The theoretically optimal investment strategy isn't the one with the highest expected return in a spreadsheet. It's the one with the highest expected return that an actual investor can implement and stick with through every kind of market she'll encounter over her time horizon. That distinction matters more than it sounds like it should. The gap between a strategy's theoretical return and what a human investor actually earns from it, once she abandons it during a stretch of stress, can dwarf the gap between a good strategy and a great one.
The evidence sits in a finding that shows up in nearly every category of fund: investor returns trail fund returns. Funds report time-weighted returns, meaning what a dollar invested at inception would have earned by holding straight through. Investors earn something different, dollar-weighted returns, which reflect the actual experience of people putting money in and pulling it out at various points along the way. That gap is consistently negative, and it's not small, because investors tend to add money after a run of strong performance (buying high) and pull money out after a stretch of poor performance (selling low). The fund's strategy may be perfectly sound. The investor's execution of it isn't, and the reason is that execution gets disrupted by the emotional weather of markets.
A strategy that models out to twelve percent a year but demands that its practitioner hold through fifty percent drawdowns will, for most people, deliver a lower actual return than a strategy that models out to ten percent but that she can psychologically live with across every kind of market. The first strategy gets abandoned, or quietly modified, during its worst stretches, which is precisely when staying invested matters most for capturing its full return. The second strategy gets maintained. It keeps producing something close to its ten percent, year after year, without the costly detours the theoretically superior strategy invites.
What this means for choosing a strategy is that an investor's own psychology belongs inside the optimization, not outside it. What asset allocation can she genuinely hold through a forty percent decline without blowing it up? How much volatility can she sit with before panic selling takes over? How much underperformance against her peers can she stomach before the social pressure to change course becomes too strong to resist? None of that is theoretical. Those are practical design inputs, every bit as important as expected return or historical volatility.
The strategy that actually scores highest on sustainability tends to be simpler than the theoretically optimal one: fewer moving parts, less upkeep, fewer decision points, a logic plain enough to be understood and internalized deeply enough to hold up under pressure. An investor who understands why her strategy works, not just that it happened to work historically but why its underlying logic implies it should keep working, is far better positioned to stick with it through a bad stretch than someone holding a strategy she adopted purely because a backtest looked good. Conviction built on understanding, rather than on recent performance, is the raw material patience is made of. And patience is what good investing actually requires.