Are these dots purple or blue?

Every day, humans make dozens of judgements, 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  in Science. That is, their concept of the color blue had expanded to also include shades of purple.

Look at a video