Eyes that change color, a mysterious phenomenon

It’s always amazing how drastically a newborn’s eye color can change in just two years. Amazing and sometimes disconcerting, because it is a physical characteristic to which many of us place great importance. It’s science: eye color can even impact how much we trust a person.

A BBC journalist was interested in these color changes, which occur so often during the first years of life, but can also occur at other times of existence. These changes can be explained by many factors, including excessive sun exposure, infection or injury – this was the case for David Bowie, whose oddball eyes are not natural. But the color change can also occur spontaneously, without any real reason.

Going back to babies, an American study shows that the vast majority of those born with brown eyes have retained the same color at the age of 2 (on the sample studied, this represents nearly 95% of cases ). In contrast, babies with blue eyes are more prone to mutations: two years later, their eyes may have turned brown, hazel or green.

Another American study on the age up to which the color change can take place established that for 80 to 90% of individuals, the color was definitively fixed before the age of 6 – with a few imperceptible nuances. In the rest of the cases, the eyes can continue to change color during adolescence, or even in adulthood. It has also been observed that in heterozygous twins, similar eye colors at the start can end up diverging.

Like a blue sky

The color of the eyes depends in particular on the amount of melanin, this pigment which is also responsible for the color of the skin. It plays an important role in protection against the sun, which explains why the lightest eyes are the most fragile, while black or brown eyes will be more resistant.

In irises with little melanin, explains journalist Martha Henriques, the blue color is due to the way collagen fibers spread light. The phenomenon is similar to that which occurs in the sky: it is the way in which the light is dispersed in the atmosphere which makes it blue.

All this does not explain why so many children produce more melanin as they grow up, and therefore why their eyes tend to darken. “We don’t know what influences these color changes”concedes David Mackey, Australian professor of ophthalmology. “As often, it is the result of the combined interaction between genetics and the environment. But what environmental factors exert an influence? We don’t have that kind of data.”

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