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To rebalance or not to rebalance
An asset allocation formula is useful because it provides a way to manage risk. Rebalancing preserves your asset allocation. The investment risk of a well-diversified portfolio (with a few ETFs or mutual funds in there to get a wide range of stocks, bonds, and international exposure) is mostly proportional to the asset class distribution. If you started out with half-stocks and half-bonds, and stocks surged 100% over the past few years while bonds have stayed flat, then you may be left with (say) 66% stocks and 33% bonds. Your portfolio is now more vulnerable to future stock market drops (the risk associated with stocks). (Most asset allocation recommendations are a little more specific than a stock/bond split, but I'm sure you can get the idea.) Rebalancing can be profitable because it's a formulaic way to enforce you to "buy low, sell high". Massive recessions notwithstanding, usually not everything in your portfolio will rise and fall at the same time, and some are actually negatively correlated (that's one idea behind diversification, anyway). If your stocks have surged, chances are that bonds are cheaper. This doesn't always work (repeatedly transferring money from bonds into stocks while the market was falling in 2008-2009 could have lost you even more money). Also, if you rebalance frequently, you might incur expenses from the trading (depending on what sort of financial instrument you're holding). It may be more effective to simply channel new money into the sector that you're light on, and limit the major rebalancing of the portfolio so that it's just an occasional thing. Talk to your financial adviser. :)