On the surface, attraction seems to have no rhyme or reason. When you like someone, you just like them, and often you can’t put your finger on why. But in terms of biology, we do actually have specific reasons why we find certain people attractive and others not so much, and those reasons may be directly affected by our genetic makeup.

In terms of non-romantic attraction, simply choosing people that we’d like to work with or spend time with, we like to think that that we don’t prejudge, and that we give everyone a fair chance. But studies show that people whose physical appearance appeals to the widest range of tastes are more likely to make friends, get job offers, and have special treatment in a variety of situations (the best tables at restaurants, being able to get away with bending rules, and so forth). Subconsciously, our brains are wired to judge people based on how healthy they appear to be, and the way the brain judges this is by measuring proportion and symmetry in the face.

As for romance, you may think that your attraction to intelligence, a kind personality, and a generous nature are all choices that you have made, but biology is at work there, as well. Even if on the surface you are not thinking about having children when you’re flirting with someone, the optical nerve and the brain cooperate to decide which potential partners have the traits we’d most like to pass to our offspring. This is nature’s way of trying to strengthen certain genetic traits that are beneficial to us.

The issue is more complicated with women, whose brains can judge differently depending on hormone levels. Women who are ovulating may choose a different kind of partner than they would at other times of the month. Have you ever thought you fancied someone, and then a week later wondered what you were thinking? It’s possible that hormones were to blame.

We have also developed a culture of beauty that changes with the times. For example, what was thought of as attractive or fashionable a hundred years ago may not be the same as what we find attractive now. Clothing, hair styles, and even body shape are subject to trends. If we are raised in a society that thinks certain features are beautiful, then we tend to condition our brains to accepting that standard of beauty.

Sometimes when the attraction our brain feels gets out of control, we behave in ways that defy logic or reason to get closer to the person we are attracted to. Most of us have been in the grips of a crazy infatuation at one time or another, and it seems to transcend all boundaries of intelligence and common sense. It can even cross over the border into frightening stalker behavior, when extreme attraction meets other emotional imbalances and heads in the wrong direction. For the most part, though, we know that crushes and infatuation are normal things we all go through, and either the feelings will dissipate or they will develop into something more concrete.

There is some scientific disagreement about whether there is a single biological or chemical law that determines human attraction. Whether the reasons are hormonal, cultural, or emotional, the equation is clearly very complicated, as we never seem to be able to predict who we will like and who we won’t. So next time you find yourself drawn to someone that doesn’t seem to make “sense” in the context of what you believe yourself to find attractive, remember that there’s more than just your personal opinions at work— your brain works in mysterious ways, and sometimes it doesn’t seem to care whether you agree with its choices or not.