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Is there any drawback in putting all my 401K into a money market fund?
(After seeing your most recent comment on the original question, it looks like others have answered the question you intended, and described the extreme difficulty of getting the timing right the way you're trying to. Since I've already typed it up, what follows answers what I originally thought your question was, which was asking if there were drawbacks to investing entirely in money market funds to avoid stock volatility altogether.) Money market funds have the significant drawback that they offer low returns. One of the fundamental principles in finance is that there is a trade-off between low risk and high returns. While money market funds are extremely stable, their returns are paltry; under current market conditions, you can consider them roughly equivalent to cash. On the other hand, though investing in stocks puts your money on a roller coaster, returns will be, on average, substantially higher. Since people often invest in order to achieve personal financial stability, many feel naturally attracted to very stable investments like money market funds. However, this tendency can be a big mistake. The higher returns of the stock market don't merely serve to stoke an investor's greed, they are necessary for achieving most people's financial goals. For example, consider two hypothetical investors, saving for retirement over the course of a 40-year career. The first investor, apprehensive Adam, invests $10k per year in a money market fund. The second investor, brave Barbara, invests $10k per year in an S&P 500 index fund (reinvesting dividends). Let's be generous and say that Adam's money market fund keeps pace with inflation (in reality, they typically don't even do that). At the end of 40 years, in today's money, Adam will have $10,000*40 = $400,000, not nearly enough to retire comfortably on. On the other hand, let's assume that Barbara gets returns of 7% per year after inflation, which is typical (though not guaranteed). Barbara will then have, using the formula for the future value of an annuity, $10,000 * [(1.07)^40 - 1] / 0.07, or about $2,000,000, which is much more comfortable. While Adam's strategy produces nearly guaranteed results, those results are actually guaranteed failure. Barbara's strategy is not a guarantee, but it has a good chance of producing a comfortable retirement. Even if her timing isn't great, over these time scales, the chances that she will have more money than Adam in the end are very high. (I won't produce a technical analysis of this claim, as it's a bit complicated. Do more research if you're interested.)