What Drift Means
A portfolio is set to a target allocation — say, 60% equities and 40% bonds — based on a deliberate assessment of goals, time horizon, and risk tolerance. That allocation is appropriate on day one. But it does not stay at 60/40 by itself. Different asset classes produce different returns over time, and the portfolio's weights shift accordingly. This is drift: the silent, automatic departure from intended allocation.
After a strong equity bull market, the same portfolio might become 75% equities and 25% bonds. The investor has not made any decision to take more risk. They have not changed their time horizon or financial situation. Yet they are now exposed to significantly more equity volatility than they signed up for.
The Mathematics of Drift
The numbers are straightforward. Start with a £100,000 portfolio: £60,000 in equities, £40,000 in bonds. Assume equities return 10% per year and bonds return 3% per year, with no rebalancing.
After one year: equities are worth £66,000, bonds £41,200. Total portfolio: £107,200. Equity weight: 61.6%.
After five years: equities have grown to about £96,630, bonds to £46,370. Total: £143,000. Equity weight: 67.6%.
After ten years: equities are approximately £155,620, bonds approximately £53,760. Total: £209,380. Equity weight: 74.3%.
The portfolio that started at 60/40 has drifted to 74/26 in a decade, simply through normal operation. This is not an edge case — it is what happens to any portfolio that is not periodically rebalanced.
Why Drift Is a Systematic Problem
The important point is that drift is not random. It is systematic and directional: portfolios tend to drift toward whichever asset class has performed best. This creates a subtle but serious distortion. A portfolio that drifts will have the highest exposure to the asset class that has risen most — which is typically also the most expensive relative to its historical valuation.
In other words, drift causes portfolios to automatically buy high and hold more of what has become expensive. This is the opposite of what a disciplined investor should do. The unmanaged portfolio ends up concentrated in recent winners precisely at the point when those winners may have less remaining upside.
Drift is also insidious because it feels comfortable. A portfolio that has drifted to 75% equities after a bull market has also grown in value — so the investor may not notice the increasing risk because they are focused on the increasing balance. The mismatch between actual and intended risk only becomes visible when the market falls.
The Solution
The only solution to drift is deliberate rebalancing: periodically reviewing the portfolio's actual weights against the target and trading back toward the intended allocation. How often and at what thresholds to rebalance are separate questions, but the logic rests entirely on the observation that drift is both inevitable and directionally predictable.