Rebalancing

Rebalancing is an investment tactic used to maintain a consistent mix of asset classes (most commonly equities vs. fixed income). This is accomplished by transferring funds from higher-performing classes to lower-performing classes. While potentially counterintuitive, rebalancing ensures that investors "Buy Low" and "Sell High".

Example

 * You own two funds (Fund A and Fund B)
 * You have a $10,000 portfolio and each fund represents 50% of the total ($5000 each)
 * Over time, Fund A rises in value to $6000 while Fund B declines in value to $4000 - you now have a 60/40 split instead of 50/50
 * To maintain your asset allocation, you would rebalance your portfolio by taking $1000 from Fund A and transferring it to Fund B
 * After doing so, your asset allocation is once again 50/50

Different rebalancing approaches
There are a variety of ways in which investors determine it is time to rebalance:
 * When asset classes deviate from their target by a certain percentage. For example, if your target asset allocation is 60% equities and 40% fixed income and your rebalancing threshold is +/- 5%, you would rebalance your portfolio when your portfolio reaches 65% equities / 35% fixed income or 55% equities / 45% fixed income
 * When asset classes deviate from their target by a certain dollar amount. For example, if you hold $6000 in equities and $4000 in fixed income and your rebalancing threshold is +/- $1000, you would rebalance your portfolio when either of your holdings deviates from their target asset allocation of 60% equities / 40% fixed income by at least $1000.
 * When contributing to or withdrawing from your portfolio. For example, if your target allocation is 60% equities and 40% fixed income, you hold $7000 of equities and $3000 of fixed income, and you wish to contribute $1000 to your portfolio, you would simply buy $1000 worth of fixed income assets. This would bring you to an allocation of 64% equities / 36% fixed income. This approach minimizes transaction costs, effort, and taxes. This portfolio rebalancing calculator can help in cases where it isn't obvious how much of a new contribution or withdrawal to allocate to each asset.

Other considerations

 * Transaction costs should be noted when making deciding whether or not to rebalance. Since many transactions have costs associated with them, many investors choose to wait for their portfolio to pass a significant threshold of deviation (whether defined by percentage or dollars) before rebalancing
 * Many investors find it difficult "selling winners" to "buy losers". To help remove emotions from the decision, many choose a specific date to rebalance (e.g. Birthday, Tax Day, etc.). Note: Investors who find the prospect daunting may want to consider Target Retirement Funds, which automatically rebalance as necessary to maintain a consistent asset allocation.

Papers

 * Portfolio Rebalancing in Theory and Practice by Vanguard Institutional Research
 * Best Practices for Portfolio Rebalancing by Vanguard Institutional Research, (7/21,2010)
 * Opportunistic Rebalancing: A New Paradigm For Wealth Managers, Gobind Doryanini, Journal of Financial Planning, (2007).