Looking directly into another person's eyes can reveal a lot about them. In fact, poker players often cover their eyes with sunglasses for fear of giving a tell, and some studies have shown that staring into another person's eyes can create deep feelings of intimacy for those involved.

Now, it turns out, a person's eyes might also reveal something unexpected and controversial: their level of intelligence, reports Discover.

In the new study, psychologist Jason S. Tsukahara and colleagues found a positive correlation between pupil size and cognitive ability. Of course, dilated pupils occur when the eye needs to let in more light, such as when you need to adjust to the dimmer light of a dark room. But what could having larger-than-average pupil size have to do with IQ? It turns out, changes in pupil size happen in a number of circumstances that don't necessarily relate with changes in light levels.

"Starting in the 1960s, it became apparent to psychologists that the size of the pupil is related to more than just the amount of light entering the eyes. Pupil size also reflects internal mental processes," wrote the researchers. "For instance, in a simple memory span task, pupil size precisely tracks changes in memory load, dilating with each new item held in memory and constricting as each item is subsequently recalled."

So could this mean that people with generally larger pupils have more active minds? It's certainly possible, but as any serious scientist will tell you, correlation does not necessarily equal causation. The researchers suggested another possibility: that both intelligence and pupil size might be influenced by some other shared factor.

"Neuroscience research has shown a close association of pupil size with activity in the locus coeruleus-norepinephrine system. [In the brain, norepinephrine] modulates the gain of target neurons to be more sensitive to incoming signals (both excitatory and inhibitory)… this modulation of neural gain has an effect on the strength of functional connectivity throughout the brain," they wrote.

In other words, it's possible that the key factor here is higher norepinephrine signaling. It would make sense that more intelligent people should be more sensitive to excitatory and inhibitory signals in the neural system. If higher norepinephrine levels are also correlated with larger pupil size, then the connection makes sense.

Of course, the study will need to be expanded to make sure the correlation does, in fact, exist. And it should also be noted that there are many other factors that relate to IQ than pupil size or norepinephrine signaling, so these factors hardly make for a conclusive intelligence test. Still, it's an interesting twist on the old notion that the eyes are the windows to the soul.