Your eyes can be a window into your soul, and some say that your eyes — be them baby blue, sexy green or deep, mysterious brown — may reveal even more.

Like how well you tolerate pain. Or your tendency toward alcohol dependence. Or just how generally agreeable you might be.

Eye color and what eye color really means are a constant source of fascination among scientists, academicians and that guy or girl across the bar. As is often the case with these things, though, it's not that simple. In fact, it gets pretty complicated.

What you see is what you get

"This general question of the relationship between, say, a visible trait — height or body size, or skin color or eye color or hair color — and anything else, whether it’s a disease trait or whether it's a visible trait, is something that geneticists think about and talk about all the time. And it's a topic of understandable popular interest," says Greg Barsh, a faculty investigator at HudsonAlpha Institute for Biotechnology, a nonprofit based in Huntsville, Alabama, and a professor emeritus of genetics at Stanford University.

"We do not think, we do not believe that there is a direct connection of eye color with specific diseases. We do not think that there is a relationship, say, between eye color and diabetes, or eye color and cancer, or eye color and behavior."

Still, many people are all too willing to make that link between the color of someone's eyes and, say, how well they react in a motor skills test. (Men with dark eyes reportedly performed better than those with lighter-colored eyes — but only when they blue racquetballs, rather than green or yellow.)

Are light-eyed people from a certain region, for example, really less agreeable than the dark-eyed population of the same region, as one study suggests?

It's easy to accept the hypothesis of this study, which concludes that "light-eyed individuals have a higher prevalence of [alcohol dependency] than dark-eyed individuals." Or this piece in Cosmopolitan, based on several popular papers, that concludes:

  • Brown-eyed people are prone to anxiety.
  • Green-eyed folks have a higher tolerance for pain.
  • Those with blue eyes have a lower risk of anxiety and depression, yet are more likely to be dependent on alcohol.

Easy, sure. But believable? Can you really make that jump, that generalization, based solely on the color of someone's eyes? Or is it, as Barsh suggests, more complicated?

Ancestry, cause and correlation

"Most individuals with blue eye color are of North European ancestry. But there are many other traits that are also correlated with North European ancestry," Barsh says. "So when someone says, 'Okay, I looked at a bunch of people with blue eyes and I also discovered that they drive faster, or they die sooner, or that they have a difference in pain tolerance,' ... the default question that always must be asked is, 'How do you know that isn't a difference that is genetic and happens to be related to their North European ancestry?'"

Barsh cites an old example popular among geneticists: A discovery is made that people in the Bay Area of San Francisco are more adept, as a whole, at using chopsticks than people in many other areas of the country. Is that because, simply, they[re from the Bay Area?

Or is it because many people from Asia, or many people with ancestors from Asia, have settled in the San Francisco area, and those ancestors were adept at eating with chopsticks?

"The situation with visible traits is actually pretty similar, because visible traits are highly, highly correlated with ancestry," Barsh says. In other words, blue eyes are usually handed down from ancestors in North Europe. People with ancestors from Asia and Africa are normally dark-eyed.

Still, that doesn't mean you can come to conclusions about diseases or behaviors just based on ancestry, either.

"One of the major challenges that I think all biologists face is distinguishing correlation from causation," Barsh adds. "If you have two traits found in one group but not another, it can be very challenging to distinguish whether the relationship between the traits is that one causes the other, or they just happen to be present in the same population."

So to do this thing right, you have to dive deeply into the genetics of a given population. And genetics are a complicated thing. There's one main gene — it's called the OCA2 — responsible for eye color, for example. But several other genes contribute. So assigning a behavior, or the chance of getting a disease to, say, the OCA2 and four or five other genes (among some 20,000 in humans) falls a bit on the simplistic side.

"We know enough about the genes that control skin and eye color that [we know] that is, in fact, all that they do. They don't do other things," Barsh says. "No matter how much we learn, we're never going to learn that eye color has anything to do with intelligence. We know that it doesn't. It doesn't have anything to do with behavior. It doesn't have anything to do with disease susceptibility."

The only exception, it seems, is that those with lighter skin and lighter eyes are more susceptible to the damaging effects of ultraviolet rays from the sun, which could lead to diseases of the eyes and skin.

Other than that, though, the color of those baby blues is that and only that: A color (or lack of, or a combination of colors) based on genes handed down from your ancestors.

Anything else may just be your eyes playing tricks on you.