Planning fallacy is the well-documented tendency to underestimate the time, cost, and risk of a future task even when you know that similar tasks have run long before. Kahneman and Tversky named it, and Buehler and colleagues showed it survives direct experience — people who blew past their last deadline still predict the next one optimistically. Manipulation exploits this by leading with the rosy figure: the low quote, the “quick” timeline, the effortless onboarding, each one landing as plausible because your own mind is already primed to expect the best case.
The exploit works because the low number does its job before reality can correct it. Once you have committed — signed a contract, paid a deposit, cleared your calendar — the escalating cost meets a second bias, sunk-cost reasoning (T8.19), which makes abandoning the half-finished plan feel like waste. The lowball is therefore rarely a standalone trick; it is the entry point to a commitment pipeline, common in renovation and construction pitches, software timelines, and any subscription or loan sold on how easy it will be.
The defense is structural, not emotional: replace the inside view (this specific plan, imagined going well) with the outside view (the actual track record of jobs like this). Ask for the reference class and a distribution of real outcomes, add a genuine buffer, and refuse to let a headline estimate double as a commitment. Honest estimates get more credible when you probe the assumptions behind them; a lowball gets vaguer and more urgent.