The theoretically optimal investment strategy is not the one that produces the highest expected return in a model. It is the one that produces the highest expected return that the investor can actually implement and maintain through the full range of market conditions she will encounter over her investment horizon. This distinction is not minor. The gap between the theoretical return of an optimal strategy and the actual return of the same strategy, as implemented by a human investor who abandons it during periods of stress, can easily exceed the gap between a good strategy and a great one.
The evidence for this claim is embedded in the consistent finding that investor returns lag fund returns in virtually every category of investment product. Funds report time-weighted returns—what a dollar invested at inception would have earned by holding the fund throughout. Investors earn dollar-weighted returns—the actual experience of investors who put money in and take money out at various points. The gap is consistently negative and significant, because investors tend to add money after periods of strong performance—buying high—and withdraw money after periods of poor performance—selling low. The fund's strategy may be sound; the investor's implementation of it is not, because the implementation is disrupted by the emotional conditions of markets.
A strategy that produces twelve percent annually in a model but that requires its practitioner to hold through fifty percent drawdowns will, for most investors, produce a far lower actual return than a strategy that produces ten percent annually but that is psychologically maintainable through the full range of market conditions. The former strategy will be abandoned—or significantly modified—during its worst periods, at exactly the moments when staying invested is most important for realising the strategy's full return. The latter strategy will be maintained, producing its ten percent consistently, without the costly deviations that the theoretically superior strategy generates.
The implication for strategy selection is that the investor should incorporate her own psychological characteristics into the optimisation. What asset allocation can she genuinely hold through a forty percent market decline without making significant changes? What level of portfolio volatility can she tolerate without being driven to reduce it through panic selling? What degree of underperformance relative to her peers can she accept before the social pressure to change her approach becomes irresistible? These are not theoretical questions; they are practical inputs to strategy design that are every bit as important as expected return and historical volatility.
The strategy that scores highest on genuine sustainability is usually simpler than the theoretically optimal one—fewer moving parts, lower maintenance requirements, fewer decision points, and a clear logic that can be understood and internalised deeply enough to provide conviction during difficult periods. The investor who understands why her strategy works—not just that it has worked historically but why its fundamental logic implies that it should continue to work—is far better equipped to maintain it during periods of underperformance than one who holds a strategy she has adopted on the basis of backtested results without a deep understanding of its underlying rationale. Conviction, grounded in understanding rather than recent performance, is the psychological raw material from which the patience that good investing requires is made.