Simple Rules and Smart Rebalancing: The Final Piece of Asset Allocation

Professional asset allocation often involves optimisers, Monte Carlo simulations, and complex models. But many investors and even large institutions use simpler rules. Some of these rules have surprising empirical support. And once any allocation is set, the unavoidable question is how to maintain it as markets push the portfolio away from its targets. Rebalancing policy is the final, often underestimated, piece of the allocation puzzle.

Rule 1: 120 Minus Your Age

120 minus your age rule: A rule of thumb that says the percentage of your portfolio in equities should equal 120 minus your age. A 30-year-old holds 90% equities; a 70-year-old holds 50%

Why any age-based rule makes sense: As people age, two things change. Their human capital (future earnings) shrinks and their financial capital grows larger in relative importance. When human capital is large and bond-like (stable salary), the financial portfolio can hold more equities. When human capital is small, the financial portfolio needs to be more defensive. Age is a crude but reasonable proxy for this shift.

There is no precise theoretical basis for 120 as the starting number rather than 100 or 125. But when you plot the equity allocations of the 60 largest US target-date fund families, the actual glide paths they use closely track the 100 to 120 minus age range. These are the results of sophisticated research into human capital lifecycle dynamics. The heuristic approximates the same answer much more simply.

Rule 2: The 60/40 Split

Many investors simply hold 60% equities and 40% bonds. This is often dismissed as unsophisticated, but the empirical case for it is stronger than it looks. Research on the composition of the global financial asset market portfolio from 1990 to 2012 found that equities, private equity, and real estate combined accounted for roughly 60% of total global investable assets in most years. The 60/40 split is not arbitrary. It approximately matches what the entire global investment community, in aggregate, chooses to hold.

The fixed income component provides real diversification. Bonds and equities have historically shown negative or low correlations in market crises, precisely when diversification is most needed. A simple, well-diversified 60/40 portfolio, implemented at low cost, has delivered competitive risk-adjusted returns over most long historical periods.

Rule 3: The Endowment Model (Yale Model)

Endowment model: An approach emphasising large allocations to private equity, real estate, and illiquid alternatives. Pioneered by Yale University, it seeks to earn illiquidity premiums over very long time horizons

Yale's endowment under David Swensen built a dramatically different portfolio: roughly 33% private equity, 17% real estate, 17% absolute return strategies, 11% foreign equity, 8% natural resources, 5% bonds, and small amounts elsewhere. The rationale is that endowments have perpetual time horizons, no liquidity needs, and professional staff who can evaluate and monitor illiquid fund managers.

The endowment model works for the specific conditions of large, well-staffed university endowments. Swensen himself was clear that most individual investors should not try to replicate it. The illiquidity is real, the manager selection skill required is rare, and the fees involved are high. An ordinary investor who puts 30% of their savings into private equity funds and cannot access it for 10 years is not following a sensible adaptation of the endowment model.

The contrasting Norway model takes the opposite view. Norway's Government Pension Fund holds only publicly traded securities, follows an approximate 60/40 allocation, and invests passively. It reflects a conviction in market efficiency, transparent governance, and the value of low costs.

Rule 4: Risk Parity

Risk parity: An approach where each asset class contributes equally to total portfolio risk. Assets with low volatility (bonds) receive much larger weights than assets with high volatility (equities)

In a typical 60/40 portfolio, equities are so much more volatile than bonds that equities drive 70-80% of total portfolio risk despite holding only 60% of the money. Risk parity corrects this by dramatically overweighting bonds so that each asset contributes the same amount to volatility.

The consequence is a portfolio with a much larger bond allocation than a standard MVO would recommend. To bring returns up to an acceptable level, risk parity portfolios often use leverage. Academic back-tests have shown promising results, but critics note that these back-tests depend on a long period of falling interest rates that benefited bonds, and that the levels of leverage required may not have been available or affordable in practice.

Risk parity's primary criticism is that it ignores expected returns entirely. It allocates risk equally regardless of whether each asset offers equal compensation for that risk. In periods when bonds have very low expected returns (as in the 2010s), risk parity overweights bonds without adjustment.

Rule 5: Equal Weighting (1/N)

1/N rule: Dividing the portfolio equally among all available assets. If there are 8 asset classes, each gets 12.5%

Equal weighting sounds naive, but it has a surprising track record. Academic research comparing equal-weighted portfolios to MVO-optimised portfolios found that equal weighting frequently delivers competitive or superior Sharpe ratios in out-of-sample tests.

The explanation lies in estimation error. MVO is designed to exploit differences in expected returns, volatilities, and correlations. But these inputs are estimated from historical data and contain errors. MVO amplifies those errors by aggressively overweighting assets whose estimated returns look highest. Equal weighting sidesteps the problem by treating all assets as identical. In a world where input estimates are noisy, ignoring them and just diversifying equally often beats a complex model that puts too much faith in unreliable forecasts.

Rebalancing: Bringing the Portfolio Back on Track

Why rebalancing matters: Markets move continuously. A portfolio starts at its target weights but drifts over time. If equities have a strong year, their weight grows too large. The portfolio becomes riskier than intended. If bonds rally, their weight grows. The portfolio becomes more conservative than desired. Without rebalancing, the strategic allocation that was carefully set becomes increasingly irrelevant.

The basic cost of not rebalancing is the loss of expected utility from holding a portfolio that is increasingly misaligned with the investor's optimal risk-return trade-off. Over years this can compound into a meaningful drag on outcomes.

Two rebalancing disciplines dominate in practice. Calendar rebalancing rebalances at fixed dates regardless of how much the portfolio has drifted: monthly, quarterly, semi-annually, or annually. It is simple to implement and has low monitoring costs. Corridor rebalancing (also called percent-range rebalancing) only rebalances when an asset class drifts outside a defined band around its target. A 20% target allocation with a 5% corridor means rebalancing only occurs when the allocation falls below 15% or rises above 25%.

Corridor rebalancing is more disciplined as risk control: rebalancing is triggered precisely when risk has drifted, not arbitrarily based on the calendar. Calendar rebalancing is cheaper to operate but may allow meaningful risk drift between rebalancing dates.

Setting the Right Corridor Width

The optimal corridor width for each asset class depends on four factors, each pulling in a specific direction.

Higher transaction costs argue for wider corridors. Rebalancing is expensive, so you tolerate more drift before paying to correct it. Higher risk tolerance argues for wider corridors. The investor is less bothered by short-term deviations from target. Higher correlation between the asset and the rest of the portfolio argues for wider corridors. When assets move in sync, drifting out of line is less likely to persist or worsen. Higher volatility of the rest of the portfolio (excluding the asset being considered) argues for narrower corridors. In a volatile environment, an asset can drift far from its target quickly, and correcting it early is more valuable.

For taxable investors, capital gains taxes add a further dimension. Every rebalancing trade that realises a gain triggers a tax bill. Higher capital gains rates argue for wider corridors. One practical tool: use new cash contributions or dividends to rebalance by buying underweight assets rather than selling overweight ones. This achieves the same directional correction without triggering any taxable event.

What Rebalancing Actually Earns

Disciplined rebalancing has historically reduced portfolio risk while adding slightly to returns. Two mechanisms explain this.

The first is diversification return. The compound return of a rebalanced portfolio is greater than the weighted average of the compound returns of its components. By regularly selling what has risen and buying what has fallen, rebalancing harvests a return that buy-and-hold does not.

The second is a return from being short volatility. The payoff profile of a regularly rebalanced portfolio can be replicated by holding the portfolio statically while writing (selling) out-of-the-money options on each asset. Because options earn their sellers a premium in exchange for limiting upside and downside, a rebalanced portfolio effectively earns this implicit option premium over time.

Neither mechanism requires any market forecasting skill. They are structural features of disciplined, rules-based portfolio management. This is why experienced wealth managers treat rebalancing not as a maintenance chore but as a return-generating discipline in its own right.