I put 100% of my 401k into an S&P index fund.
Do I need to rebalance that, or is rebalancing only necessary when you have multiple asset allocations such as 80% in stock, and 20% in bonds and then you rebalance to keep that same allocation.
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There's nothing to rebalance, the index fund rebalances itself to continue matching the index.
However, you need to understand that such an investment is not diversified and you only invest in a very specific market, and very specific stocks on that market. S&P 500 is large (500 different companies, most of the time), but still not as broadly diversified as your retirement investment portfolio should be.
You should talk to a financial adviser (CFP for example), many companies provide access to these for 401k plan participants. But in any case, I'd suggest considering "target date" funds - funds that are investing based on your expected retirement year, and become more conservative as you get closer to that year.
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.