I agree with NYCLurker, and I have been involved in the industry of managing retirement and other long-term funds for 20 years – and, no, I’m not a blind believer in the endless high growth of equity values.
If you want to avoid the risk of investing in equities, you usually can choose a more conservative 401k option that has a more stable value. Personally, I would suggest that you invest at least some in some equity funds, especially if you are young. You need to form a strategy for the next 30-50 years before you come up with tactical adjustments for the next 1-5 years. Any very long-term strategy for a young person almost certainly should invove some diversified investment in equities. This board has more than its share of extremely rsk-adverse people focused on what investments will do over the next 5-10 years, who may not provide the best advice for someone looking to get the best return over a very long period, like 30-50 years.
If you think that the next 5-10 years will be times of excessive prices for equities, then just plan to allocate your 401k money to the fixed/stable accounts for that period, but try to maximize what you contribute to all these tax-favored accounts.
