Poker players know that the eyes never lie.  Indeed your eyes almost always signal your intentions for the simple reason that you have to see what you intend to do.

This is an essential difference between communication with eye movement/eye contact and other forms of communication.  The connection between what you know and what you say is entirely your choice and of course you will always use this freedom to your advantage.  But what you are looking at and where your eyes move are inevitably linked.

Naturally your friends and enemies have learned, indeed evolved to exploit this connection.  Even the tiniest changes in your gaze are detectable.  As an example, think of the strange feeling of having a conversation with someone who has a lazy eye.

Given that Mother Nature reveals such a strong evolutionary advantage for reading another’s gaze the question then arises why we have not evolved to mask it from those who would take advantage?  The answer must be that it would in fact not be to our advantage.

With any form of communication, sometimes you want to be truthful and other times you want to deceive.  The physical link between your attention and your gaze means that, for this particular form of communication you can’t have it both ways.  Outright deception being impossible, at best Nature could hide our gaze altogether, say by uniformly coloring the entire eye.

But she chose not to.  By Nature’s revealed preference, this particular form of honesty is evolutionarily advantageous, at least on average.