Our pupils react to more than just light; they’re an involuntary window into our state of emotional thought.

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For over a century, scientists have known that our pupils respond to psychological changes. Even though they aren’t sure why the dilation itself occurs, their studies have determined the main triggers that cause your pupils to dilate.

1. You’re sexually aroused.

Your pupils expand when you’re sexually aroused or looking at someone you love.

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In fact, pupil dilation correlates with arousal so consistently that researchers use pupil size for many cognitive investigations.

2. Your brain is working too hard (otherwise known as MATH)

Research has shown that the harder your brain works, the bigger your pupils get. When you’re calculating an equation, the more difficult it is, the further your pupils will widen – and they will stay that way until you’ve finished solving it.

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3. You’ve made your decision.

If you’ve made a decision, your pupils will give it away before you openly reveal it. This study had participants press a button any time they wanted in a 10 second period, and their pupils began to dilate about one second before they pressed it.

4. You either need to attack, or run the hell away.

Your pupils expand when your nervous system stimulates your “fight or flight” response.

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5. Something is very wrong (or very, very right).

If you have brain damage or you’re on some serious drugs, your pupils will tell all. You’ve probably had a doctor shine a light into your eyes, because they’re checking that your pupils are equal, round, and reactive to light – but if your brain is broken, your eyes may behave differently. Also, taking amphetamine, cocaine, LSD, or mescaline will dilate your pupils to the point that nearby police officers will invade your eyes with their flashlights to prove that you’ve been naughty.

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If you ever get in trouble with the law for being under the influence, you could try convincing the arresting officer that you aren’t high… you’re just really into them.

You could also get punched in the face.

Actually, that can affect your pupils, too. Just look at David Bowie.