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How to rebalance a portfolio without moving money into losing investments
A strategy of rebalancing assumes that the business cycle will continue, that all bull and bear markets end eventually.  Imagine that you maintained a 50% split between a US Treasury bond mutual fund (VUSTX) and an S&P 500 stock mutual fund (VFINX) beginning with a $10,000 investment in each on January 1, 2008, then on the first of each year you rebalanced your portfolio on the first of January (we can pretend the markets are open that day). The following table illustrates the values in each of those funds with the rebalancing transactions: This second table shows what that same money would look like without any rebalancing over those years: Obviously this is cherry-picking for the biggest drop we've recently experienced, but even if you skipped 2008 and 2009, the increase for a rebalanced portfolio from 2010-2017 is 85% verses 54% for the portfolio that is not being rebalanced in the same period. This is also a plenty conservative portfolio. You can see that a 100% stock portfolio dropped 40% in 2008, but the combined portfolio only dropped 18%. A 100% stock portfolio has gained 175% since 2009, compared to 105% for the balanced portfolio, but it's common to trade gains for safety as you get closer to retirement. You didn't ask about a 100% stock portfolio in your initial question. These results would be repeated in many other portfolio allocations because some asset classes outperform others one year, then underperform the next. You sell after the years it outperforms, then you buy after years that it underperforms.