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How (or is it necessary) to rebalance a 401k with only one index fund?
Rebalance is across asset-classes which are mutually independent [like stocks and bonds; they may be inversely correlated at times as when stocks go down, bonds go up] 80%-20% (stock-bond) split is good for a young investor [say in 30s, some suggest 110-age as a good stock allocation percentage]. Here rebalance is done when say the asset-allocation(AA) strays away more than say 3 to 5% (again just a rule of thumb). E.g. if due to a recent run-up in stocks, AA could become 85%-15%. Then you sell stocks to buy bonds to make the AA 80%-20% And since this method always sells the winner -- you automatically make gains [selling high and buying low] S&P 500 index gives decent diversification within stocks; you want a total-bond-fund to take care of the bond side of your AA.