How to rebalance a passive portfolio if I speculate a war is coming?
At a risk of stating the obvious: a passive portfolio doesn't try to speculate on such matters. Normally, in a war everybody suffers and the entire economy goes down. Military contractors do better than average, but the average sucks. The way to take advantage of knowing a war is coming is to leave as soon as possible. There are strategic materials that can become valuable in a war, but such investments are generally very specialized and not something an ordinary investor would be in a position to exploit.  The most profitable businesses in war are food, oil, and ammunition.
"Not a bond holder, but when we get dividends we usually just buy up a benchmark index tracking ETF unless/until we're ready to rebalance our portfolio. Most of the trades in the day are earmarked with the reason ""spending cash"". I'd assume it's similar for bond holders and coupons." "If the stock market dropped 30%-40% next month, providing you with a rare opportunity to buy stocks at a deep discount, wouldn't you want to have some of your assets in investments other than stocks? If you don't otherwise have piles of new cash to throw into the market when it significantly tanks, then having some of your portfolio invested elsewhere will enable you to back up the proverbial truck and load up on more stocks while they are on sale. I'm not advocating active market timing. Rather, the way that long-term investors capitalize on such opportunities is by choosing a portfolio asset allocation that includes some percentage of safer assets (e.g. cash, short term bonds, etc.), permitting the investor to rebalance the portfolio periodically back to target allocations (e.g. 80% stocks, 20% bonds.) When rebalancing would have you buy stocks, it's usually because they are on sale. Similarly, when rebalancing would have you sell stocks, it's usually because they are overpriced. So, don't consider ""safer investments"" strictly as a way to reduce your risk. Rather, they can give you the means to take advantage of market drops, rather than just riding it out when you are already 100% invested in stocks. I could say a lot more about diversification and risk reduction, but there are plenty of other great questions on the site that you can look through instead." The first step I would do is determine the asset class mixture for your current portfolio and the mixture for your new one. If they are the same and all you are doing is changing the funds that you use to invest in that mixture of asset class then just do the change all at once. In this case there is no market risk as you are just swapping funds (hopefully to ones that you feel will better track the underlying asset classes).  If you are also changing your asset class mixture, then it depends on how large the change is. I would still do the whole change at once. But if you are worried about fluctuations then you could slowly rebalance into your final position by taking a couple of intermediary steps. I would still change all of the fund first but maybe in a mix closer to your current asset mix and then over the next couple of months adjust the ratios to reach your final desired asset mix. Also, almost by definition rebalancing involves making more trades   than you would have otherwise; wouldn't the additional trading fees   you incurred in doing so reduce the benefits of this strategy? You forgot to mention taxes. Rebalancing does or rather can incur costs. One way to minimize the costs is to use the parts of the portfolio that have essentially zero cost of moving. These generally are the funds in your retirement accounts. In the United States they can be in IRAs or 401Ks; they can be regular or Roth. Selling winners withing the structure of the plan doesn't trigger capital gains taxes, and many have funds within them that have zero loads.  Another way to reduce trading fees is to only rebalance once a year or once every two years; or by setting a limit on how far out of balance. For example don't rebalance at 61/39 to get back to 60/40 even if it has been two years.  Given that the ratio of investments is often rather arbitrary to begin   with, how do I know whether I'm selling high and buying low or just   obstinately sticking with a losing asset ratio? The ratio used in an example or in an article may be arbitrary, but your desired ratio isn't arbitrary. You selected the ratio of your investments based on several criteria: your age, your time horizon, your goals for the money, how comfortable you are with risk. As these change during your investing career those ratios would also morph. But they aren't arbitrary.  These decisions to rebalance are separate from the ones to sell a particular investment. You could sell Computer Company X because of how it is performing, and buy stock in Technology Company Y because you think it has a better chance of growing. That transaction would not be a re-balancing. Selling part of your stock in Domestic Company A to buy stock in international Company B would be part of a re-balancing. "There are ways to mitigate, but since you're not protected by a tax-deferred/advantaged account, the realized income will be taxed. But you can do any of the followings to reduce the burden: Prefer selling either short positions that are at loss or long positions that are at gain. Do not invest in stocks, but rather in index funds that do the rebalancing for you without (significant) tax impact on you. If you are rebalancing portfolio that includes assets that are not stocks (real-estate, mainly) consider performing 1031 exchanges instead of plain sale and re-purchase. Maximize your IRA contributions, even if non-deductible, and convert them to Roth IRA. Hold your more volatile investments and individual stocks there - you will not be taxed when rebalancing. Maximize your 401K, HSA, SEP-IRA and any other tax-advantaged account you may be eligible for. On some accounts you'll pay taxes when withdrawing, on others - you won't. For example - Roth IRA/401k accounts are not taxed at all when withdrawing qualified distributions, while traditional IRA/401k are taxed as ordinary income. During the ""low income"" years, consider converting portions of traditional accounts to Roth." "To answer your question directly..  you can investigate by using google or other means to look up research done in this area.  There's been a bunch of it Here's an example of search terms that returns a wealth of information. effect+of+periodic+rebalancing+on+portfolio+return I'd especially look for stuff that appears to be academic papers etc, and then raid the 'references' section of those.  Look for stuff published in industry journals such as ""Journal of Portfolio Management"" as an example. If you want to try out different models yourself and see what works and what doesn't, this Monte Carlo Simulator might be something you would find useful The basic theory for those that don't know is that various parts of a larger market do not usually move in perfect lockstep, but go through cycles.. one year tech might be hot, the next year it's healthcare.  Or for an international portfolio, one year korea might be doing fantastic only to slow down and have another country perform better the next year.    So the idea of re-balancing is that since these things tend to be cyclic, you can get a higher return if you sell part of a slice that is doing well (e.g. sell at the high) and invest it in one that is not (buy at the low) Because you do this based on some criteria, it helps circumvent the human tendency to 'hold on to a winner too long' (how many times have you heard someone say 'but it's doing so well, why do I want to sell now""? presuming trends will continue and they will 'lose out' on future gains, only to miss the peak and ride the thing down back into mediocrity.)  Depending on the volatility of the specific market, and the various slices, using re balancing can get you a pretty reasonable 'lift' above the market average, for relatively low risk. generally the more volatile the market, (such as say an emerging markets portfolio) the more opportunity for lift. I looked into this myself a number of years back, the concensus I came was that the most effective method was to rebalance based on 'need' rather than time.  Need is defined as one or more of the 'slices' in your portfolio being more than 8% above or below the average.  So you use that as the trigger. How you rebalance depends to some degree on if the portfolio is taxable or not.  If in a tax deferred account, you can simply sell off whatever is above baseline and use it to buy up the stuff that is below.   If you are subject to taxes and don't want to trigger any short term gains, then you may have to be more careful in terms of what you sell.  Alternatively if you are adding funds to the portfolio, you can alter how your distribute the new money coming into the portfolio in order to bring up whatever is below the baseline (which takes a bit more time, but incurs no tax hit)  The other question is how will you slice a given market?  by company size?  by 'sectors' such as tech/finance/industrial/healthcare, by geographic regions?" "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." You are correct that over a short term there is no guarantee that one index will out perform another index. Every index goes through periods of feat and famine. That uis why the advice is to diversify your investments. Every index does have some small amount of management. For the parent index (the S&P 500 in this case) there is a process to divide all 500 stocks into growth and value, pure growth and pure value. This rebalancing of the 500 stocks occurs once a year. Rebalancing The S&P Style indices are rebalanced once a year in December. The   December rebalancing helps set the broad universe and benchmark for   active managers on an annual cycle consistent with active manager   performance evaluation cycles. The rebalancing date is the third   Friday of December, which coincides with the December quarterly share   changes for the S&P Composite 1500.  Style Scores,   market-capitalization weights, growth and value midpoint averages, and   the Pure Weight Factors (PWFs), where applicable across the various   Style indices, are reset only once a year at the December rebalancing. Other changes to the U.S. Style indices are made on an as-needed   basis, following the guidelines of the parent index. Changes in   response to corporate actions and market developments can be made at   any time. Constituent changes are typically announced for the parent   index two-to-five days before they are scheduled to be implemented. Please refer to the S&P U.S. Indices Methodology document for   information on standard index maintenance for the S&P 500, the S&P   MidCap 400,the S&P SmallCap 600 and all related indices. As to which is better: 500, growth,value or growth and value? That depends on what you the investor is trying to do.