Human eye color has always been fascinating and mysterious. Babies are usually born with blue eyes. Those mostly change within the first three years to other colors. Some people have different colored eyes; they were celebrated back in July with “Different Colored Eyes Day.” Some emotions can cause the eyes color to change, such as when you’re angry.

Scientists still don’t know why some people’s eyes are a certain color. It is known that human eye colors originate with three genes, obviously passed down from your parents. Thing is, only two of those three genes are understood. Those two give people the common eye colors of green, blue, and brown. But scientists don’t understand why other people have gray, hazel, or eyes with multiple colors in them.

Back in the 60s the common thinking was that brown eyes were “dominant” gene traits and blue eyes were “recessive,” but that’s too simplistic. Eye colors don’t turn out to be a blend of your parents’ eyes. Really, the only predictor is if the parents both have brown eyes, then the baby will likely also have brown eyes. Dark colors dominate more, so brown wins out over green, which wins out over blue.

Baby eyes are interesting in themselves. Most babies are born with blue eyes and everyone watches to see if those eyes are going to another color. This darkening is due to melanin, just like in the skin. Babies don’t have melanin in their eyes at birth, but it develops with age and can make the eye color darken.

Some children are born with irises that don’t match in color. This can be caused by trauma in the womb or shortly after birth, a benign genetic disorder, or a developmental issue in the transport of pigment. Although, an eye doctor should check this, there is usually nothing serious about the condition. Hence, you get Kate Bosworth with her one hazel eye and one blue eye.

Change your mood, change your color

The iris is the part of the eye that carries the color. It is a muscle that expands and contracts to control the size of the pupil. The pupil gets bigger in low light and smaller in bright light. It also changes shape based on what you are doing, shrinking when you are doing close up things such as reading.

The cool thing is that when the pupil changes size, the pigments in the iris compress or spread apart. This changes the eye color to a degree.

The pupil can change size with certain emotions, thus changing the iris color dispersion and the eye color. You’ve probably heard people say your eyes change color when you’re angry, and that probably is true.

Your eyes can also change color with age. They usually darken somewhat. This happens more to white people (10 to 15%) because they tend to have lighter eye colors to begin with.

If you have dramatic changes in eye color, or if one eye changes, you need to see an eye doctor. Such changes can be warning signs of certain diseases such as pigmentary glaucoma.

And what would you think is the most attractive eye color? In a website poll of over 66,000 respondents, 20% said green was the most attractive, followed by hazel and light blue at 16%. Brown was far and away voted the least attractive (6%). But don’t take it personally, brown-eyed people; you can always opt for colored contact lenses, just like they do in the movies.