Two new studies suggest that sex and eye color influence the risk of developing seasonal affective disorder (SAD). The researchers also put forth some interesting explanations for why this may be the case.

SAD, a psychiatric condition, is often characterized by feelings of hopelessness and acute sadness that occur during the fall and winter months.

A form of depression, SAD is estimated to affect 5% of the United States population. And of these, women are thought to be at a higher risk.

In fact, 4 in 5 people living with the condition are thought to be women.

Previously, researchers found that the strong prevalence of SAD among women is independent of social or lifestyle factors, suggesting that perhaps there are biological sex-specific differences that account for the predisposition.

Recent research confirms that women are more prone to the condition, but it adds an interesting element to the mix: eye color.

Additionally, the two new studies provide intriguing novel explanations for why sex and eye color can influence the risk of SAD.

The team's findings were presented at the annual conference of the British Psychological Society in Nottingham, United Kingdom, by Lance Workman, who is a professor at the University of South Wales, also in the UK.

Why 'blue eyes keep the blues away'

The first study to be presented by Professor Workman—aptly titled 'Blue eyes keep the blues away: the relationship between SAD, lateralized emotions, and eye color'—surveyed 175 students from the University of South Wales and the Girne American University in North Cyprus.

The results of the questionnaires revealed that participants with brown eyes were significantly more likely to experience shifts in mood compared with blue-eyed participants.

Professor Workman has an interesting explanation for this. He says, "We know that light entering the brain causes a decrease in levels of melatonin."

"As blue eyes allow more light into the brain, it may be that this leads to a greater reduction in melatonin during the day and this is why people with lighter eyes are less prone to SAD."

–Professor Lance Workman

"Individuals with blue eyes appear to have a degree of resilience to SAD," explain the authors.

"This," they add, "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."

People with SAD use their right brain

The team also asked the participants with SAD to take part in an additional test that examined how their two brain hemispheres responded when they were trying to recognize different emotional expressions on other people's faces.

This test revealed that people with SAD tended to use their left visual field when recognizing facial expressions and use their brain hemisphere to "decode" these expressions.

As Professor Workman explains, "This tendency to use the left visual field and right side of the brain for identifying facial expressions is present in the general population, whether they [live with] SAD or not."

"But," he continues, "people who [have] more conventional forms of depression generally lose this right hemisphere advantage."

"In the case of SAD, we found this left visual field advantage was actually increased. This suggests SAD has different causes than, say, bipolar depression," adds Professor Workman.

Why women may be at a higher risk

The second study presented at the conference surveyed a much larger sample of 2,031 people. Of these, 8% had a chronic form of SAD, while 21% had a milder form of the illness.

Women were at a particularly high risk—in fact, they were 40% more likely to develop the condition than men. The study also points out that SAD is more severe when women are of reproductive age.

This made Professor Workman venture another possible evolutionary explanation for the findings. He speculates that the disorder is nothing but an energy-preserving mechanism gone haywire.

During a woman's reproductive years, he says, the mother would have to conserve energy to ensure survival of both her and her offspring, particularly during the winter months.

This seems to be supported by the fact that symptoms of SAD also include a craving for carbs, and putting on weight during the winter months may have also helped our ancestors to cope with the cold, the researcher says.