Underestimating time, costs, or risks of future actions

Explanation

The planning fallacy denotes the systematic tendency to underestimate the time, cost, and risk required to complete a future task, while overestimating its benefits, even among individuals who possess detailed knowledge that comparable past tasks overran their forecasts. Daniel Kahneman and Amos Tversky formalized the phenomenon in their 1979 chapter “Intuitive Prediction: Biases and Corrective Procedures,” distinguishing an inside view that constructs a specific, optimistic scenario of the steps required from an outside view that consults the statistical distribution of outcomes for similar completed projects. Planners default to the inside view because a coherent narrative of intended actions feels more diagnostic than base-rate data; conjunctive probabilities are overweighted, unforeseen interruptions are treated as exceptional rather than typical, and past overruns are attributed to transient external causes rather than structural features of the class of tasks. Neuroimaging work by Tali Sharot and colleagues (2011) links the bias to asymmetric belief updating: the right inferior frontal gyrus tracks desirable estimation errors more robustly than undesirable ones in highly optimistic individuals, while the anterior cingulate cortex supports the simulation of positive future episodes, thereby sustaining the inside-view narrative even after disconfirming evidence arrives.

Empirical demonstrations confirm the pattern across scales. In a 1994 study by Roger Buehler, Dale Griffin, and Michael Ross, thirty-seven honors students predicted an average of 33.9 days to finish their theses; the thirty-three who completed did so in 55.5 days on average, and only 29.7 percent finished by their own best-guess date. Large-project analyses by Bent Flyvbjerg and colleagues document parallel underestimation in infrastructure, where the inside view of a detailed engineering plan systematically neglects the distributional evidence of historical cost and schedule overruns.

Examples

• Suez Canal’s Six-Year Promise: Ferdinand de Lesseps, the French diplomat who obtained the 1854 concession from Egyptian viceroy Muhammad Sa’id, repeatedly assured investors that the canal linking the Mediterranean to the Red Sea could be excavated in six years for a maximum of roughly 200 million francs. Excavation began on 25 April 1859 at Port Said; the waterway opened only on 17 November 1869 after approximately ten and a half years, at a final recorded cost of 432.8 million francs. Contemporary company reports and the 1869 inauguration accounts document the shift from forced corvée labor to mechanical dredging after an 1864 arbitration, yet de Lesseps’s early prospectus still projected five-to-six-year completion. Over-reliance on the inside-view narrative of sequential excavation stages, which ignored the base rates of prior isthmian attempts and the documented difficulties of desert logistics, left no contingency for disease, political interference, or technical redesigns; a distributional forecast drawn from earlier canal and railway projects would have allocated additional years and capital, potentially avoiding the near-bankruptcy that required emergency French share issues and Egyptian subsidies.
• Sydney Opera House’s Unsolved Shells: Danish architect Jørn Utzon’s 1957 competition entry for the performing-arts complex on Sydney’s Bennelong Point was estimated by quantity surveyors at A$7 million (then A£3.5 million) with an opening on Australia Day 1963. Construction started 2 March 1959 before the roof geometry was solved; the spherical solution for the shells was found only in 1962, the podium finished in early 1963, and Utzon resigned in February 1966 after payment disputes with the New South Wales government under Minister Davis Hughes. The building opened on 20 October 1973 at a final cost of A$102 million—more than fourteen times the original figure—after 496 lotteries sold 86.7 million tickets. Queen Elizabeth II, at the opening, remarked that “its construction has not been totally without problems.” Beginning work on an unengineered design treated the unique architectural vision as self-executing and ignored the historical distribution of delays on complex concrete shell structures; allocating resources according to comparable postwar civic buildings would have deferred groundbreaking until structural resolution and built explicit schedule buffers, reducing the later interior redesigns that consumed most of the overrun.
• Boston’s Central Artery/Tunnel Project: The Big Dig, formally the Central Artery/Third Harbor Tunnel Project, was authorized with a 1982–1985 estimate of approximately $2.6–2.8 billion and a target completion near 1998. By the July 1996 testimony of the U.S. General Accounting Office, official figures already reached $7.8 billion while GAO calculated at least $10.4 billion; the project ultimately closed in December 2007 at roughly $14.6–14.8 billion in construction costs (higher when interest is included). Project manager Bechtel/Parsons Brinckerhoff’s internal 1994 projections already approached $14 billion, yet public statements maintained lower caps. The inside-view reliance on detailed tunnel and bridge designs treated subsurface conditions, environmental mitigation (1,500 separate agreements), and coordination among dozens of contracts as manageable exceptions rather than typical of urban megaprojects; an outside-view reference class drawn from prior U.S. highway tunnels would have required larger contingencies and independent cost reviews, limiting the cascading change orders and the 2006 ceiling-panel collapse that added tens of millions in remediation.
• Denver International Airport’s Automated Baggage System: Denver International Airport was planned to open in late 1993 with a highly automated, multi-airline baggage-handling system intended to process thousands of bags per hour via computer-controlled carts. The system’s integration failures—jams, lost bags, and mechanical breakdowns—delayed the airport’s opening by sixteen months to 28 February 1995. Contemporary engineering assessments and city audits placed the direct cost of the baggage-system overruns and associated delay financing in the range of $500–560 million, part of a larger airport cost growth. Airport officials and contractor BAE Automated Systems had projected near-flawless operation based on the internal logic of the software and mechanical design. Treating the novel system as an extension of simpler existing conveyors ignored the base rates of software and mechanical integration failures on large transportation projects; adopting an outside view that applied historical uplift factors from comparable automated logistics installations would have staged a simpler manual backup earlier and reduced the months of idle infrastructure and interest charges.

Conclusion

The planning fallacy compounds individual miscalibration into institutional underinvestment in detection and response capacity. Because the prefrontal mechanisms that sustain optimistic simulation preferentially update on favorable information, organizations that rely solely on detailed internal plans systematically under-allocate schedule contingency, independent review, and adaptive reserves. Reference-class forecasting, operationalized by Bent Flyvbjerg and adopted in British Treasury guidance after 2003, counters the bias by first locating a project in the historical distribution of similar completed efforts and only then adjusting for unique features. Pre-mortem exercises and explicit outside-view prompts further reduce the asymmetry documented in frontal-lobe imaging. Societies that continue to treat each new megaproject as uniquely immune to the statistical record of its predecessors will continue to transfer the costs of optimism onto taxpayers and users; those that institutionalize distributional forecasting will convert the same neurocognitive tendency into more resilient infrastructure and more credible public promises.

Quick Reference

• Synonyms: optimism bias in forecasting; underestimation of task duration; inside-view bias
• Antonyms: outside-view forecasting; reference-class forecasting; realistic base-rate prediction
• Related Biases: optimism bias; overconfidence effect; anchoring and adjustment; conjunction fallacy

Citations & Further Reading

• Buehler, R., Griffin, D., & Ross, M. (1994). Exploring the “planning fallacy”: Why people underestimate their task completion times. Journal of Personality and Social Psychology, 67(3), 366–381.
• Flyvbjerg, B. (2008). Curbing optimism bias and strategic misrepresentation in planning: Reference class forecasting in practice. European Planning Studies, 16(1), 3–21.
• Flyvbjerg, B., Holm, M. K. S., & Buhl, S. L. (2002). Underestimating costs in public works projects: Error or lie? Journal of the American Planning Association, 68(3), 279–295.
• Kahneman, D., & Tversky, A. (1979). Intuitive prediction: Biases and corrective procedures. In S. Makridakis & S. C. Wheelwright (Eds.), Forecasting (TIMS Studies in the Management Sciences, Vol. 12, pp. 313–327). North-Holland.
• Sharot, T., Korn, C. W., & Dolan, R. J. (2011). How unrealistic optimism is maintained in the face of reality. Nature Neuroscience, 14(11), 1475–1479.
• U.S. General Accounting Office. (1996). Central Artery/Tunnel Project: Financial plan (GAO/RCED-96-186). Government Printing Office.