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Why might it be a bad idea to invest 100% of your 401(k) into a stock index fund?
At your age, I don't think its a bad idea to invest entirely in stocks. The concern with stocks is their volatility, and at 40+ years from retirement, volatility does not concern you. Just remember that if you ever want to call upon your 401(k) for anything other than retirement, such as a down payment on a home (which is a qualified distribution that is not subject to early distribution penalties), then you should reconsider your retirement allocations. I would not invest 100% into stocks if I knew I were going to buy a house in five years and needed that money for a down payment. If your truly saving strictly for a retirement that could occur forty years in the future, first good for you, and second, put it all in an index fund. An S&P index has a ridiculously low expense ratio, and with so many years away from retirement, it gives you an immense amount of flexibility to choose what to do with those funds as your retirement date approaches closer every year.