Rebalancing - Restoring Target Weights and Controlling Risk

By EC Assets Research Team, Portfolio Research · Published 20 May 2026 · Updated 15 October 2026

Rebalancing: Rebalancing periodically restores a portfolio to its target weights, selling assets that have grown and buying those that have shrunk. It is primarily a risk-control discipline - stopping risk from drifting upward as winners compound - with a modest 'rebalancing premium' as a possible bonus in mean-reverting markets.

What Rebalancing Is

Rebalancing restores a portfolio to its target weights. Left alone, a portfolio drifts: assets that rise become a larger share and assets that fall a smaller one, so a 60/40 mix of equities and bonds becomes steadily more equity-heavy in a long bull market, and riskier than intended. Rebalancing sells what has grown and buys what has shrunk.

The mechanism is contrarian by construction. It trims recent winners and adds to recent losers, which feels wrong in the moment and is why rules work better than discretion.

How It Works

There are three common methods. Calendar rebalancing restores the weights on fixed dates, such as monthly, quarterly or annually. Threshold or band rebalancing acts only when a weight moves beyond a tolerance, for example 5 percentage points from target. Hybrid rules check on a schedule and trade only if a band is breached.

The trade size follows from simple arithmetic. A 60/40 portfolio in which equities gain 25 percent and bonds 2 percent ends the year at 75 and 40.8, an equity share of 64.8 percent. Restoring 60/40 means selling 5.5 of equities and buying bonds. A 5-point band would leave this portfolio alone; a 10-point rise would trigger a trade.

Band width is a trade-off. Narrow bands keep the portfolio close to target at the cost of frequent trading; wide bands save costs but allow more drift. Many institutions set bands in proportion to an asset's weight and volatility, so large, volatile allocations get wider absolute bands than small, stable ones, and check them more often than they expect to trade.

Perold and Sharpe (1988) showed that rebalancing to constant weights is a concave strategy: it does best when markets oscillate and worst when they trend. Fernholz and Shay (1982) and Booth and Fama (1992) describe the diversification return a rebalanced portfolio earns over the average of its parts:

$$\text{Diversification return} \approx \tfrac{1}{2}\left(\sum_i w_i \sigma_i^2 - \sigma_p^2\right)$$

For 60 percent equities at 16 percent volatility and 40 percent bonds at 7 percent, uncorrelated, the formula gives about 0.37 percentage points a year. It is the same arithmetic as volatility drag seen from the portfolio side, and the volatility drag calculator shows how variance eats into compounding.

Worked Example

A 60/40 portfolio of the S&P 500 price index and an approximate 10-year Treasury, from January 1990 to October 2026, without costs, taxes or dividends:

Equity returns exclude dividends, which understates equities and the drift of the unrebalanced portfolio.

The returns are within about 0.13 percentage points of each other. The difference is risk. The unrebalanced portfolio ended with 85.3 percent in equities, a mix its owner never chose; it happened to have a smaller drawdown in 2008 because the 2000 to 2002 bear market had already cut its equity share, but it entered 2026 far more exposed to the next crash. The band rule kept the equity share between 55 and 65 percent with 32 trades in 37 years.

When It Applies (and Limitations)

Trends cut the bonus. When one asset outperforms for years, rebalancing repeatedly sells it early. Over 1990 to 2026 equities trended upward and rebalancing earned slightly less than buy and hold.

Costs and taxes. Each trade incurs spreads and, for taxable investors, realised gains. The more frequent the rule, the larger the drag; using cash flows and dividends to rebalance reduces it, and tax considerations can make a band rule preferable to a calendar one.

Illiquid assets. Private equity, real estate and other illiquid holdings cannot be traded quickly, and appraisal lags make their weights stale, so rebalancing is approximate and often done through the liquid part of the portfolio.

Rules can be abandoned. Rebalancing into a falling market requires buying the asset that is hurting, which is when committees are most tempted to suspend the rule.

Why It Matters for Institutional Investors

Enforcing the strategy. A strategic asset allocation only exists if it is maintained. Rebalancing is the mechanism that keeps the portfolio the investor approved rather than the one the market has drifted into.

Risk budgets. Drift changes risk contributions, and in a 60/40 portfolio equities already dominate risk; see portfolio construction. Rebalancing keeps the risk budget close to plan.

Governance. Bands, frequency, permitted instruments and the procedure in stress belong in a written policy, so that the rule holds when markets make it hardest to follow.

Cash flows as a tool. Contributions, benefit payments, income and dividends can be steered toward underweight assets, which rebalances part of the drift without selling anything and is especially valuable for taxable investors.

Correlation regimes. The diversification return grows as correlations fall and disappears when assets move together, so the value of rebalancing varies with the market regime.

References

1. Perold, A. F., & Sharpe, W. F. (1988). Dynamic Strategies for Asset Allocation. Financial Analysts Journal, 44(1), 16-27.
2. Fernholz, R., & Shay, B. (1982). Stochastic Portfolio Theory and Stock Market Equilibrium. Journal of Finance, 37(2), 615-624.
3. Booth, D. G., & Fama, E. F. (1992). Diversification Returns and Asset Contributions. Financial Analysts Journal, 48(3), 26-32.
4. Jaconetti, C. M., Kinniry, F. M., & Zilbering, Y. (2010). Best Practices for Portfolio Rebalancing. Vanguard Research.