Planning fallacy in investing: optimistic projections of returns and timelines

An investor planning to retire in 25 years on a portfolio earning 8% per year nominal is making a confident projection that involves multiple optimistic assumptions. Each assumption is individually plausible; together they routinely produce plans that disappoint. Planning fallacy is the documented bias of projecting outcomes in the favourable corner of the achievable distribution rather than at the median, and it is one of the most consequential biases in long-term investing.

What planning fallacy is

Planning fallacy was named by Kahneman and Tversky (1979) to describe the systematic tendency to underestimate the time, cost, and risk of plans while overestimating the benefits. The bias was first studied in project-completion contexts—students predicting how long a thesis would take, contractors estimating construction times, government bodies projecting infrastructure costs—and has been replicated extensively in investing contexts since.

The bias operates through several channels. The inside view focuses on the specifics of the current plan and discounts the base-rate evidence from similar plans that ended differently. The optimism bias treats favourable scenarios as more likely than they are. The overconfidence bias treats one's own ability as above average even when it is not. The combination produces projections that are systematically biased toward the upper end of the achievable distribution.

For investing, the bias's most consequential expression is in long-horizon projections of returns. An 8% nominal return assumption applied over 25 years implicitly assumes that the next 25 years will resemble the historical average—and that the investor will not panic out at any of the drawdowns the historical average includes. Both assumptions are individually optimistic; the combination is more so.

How it manifests in investing

The most direct expression is in retirement planning. The Vanguard, Fidelity, and similar consumer-facing planning tools all use defined return assumptions to project terminal balances, and the assumptions tend to favour the upper end of historical experience rather than the median. An investor projecting 8% nominal equity returns and saving accordingly is implicitly betting on a favourable scenario; if the realised return is 5–6%, the terminal balance is meaningfully smaller than the projection suggested.

A second expression is in business-cycle and tactical projections. An investor evaluating a tactical strategy that earned 12% per year over a backtest period typically projects forward similar returns, ignoring the well-documented decay of factor premia after publication and the broader pattern that backtested returns systematically overstate live performance. The projection is the specifics of the strategy's recent history; the base-rate evidence about strategy decay is discounted.

A third expression is in goal-based planning. An investor with a defined goal—paying for a child's university, funding a property purchase, supporting parents in their later years—typically projects the cost in current-currency terms with an inflation adjustment that understates the actual price growth in the relevant sectors (education, housing, healthcare have all run materially above headline CPI for sustained periods). The goal is therefore systematically under-funded in the planning.

The cost

The cost of planning fallacy is the gap between the projected outcome and the realised outcome—and that gap is asymmetric in the direction of disappointment. An investor whose plan assumed 8% returns and earned 6% reaches retirement with a portfolio that is materially smaller than expected, with limited time to course-correct. The plan was internally coherent; the assumptions were mildly optimistic; the cumulative consequence is a meaningful shortfall.

Empirically, the gap between planned and realised retirement outcomes has been documented in many contexts. The Morningstar "Mind the Gap" series shows that investor returns systematically lag fund returns by 1.0–1.7 percentage points per year, partly because of planning-fallacy-driven savings decisions that under-prepared for actual market behaviour. Compounded over decades, the gap accumulates to substantial fractions of total terminal wealth.

The cost is most acute for investors approaching retirement with a planning horizon that has already passed. The investor with 30 years to go has time to discover the planning error and increase savings or extend working life; the investor with 5 years to go has limited remaining capacity to course-correct. The planning fallacy compounds over time and is hardest to address when its consequences are most imminent.

What helps

The structural remedy, due to Kahneman, is the outside view—using base-rate evidence from comparable plans rather than the inside view of the specific current plan. For return projections, this means using long-run historical averages from comparable strategies and asset classes, not the upper end of the recent observed range. The Dimson-Marsh-Staunton long-run data provides the relevant base rates for major asset classes.

For specific plans, sensitivity analysis around the base assumption captures the implied dispersion. A retirement plan tested at 4%, 5%, 6%, and 7% real returns reveals which scenarios produce comfortable outcomes and which produce shortfalls. The investor can then size the savings rate to be acceptable across the range rather than being optimised to the favourable case.

Planning fallacy in pfolio

pfolio's analytics report historical performance, drawdown, and risk metrics for any portfolio configuration. These statistics provide the empirical anchor against which planning assumptions can be checked, replacing optimistic projections with the actual return distribution the portfolio has historically delivered.

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• Overconfidence in investing: why most active investors underperform the market
• Goal-based investing: building portfolios around the investor's defined objectives
• Sequence of returns risk: why the order of returns matters as much as the average