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Everyone loves a good optical illusion. There are ones that play with your perception of color, or look like they're swirling even though they're still.

And how about the Müller-Lyer illusion? It's named after the German sociologist, Franz Carl Müller-Lyer, who came up the image in 1889. Basically, it's a couple of arrow-like figures.

How do you think the lines compare to each other? Even if you've seen this illusion before, chances are the lines look like they're slightly different lengths—but they're actually the same. But not everyone falls for this trick.

A lot of it might actually have to do with where you're from and what you're used to seeing, because optical illusions are designed to trick your brain. The brain's visual cortex processes everything your eyes see, starting with two-dimensional images from your retinas, and figuring out depth somehow. We think the brain calculates depth using binocular disparity— the difference between what your right eye sees and what your left eye sees— along with other cues, like the size of different things and angles in a space.

Psychologists think the Müller-Lyer illusion hinges on those other visual cues that hint at three dimensions, and works best on brains that are used to seeing a bunch of right angles. Many of us live and work in spaces that are chock full of right angles, so we're used to interpreting depth in boxy rooms. If you're standing on one side of a long table, for instance, you know that the side closest to you is the same size as the opposite.

But the closer side looks bigger, and the farther side looks smaller. That's thanks to perspective. The Müller-Lyer illusion plays with our brain's sense of perspective.

The arrow-like caps on the ends of each line might trick our brains into interpreting both of them as having some depth. So the line with inward-pointing arrow heads is being interpreted as farther away, and the line with outward-pointing arrow heads is being interpreted as closer. Even though our eyes might detect that the lines are the same length, our brains might get confused about the perspective.

You brain might perceive the farther line as smaller, like that far end of a table in a 3D world, and therefore think it's really a longer line. Not all people are surrounded by so many 90-degree angles, though. And those who aren't seem to be less susceptible to the Müller-Lyer illusion.

In the 1960s, a group of researchers led by psychologist Marshall H. Segall set out to test what they called the “carpentered world” hypothesis and the “experience with two-dimensional representations of reality” hypothesis. The researchers proposed that some peoples' experience with “carpentered” or box-like spaces and 2D images of 3D spaces, like photographs of rooms, helped the illusion trick their brains.

They collected data from individuals in 17 groups of people around the world, showing them sets of two arrow-like lines— one with the caps pointing out, labeled a, and one with the caps pointing in, labeled b. Then, they gathered data about how much longer line a needs to be than line b before people perceived them as equal. The greater the difference, the more susceptible the group was to the illusion.

The most susceptible group was from Evanston, Illinois in the United States. And the group of Europeans living in Johannesburg, South Africa was pretty susceptible too. On the other hand, societies of agriculturalists and foragers, like the San from the Kalahari Desert or Bete from the Ivory Coast, were less tricked by the illusion.

And they probably didn't spend as much time in right-angle-filled environments. Of course, one study isn't definitive proof of a phenomenon. A different study from 1973 involved one group of Americans and five groups of Zambians.

And a difference was found between Zambians living in rural and urban environments, where those living in urban environments were more susceptible to the illusion. This supported the idea that being tricked by the illusion could involve factors like people's immediate surroundings. And as psychologists have studied this illusion more, it seems like how you perceive the lines in the Müller-Lyer illusion is at least partially influenced by what you see all the time, because that's what your brain is used to interpreting.

Other researchers have tried to look into different cases where visual perception seems to be influenced by your surroundings and culture. A 2005 study, for instance, found that Japanese and American undergraduates noticed changes in images differently. They were shown pictures of different scenes like an American city, a Japanese city, or something without cultural markers like a generic construction site.

And they were given a change blindness task, where they had to pick out small changes between very similar sets of pictures. With American scenery, all participants were generally better at seeing changes that involved prominent objects. And with Japanese scenery, all participants were generally better at noticing changes in the background.

But with the neutral scenery, it seemed like cultural differences showed up:. Japanese students noticed more contextual changes, and American students noticed more focal object changes. Citing other studies that tracked participants' eye movements, the researchers proposed that these differences could be because different people pay attention to their surroundings differently.

Or it could mean that different environments encourage different kinds of interpretation by our eyes and brains. It's important to remember, though, that there might be other factors in play in these perception experiments. But if these kinds of studies tell us anything, it's that brains are complicated, and everyone is probably seeing things from a slightly different perspective.

Thanks for using your eyes and brains to watch this episode of SciShow Psych! If you want to learn more about weird phenomena involving the human mind, you can go to youtube.com/scishowpsych and subscribe. [OUTRO ♪].