beat it. That said, no-one promises that investing in S&P500 is better than investing in DJIA, for example. These are two different indexes tracking different stocks and areas. So when advisers say ""diversify"" they don't mean it that you should diversify between different stocks that build up the S&P500 index. They mean that you should diversify your investments in different areas. Some in S&P500, some in DJIA, some in international indexes, some in bond indexes, etc. Still, investing in various indexes will likely yield better results than actively managing the investments trying to beat those indexes, but you should not invest in only one, and that is the meaning of diversification. In the comments you asked ""why diversify at all?"", and that is entirely a different question from your original ""what diversification is?"". You diversify to reduce the risk of loss from one side, and widen the net for gains from another. The thing is that any single investment can eventually fail, regardless of how it performed before. You can see that the S&P500 index lost 50% of its value twice within ten years, whereas before it was doubling itself every several years. Many people who were only invested in that index (or what's underlying to it) lost a lot of money. But consider you've diversified, and in the last 20 years you've invested in a blend of indexes that include the S&P500, but also other investments like S&P BSE SENSEX mentioned by Victor below. You would reduce your risk of loss on the American market by increasing your gains on the Indian market. Add to the mix soaring Chinese Real Estate market during the time of the collapse of the US real-estate, gains on the dollar losing its value by investing in other currencies (Canadian dollar, for example), etc. There are many risks, and by diversifying you mitigate them, and also have a chance to create other potential gains. Now, another question is why invest in indexes. That has been answered before on this site. It is my opinion that some methods of investing are just gambling by trying to catch the wave and they will almost always fail, and rarely will individual stock picking beat the market. Of course, after the fact its easy to be smart and pick the winning stocks. But the problem is to be able to predict those charts ahead of time." No, there is no real advantage. The discrepancies in how they track the index will (generally) be so small that this provides very, very limited diversification, while increasing the complexity of your investments. "There is actually a recent paper that attempted to decompose Buffett's outperformance. I've quoted the abstract below: ""Berkshire Hathaway has realized a Sharpe ratio of 0.76, higher than any other stock or mutual fund with a history of more than 30 years, and Berkshire has a significant alpha to traditional risk factors. However, we find that the alpha becomes insignificant when controlling for exposures to Betting-Against-Beta and Quality-Minus-Junk factors. Further, we estimate that Buffett’s leverage is about 1.6-to-1 on average. Buffett’s returns appear to be neither luck nor magic, but, rather, reward for the use of leverage combined with a focus on cheap, safe, quality stocks. Decomposing Berkshires’ portfolio into ownership in publicly traded stocks versus wholly-owned private companies, we find that the former performs the best, suggesting that Buffett’s returns are more due to stock selection than to his effect on management. These results have broad implications for market efficiency and the implementability of academic factors."""