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Of the many branches found on the tree of science, biology is probably the one that I get least excited about… Kind of a strange statement considering I am (as far as you know) a human being. Still, once in a while I come upon an interesting find from the realm of biology that totally engages me. Typically it’s a new development or discovery related to something that I probably should already have a base knowledge of, but certainly don’t. For example, our eyes… Or more specifically, the pupils of our eyes.

I’m not alone in my ignorance though… Not entirely anyway. As it turns out, there is a lot that scientists still don’t understand about the complicated workings of our peepers. For example, we have long observed that our pupils don’t just dilate and contract is response to light levels, but also to a number of emotions… Why they do this has been a mystery to scientists for a long while.

That type of thing doesn’t fly with most researchers. Try telling an obsessively curious mind that there is no solution to a specific problem, and to move on… Not gonna happen. It shouldn’t be surprising that this phenomena has and still is intensely studied and with persistence we’re getting close.

“Nobody really knows for sure what these changes do,” says Stuart Steinhauer, director of the Biometrics Research Lab at the University of Pittsburgh School of Medicine. Steinhauer views the behaviour as a by-product of the nervous system and how it processes important information. It’s a bit to wrap your mind around, especially on a Sunday morning, but the explanation from Scientific American is this:

Stimulation of the autonomic nervous system’s sympathetic branch, known for triggering “fight or flight” responses when the body is under stress, induces pupil dilation. Whereas stimulation of the parasympathetic system, known for “rest and digest” functions, causes constriction. Inhibition of the latter system can therefore also cause dilation. The size of the pupils at any given time reflects the balance of these forces acting simultaneously.

The pupil response to cognitive and emotional events occurs on an even smaller scale than the light reflex, with changes generally less than half a millimeter. By recording subjects’ eyes with infrared cameras and controlling factors that might affect pupil size, such as ambient brightness, color and distance, scientists can use pupil movements as a proxy for other processes, like mental strain.

So, our pupils give us away: They dilate when we’re having trouble with a math problem, they dilate when you’re dead, and they dilate during orgasm.

In fact, pupil dilation correlates with arousal so consistently that researchers use pupil size, or pupillometry, to investigate a wide range of psychological phenomena.

There you have it. If you’re planning on faking an orgasm or pretending you’re better at math than you actually are, think twice. Your pupils might give you away.