You can fake longer lashes with mascara, a curler, or extensions, but a new study suggests that the actual length of your lashes has already been set by a biological ratio. Plus, there's also a pretty good mathematical reason to lay off those extra long extensions.

So what is the formula? The length of eyelashes on all 22 species of mammals studied was 1/3 of the width of the eye. To get to that formula, researchers at the Georgia Institute of Technology used two methods: First, they hit the American Museum of Natural History to start measuring all the eyes and lashes that they could. That's where the mathematical ratio was initially noticed, as you can see in this chart of some of the different eyes they looked at.

But what they did next gave a hint as to why that was the case — and it also shows why you may want to skip the extensions.

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The researchers set up a wind tunnel with a plate serving as an "eye" at the end of it and some improvised mesh "lashes" of varying lengths over it. The best results at the mesh diverting dust in the wind tunnel came in at 1/3 the width of the plate. Under the 1/3 ratio, the lashes didn't protect the plate as well. But once the "lashes" exceeded one 1/3, something strange happened. Not only was there no additional protection, the effects actually became worse, with the extra length acting almost like a funnel to direct dust into the eye. Lead author on the study Guillermo Amador explained it in a statement like so:

When eyelashes are shorter than the one-third ratio, they have only a slight effect on the flow. Their effect is more pronounced as they lengthen up until one-third. After that, they start funneling air and dust particles into the eye

You can check out the whole paper over at the Journal of the Royal Society Interface.

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Image: Amador et. al, Journal of the Royal Society Interface, Top image: Inga Ivanova / Shutterstock.