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I don’t mean to salt old wounds by reintroducing a debate that ripped through our culture, creating lasting fissures and ruining lives in the process, buuuuuut: remember the dress? You know, the dress. The one you were right about (because you saw it as white and gold), even when you found out that you were wrong about it (it’s actually blue and black), because, honestly, that dress is white and gold and anyone who says otherwise is an oppressive person who disseminates fake news and just wants to brainwash you.

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(It is actually blue and black. Or is it?)

Over two years after that obscure object of debate changed our lives permanently, a vision scientist named Pascal Wallisch thinks he’s figured out why some people saw white and gold and others saw blue and black. In a piece on Slate, “Two Years Later, We Finally Know Why People Saw ‘The Dress’ Differently,” Wallisch writes that the dress phenomenon stumped vision experts who “thought we basically knew how color vision worked, more or less.” He compares the divide to “discovering a new continent.”

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Through his research, though, Wallisch believes that the white-and-gold versus black-and-blue debate comes down to perception and whether or not people assumed they were viewing the dress in shadow or not. If they did make that assumption, they were more likely to see the dress as white and gold because their brains were correcting their perception by “mentally subtracting a short wavelength light.”

As to why some people assumed a shadow and others did not, Wallisch says this may come down to whether or not the person is a “lark” (people who wake up early and get to bed relatively early, and therefore are exposed to daylight) than those who tend to go to bed later and sleep later, or “owls.” He writes:

And this is exactly what I found—the effect is subtle but statistically reliable and dose-dependent; in other words the more someone self-identified as a lark, the more likely he or she was to see the dress image as white and gold. Moreover—and to tie everything up with a bow— owls were more likely to assume that the lighting was artificial, not natural.

As the effect is subtle—really a proxy for illumination exposure history that cannot be expected to correlate perfectly, one should not expect this to hold for every individual observer. Even if someone spends most of their waking time at night, he or she might not use incandescent lighting. But on average, this group will be exposed to more incandescent light than the larks.

Wallisch’s observations come from a survey of 13,000 people. He’s shared them in a more formalized manner in the paper “Illumination assumptions account for individual differences in the perceptual interpretation of a profoundly ambiguous stimulus in the color domain: ‘The dress’,” which was published in this month’s issue of the Journal of Vision.

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So now you know why people saw what they did and you never have to think about the dress again, sorry for making you do that.