It has long been said that the eyes are the window to the soul, but behavioral scientists have long said that our body language and facial expressions are actually the best way to convey emotions. However, new research has revealed that humans and chimpanzees alike both involuntarily match the pupil size of those they are interacting with, especially if there is some kind of emotional connection - namely empathy.

Do you remember that unbelievably sad Sarah McLachlan commercial promoting the BC SPCA End Animal Cruelty campaign that aired nearly a decade ago? Yea, we still remember too. And while your eyes began to tear up just a little, your pupils were also likely on the fritz.

When humans are exposed to stress, their eyes have been known to dilate, perhaps to help take in more light and assess the stressful situation more clearly. Now, researchers are suggesting that your eyes could have also been adjusting to boast the same pupil size as McLachlan's, even as she calmly told you how to help end animal abuse.

It's actually not clear if this occurs when watching television, but according to a study recently published in the journal PLOS One, it certainly occurs even for photos.

In a study that assess both human and chimpanzee pupil mimicry for the first time, Marisaka Kret and a team of researchers asked volunteers and chimps to view images of chimp and human eyes for only four seconds at a time. These images were digitally manipulated so that the pictured pupil would either dilate or contract within the first second, and remain stable for the last 3 seconds.

The researchers tracked the participants' pupils while they viewed the pictures. Amazingly, the participants' pupils did indeed dilate or contract accordingly (as to match the pictured pupil), but only when viewing the eyes of their own species.

Kret speculates that the involuntary changing of pupil size may be a subconscious attempt to reinforce bonding and share an emotional state between parties in eye-contact.

Neil Harrison, who has been involved in similar work, told New Scientist that this might also hint at a new reason as to why humans evolved sclera, the whites of our eyes.

"Traditionally, it's been thought that the evolution of white sclera was driven by its enhanced ability to indicate gaze direction, and hence share attention," he said.

Now it can be suggested that sclera actually help humans better detect pupil change.