Lovallo and Kahneman’s 2003 Harvard Business Review paper used a word that academic papers rarely use about intelligent, capable people. Delusional. Their argument was not that business leaders are incompetent. It was that specific, documented cognitive mechanisms produce overestimation so consistently and so structurally that the word is accurate.

The planning fallacy

Kahneman and Tversky first described the planning fallacy in 1979. The finding is simple and uncomfortable: people systematically underestimate the time, cost, and risk of future actions while overestimating their benefits. This is not random error. It is directional. It runs consistently in one direction, across individuals, industries, and cultures.

The mechanism is presentism — forecasting the future from a calm, optimistic present. When you build a business plan, you are imagining execution while feeling focused and motivated. You simulate a best-case scenario because the obstacles are abstract and your current state is not. Past delays, historical overruns, and comparable failures in similar projects are available information. They are not the information that feels most relevant in the moment.

Buehler, Griffin, and Ross’s 1994 experimental research established this precisely. People consistently predicted task completion times more optimistically than their own past performance in similar tasks warranted. When asked explicitly to recall how long similar tasks had taken previously, the overestimation reduced — but did not disappear.

For entrepreneurs, the planning fallacy manifests most destructively in revenue timing. Year one projections almost universally overestimate how quickly customers arrive and underestimate how long product-market fit takes to establish. This isn’t a planning failure, but it is the predictable output of constructing a plan from inside an optimistic present state.

The inside view problem

The second mechanism is the more structurally important one. Lovallo and Kahneman’s inside view versus outside view distinction is the clearest diagnostic framework in the forecasting literature.

The inside view is how virtually every business plan is built. You focus on your specific situation — the team, the opportunity, what makes this different. You construct a narrative of how execution will unfold. Every detail feels relevant and every assumption feels considered.

The outside view asks a categorically different question: what happened to all the other businesses that attempted something similar? Not your business. The reference class. What percentage of companies at your stage, in your category, achieved this revenue milestone within this timeframe?

Most plans never engage with that question at all.

In one study, students expected to perform better than 84% of their peers — a statistical impossibility. When given simple outside-view information before making their predictions, average overconfidence reduced by 20%. Still overconfident. Significantly less so.

The correction is reference class forecasting: before finalising any projection, identify comparable businesses and find the distribution of their outcomes. Position your plan relative to that distribution rather than constructing it independently from the inside. The outside view will almost always produce a more conservative and more accurate forecast than the inside view — not because ambition is wrong, but because the inside view structurally ignores the base rate.

Your plan is not being evaluated in isolation. It is being executed in the same environment where similar plans have a documented track record. That track record is the most relevant information you are probably not using.

Competitor neglect

The third mechanism is the most practically underappreciated. Business plans routinely project revenue and market share on the assumption that competitors will not respond. They are mentioned in a slide. They are not modelled in the plan.

A founder projecting 5% market share in three years is implicitly assuming that existing players will not adapt, that new entrants will not emerge, and that customer acquisition will proceed on an uncontested basis. In a competitive market, every one of those assumptions is false by construction.

The cognitive mechanism producing this is the inside view operating at the competitive level. It is significantly easier to simulate your own execution in detail than to model the adaptive responses of multiple competitors across multiple scenarios. The asymmetry is not laziness — it is a structural feature of how the mind constructs narratives about the future.

Lovallo and Kahneman identified competitor neglect alongside optimism bias as jointly producing the most damaging category of business plan errors. The practical correction is explicit: model at least one competitor response scenario. What does the plan look like if the market leader drops prices in response to your entry? What if a well-funded competitor enters the same space six months after you launch? Explicit modelling does not guarantee accuracy. It interrupts the neglect.

One honest caveat Flyvbjerg added in his 2003 commentary on Lovallo and Kahneman: not all business plan overoptimism is cognitive bias. Some of it is deliberate. Forecasts are sometimes inflated intentionally to secure funding or internal approval. The cognitive bias is self-deception. Flyvbjerg’s observation is that some of it is other-deception. Both are worth naming because they require different responses.

If the patterns described here map onto a persistent tendency to commit to plans before stress-testing the assumptions — and if that pattern is affecting your decision-making in ways that feel difficult to interrupt — that is worth exploring with someone who can look at the specific dynamic rather than the general mechanism.

A book worth reading alongside this

How Big Things Get Done by Bent Flyvbjerg and Dan Gardner is the most directly applicable starting point. Flyvbjerg has spent 30 years researching why large projects consistently fail — and the answer is almost always planning fallacy, optimism bias, and base rate neglect. His reference class forecasting methodology is the empirically validated correction. For any entrepreneur building a business plan and wanting to understand how to make it more accurate rather than more optimistic, it is the most honest and practically useful book available.

This article discusses psychological patterns documented in research on forecasting, cognitive bias, and entrepreneurial decision-making. It is not designed to identify, diagnose, or assess any psychological condition, and it is not a substitute for professional support. The patterns described here are well-documented features of human cognition — recognising yourself in them is not a cause for alarm. If, however, you find that these patterns are significantly affecting your work, relationships, or wellbeing, speaking with a psychologist or therapist can provide personalised guidance that an article cannot.

This article is for educational and informational purposes only. It is not a substitute for professional psychological advice, diagnosis, or treatment. If you are experiencing significant psychological distress, please consult a qualified mental health professional.

Sources: Kahneman, D. & Tversky, A. (1979), TIMS Studies in Management Science, 12. Lovallo, D. & Kahneman, D. (2003), Harvard Business Review, July–August. Buehler, R., Griffin, D. & Ross, M. (1994), Journal of Personality and Social Psychology, 67(3). Flyvbjerg, B. (2003), Harvard Business Review, December. Flyvbjerg, B. (2008), European Planning Studies, 16(1).