Why People With Brown Eyes May Be at Higher Risk for SAD

Why People With Brown Eyes May Be at Higher Risk for SAD

Way back in 1977, country singer Crystal Gayle had a big hit with a song called “Don’t It Make My Brown Eyes Blue.” Today, researchers have a slightly different take: Brown eyes may increase the risk of the blues – specifically, seasonal affective disorder, or SAD.

One particular researcher, in fact, has found a correlation between brown eyes and SAD, a form of depression that typically begins as the days grow shorter in fall and winter and resolves on its own when the longer days of spring return. Lance Workman, a professor of psychology at the University of South Wales in the U.K presented a paper at the annual conference of the British Psychological Society earlier this year. Equally unable to resist a pun, he titled his paper “Blue eyes keep the blues away: the relationship between SAD, lateralized emotions, and eye color.” When Workman surveyed 175 students from the University of South Wales and the Girne American University in North Cyprus he found that those with brown eyes were significantly more likely to experience seasonal mood shifts than those with blue eyes. Put another way, “Individuals with blue eyes appear to have a degree of resilience to SAD,” the paper concludes.

About 5 percent of the U.S. population experiences SAD. Women are about four times as likely to have the condition, but no other risk factor had been known until now, says Workman, who grew interested in SAD about 25 years ago. “I noticed that a number of my students who were the ‘life and soul’ of the party during spring and summer were often also those most likely to withdraw during the winter months. Some years later I began to notice that seemed to be associated with having dark eyes. So, I began to investigate this,” he says.

Evolutionary Psychology at Work
It had been thought that mood variation might be associated with distance away from the equator – the latitude hypothesis, Workman says. To test this, his team gathered self-reported data from two samples living at two different latitudes (51 degrees north and 35 degrees north). They also examined the relationship between eye coloration and levels of reported mood variability with the seasons. They found no significant differences between the samples based on latitude, but they did uncover a significant difference between blue- or light-eyed and brown- or dark-eyed individuals across the samples, “with the latter self-reporting higher levels of mood variability with the seasons,” he says.

Though 175 subjects comprise a small sample, Workman says that the statistical procedure known as a “power analysis,” which determines the sample size to test a given hypothesis, suggested a sample of this size would be sufficient for this study. Other studies have considered eye color and SAD, he says, “but they did so as a side issue. We wanted to make this the main feature of our study and to relate this to evolutionary psychology.” Workman is thinking about researching whether eye color is related to other forms of mood disorder, such as major depression or bipolar disorder.

Which takes us to the logical next question: Why would eye color matter? Workman concludes in the paper that his findings, “may be taken as suggestive that the blue eye mutation was selected as a protective factor from SAD as sub-populations of humans migrated to northern latitudes.” The assumption had been that the mutation that led to blue eye color, around 10,000 years ago, was due either to it being seen as attractive or as a part of “the general package of pale skin in northern latitudes,” he says. “It seemed to me that, given that frequencies of blue eye coloration reach their highest proportions in the most northerly latitudes of Europe, and given SAD rates reach their highest figures at the most northerly latitudes, then another possibility is that the blue eye mutation is maintained in such areas in order to alleviate the effects of SAD.”

The Light-Mood Connection
That makes sense to the scientist who first described seasonal affective disorder, back in 1984. Dr. Norman E. Rosenthal, clinical professor of psychiatry at Georgetown University Medical School and author of “Winter Blues: Everything You Need to Know to Beat Seasonal Affective Disorder,” says that “we all evolve according to our environment.” Northern peoples developed fair skin, accompanied by blue eyes, because the lack of sunlight meant that the skin needed to allow more of the sun’s rays to penetrate it in order to produce vitamin D. More southerly ancestors, he says, evolved darker skin and eyes to keep out the burning rays of the direct sun. “For people in the tropics, it was more important for survival to be protected against the ultraviolet light effects,” he says. And hours of daylight don’t vary as much from summer to winter there as they do up north.

Sunlight, it is now known, doesn’t only work through the skin to synthesize vitamin D, it also works through the eyes to affect vitality and mood. Thus, keeping the sun out to protect the skin may also prevent enough light absorption to keep mood elevated in the darker months. “It is perfectly sensible,” Rosenthal says. “The hypothesis is interesting and plausible.” Rosenthal says that he had in fact always wondered whether there might be an association between SAD and blue eyes. “We looked at that in our [studies], but I guess we weren’t looking with a broad enough lens to consider that it might be the opposite way.”

Anecdotally, Rosenthal remembers an older male patient who developed SAD. “It is very unusual to first appear in a middle-aged man. I had never seen it before,” he says. When he suggested trying light therapy, the man asked if that would be a problem considering he had cataracts. “He had trauma to his eyes, so the idea that the amount of light entering the eye could be salient is very plausible,” he says. “It’s just an interesting fact and puts one more block in place that there is a light-mood connection.”

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