Science 360, 1465–1467 (2018)

How people classify what they observe into categories to make sense of the world and judge their surroundings has been a longstanding question in psychological research. These categorizations span from low-level decisions — is this shade of purple rather red or blue — to high-level judgements, such as whether aggression translates to physical assault, or includes angry stares and hurtful remarks.

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In a set of studies, David Levari, of Harvard University, and colleagues established an intriguing effect: if members of a category become rarer, participants expand the concept and assort items they previously classified differently to the now rarer category. Remarkably, this behaviour was not only established for low-level concepts — when blue items became rarer, participants started to call purple items blue — but also to high-level decisions. Participants judged faces as threatening or not, and when the number of threatening faces was reduced, some faces that had previously been categorized as neutral were judged to be threatening.

This prevalence-induced concept change offers an interesting explanation for why concepts expandover time. A central aim of further research will be to test whether this explanation holds for real-world changes in how societies classify behaviour.