Our eyes begin life at their literal brightest. At birth, the levels of melanin – the pigment responsible for our hair and skin colouration – in our bodies is low and its saturation in our eyes is usually consistent with the colour skin we are born with; the darker the skin, the darker the eyes. By around the age of three, melanin levels in our eyes plateaus, leaving the colour we carry through the rest of life.
But the greens, blues, hazels, and dark browns are not set in stone. Eyes colour can, and does, change. The next question then is why?
How Bright It Is
Our pupils are constantly expanding and contracting to regulate how much light they let in at any one time. As they move, the irises encircling them and stretched and contracted with them. When the iris stretches, the melanin in its connective tissue at the front gets pulled over a greater area causing brightening. When they contract, that melanin is pulled into higher densities causing darkening.
But brightness isn’t the only way our pupils react to external conditions. When we are angry, scared, or anything else that invokes a fight-or-flight instinct, our pupils distil to absorb as much visual information as they can(( about the situation we are in. The surrounding iris again becomes larger or smaller accordingly, causing small variations in colour brightness.
While the eye facilitates the input of visual information, it is the brain which acts as the processor of that information. As it does so, the brain constantly plays around with its editing capabilities to turn what the eye picks up to what we “see”. On the one hand, this allows us to pick out colours in both the midday sun and the dark of night. On the other, it leads to mind-hurting optical illusions where two objects of a single colour can seem radically distinct when placed amongst different lighting and background colours.
Health Conditions or Injury
A variety of diseases can cause changes in eye colour including pigmentary glaucoma and pigment dispersion syndrome which release excessive amounts of melanin in the iris.
A more idealised condition characterised by two differently coloured eyes is known as heterochromia. It too may come about as a byproduct of a disease but can also arise after traumatic injury which disrupts the production of melanin in the iris.
For heterochromes who didn’t get the condition through injury, a relatively rare combination of genetic traits can also cause it. But, while this can be fascinating to observe, it is a trait carried from birth and not one developed through time. To find those sorts of changes, we turn to twins.
A 1996 study looked into differences between monozygotic (“identical”) and dizygotic (fraternal) twins. They found that 4 out of the 82 monozygotic twins studied (5%) exhibited minor differences in eye colour. Of the 148 dizygotic twins, 33% had minor differences identified and another 34% had different colours altogether. The researchers went on to outline that complete differences in eye colour are a significant indicator of genetic dissimilarity. They also recognised minor differences being a possible indicator but suggest it be used alongside other features.
Surgical and Cosmetic Approaches
Recent years have seen some artificial additions to the list of reasons someone’s eyes have changed colour. Their introduction has been largely driven by aesthetics, be it contacts for a modelling gig or corrective surgery after a health problem.
So, as for the overarching question of can our eye colour change – yes it can. Is it likely to change that much? No, probably not.