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Are these dots purple or blue? Your answer might not be as reliable as you think

Every day, humans make dozens of judgments, from deciding whether our clothes match to determining whether a shady character in the street is a threat. Such decisions aren’t based on hard-and-fast rules, a new study reveals. Instead, our concept of “threat”—and even of the color “blue”—is all relative.

To make the find, researchers showed non–color-blind participants a series of 1000 dots ranging from very blue to very purple, and asked them to judge whether each dot was blue. For the first 200 trials, participants saw an equal number of dots from the blue and purple parts of the spectrum, but then the prevalence of blue dots gradually decreased to just a fraction of what it was before. By the end of the study, participants’ interpretation of the colors had changed: Dots that they had thought were purple in the first set of trials they now classified as blue, the authors report today in Science. That is, their concept of the color blue had expanded to also include shades of purple.

Even when the researchers forewarned participants that blue dots would become rarer and promised them money if they kept their judgments consistent, the same shift occurred. And the team found similar results in more complex versions of the task, where participants had to judge whether a face was threatening or whether a research proposal was ethical. When threatening faces or unethical research proposals became less common, people started to consider previously benign examples as posing a threat or being unethical.

These results could explain why people tend to be pessimistic about the state of the world, the authors say. Humanity has made great strides in reducing social issues like poverty and illiteracy, but as these problems become less common, previously minor issues start to seem much more problematic.