<p>Casino blackjack players learn a simple heuristic that helps them decide when to take additional cards, the most common blackjack decision. The heuristic assumes that all upcoming cards will be 10-value cards, even though that assumption is true fewer than one in three times. The heuristic results in systematic error, but it is also adaptive: it is easier to learn than the optimal strategy, the cost of using it is trivial, and its use is associated with better expected returns. The heuristic helps explain inconsistent previous findings about blackjack likelihood judgments and decision biases. The research relies on mixed methods including qualitative data from 1.5 years of ethnographic fieldwork as a blackjack dealer and player, and quantitative data from interviews with players about how they play each blackjack hand. The heuristic is used as a case to support several theoretical contentions: (a) despite established precedent, gambling is not a good domain-general metaphor for decision making under risk or uncertainty; (b) even in a small-world domain where outcome likelihoods can be calculated and monetary outcomes are unambiguous, using subjective probability to infer expected value may be both uncommon and non-normative; and (c) a focus on narrow, domain- and culture-specific heuristics and biases—despite their limited scope—offers valuable lessons about how, and how well, people make decisions.</p>