People often respond to decreases in the prevalence of a stimulus by expanding their concept of it. This “prevalence-induced concept change” occurred even when participants were forewarned about it and even when they were instructed and paid to resist it.

Prevalence-induced concept change in human judgment. David E. Levari et al. Science  Jun 29 2018: Vol. 360, Issue 6396, pp. 1465-1467. DOI: 10.1126/science.aap8731

Perceptual and judgment creep: Do we think that a problem persists even when it has become less frequent? Levari et al. show experimentally that when the “signal” a person is searching for becomes rare, the person naturally responds by broadening his or her definition of the signal—and therefore continues to find it even when it is not there. From low-level perception of color to higher-level judgments of ethics, there is a robust tendency for perceptual and judgmental standards to “creep” when they ought not to. For example, when blue dots become rare, participants start calling purple dots blue, and when threatening faces become rare, participants start calling neutral faces threatening. This phenomenon has broad implications that may help explain why people whose job is to find and eliminate problems in the world often cannot tell when their work is done.

Abstract: Why do some social problems seem so intractable? In a series of experiments, we show that people often respond to decreases in the prevalence of a stimulus by expanding their concept of it. When blue dots became rare, participants began to see purple dots as blue; when threatening faces became rare, participants began to see neutral faces as threatening; and when unethical requests became rare, participants began to see innocuous requests as unethical. This “prevalence-induced concept change” occurred even when participants were forewarned about it and even when they were instructed and paid to resist it. Social problems may seem intractable in part because reductions in their prevalence lead people to see more of them.