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Consumers routinely ignore information when making purchases. They overlook the tax attached to a price, skim past ingredient lists and often fail to process exact numbers with any precision.

This kind of inattention is well documented. What is less well understood is how businesses exploit this behavior.

Assistant professor Andreas Kraft of the University of Chicago Booth School of Business and University of Texas professor Raghunath Rao explored a specific form of inattention known as left-digit bias, which is among the most reliably documented quirks in consumer psychology. Their study, focused on the used-car market, suggests that dealerships systematically attract more inattentive buyers and profit handsomely as a result—and also that this bias could be hard to address with increased disclosure or transparency.

Consumers anchor heavily on the leftmost digit when they evaluate a number. Prices such as $1.99 or $1.90 exploit this tendency.…