Whenever we try to make a judgment on someone we meet, we hear the same thing over and over: "Trust your gut."

We might spend days, months, or even years trying to figure someone out. Is he who he says he is? Should I trust her? The wheels in our head spin as we think of all the variables and how they'll play out.

And still, we keep hearing that we should just listen to our instincts. Complicated questions, simple answer. What should we do, and where did this whole idea of the gut instinct come from, anyway?

Intuition isn't some magical, mysterious quality that we carry with us. It actually comes from the knowledge and past experiences that we all carry. Even if we're unable to explain why we feel the way we do, there's a logical explanation behind our gut feelings.

Whenever you encounter anything new, the unconscious side of your brain is constantly making assessments. It takes in certain cues, such as a smile or parts of a story, and then matches it with something similar in our database of memories to come up with a conclusion. Meanwhile, our conscious side remains unaware of this rapid process taking place.

Our instincts help us navigate our world more easily by creating mental shortcuts that help us act quickly. Instead of using energy to fully assess a situation, our brains look for fast answers.

But how trustworthy are our gut feelings?

Leadership's all in the face

It's been said that we shouldn't judge a book by its cover, but studies reveal that we can learn quite a bit just by looking at someone's face. Nicholas Rule, a psychology professor at the University of Toronto, did a series of studies on facial perception.

In 2011, Rule showed a group of people the college yearbook photos of top U.S. lawyers. These strangers successfully predicted which lawyers would eventually lead the most profitable law firms in the country. He conducted a similar study using 20 female CEOs and found a direct correlation between ratings and corporate profits.

Exactly why, though, is harder to explain. Maybe it's because we judge a person initially by their physical appearance, so they develop certain personality traits to fit their appearance. In a self-fulfilling prophecy, they end up finding positions that match their character.

Or is it the other way around? The person's personality changes their appearance as they repeat certain facial expressions. From laugh lines to glowering looks, we use these physical expressions to gauge what the person is like.

How micro-expressions help us read others' words

There's a lot to be said about reading someone's face. When we watch someone speak or react to something, we look at their face for non-verbal expressions. What might not be so obvious is that we're subconsciously reading people's micro-expressions to see how they really feel.

A micro-expression is a brief, involuntary facial expression. Unlike regular expressions, micro-expressions often only last for a fraction of a second and are difficult to fake. For instance, someone could avert eye contact briefly if they're hiding something or feel uncertain.

When someone's words don't match with the micro-expressions on their face, we sense that something is 'off' about the person. What they say doesn't match what they think. This uneasy feeling we experience can be hard to articulate, so we attribute it to our gut feeling.

Our instincts are flexible

Our instincts exist for a reason. They were built to help us assess people quickly to determine whether or not we can trust them, which increase our chances of survival. These gut feelings are built upon and modified by our past experiences and things that we've learned.

The bad news is that prejudices and memorable experiences can cloud our judgment and instinct. We might mistakenly think we know what a stranger is like by comparing them with someone else. Or, we impose personality traits on someone to suit how we want them to be.

Every one of us carries biases that alter how our gut feelings react. We need to assess how our experiences change our perceptions of people so that we can make better judgments in the future. By recognizing that our gut feelings get thrown off on occasion, we can balance our emotions with rational thought.