The observation that many newborns appear to have striking blue or slate-gray eyes is a fascinating example of human biology. For a significant portion of the population, this initial hue is temporary, not the final eye color. This phenomenon lies entirely in the delayed production of pigment within the iris, a process that begins only after birth.
Addressing the Misconception
The idea that all babies are born with blue eyes is a widespread myth that does not hold true globally. Infants of African, Asian, or Hispanic ancestry often possess brown eyes from birth, and these eyes rarely change color later on. This is because these populations naturally have higher initial levels of melanin in the iris. A 2016 study of newborns found that nearly two-thirds of infants had brown eyes, with only about one in five arriving with blue eyes. The blue-eyed appearance is largely seen in babies of Caucasian descent, where pigment development is slower to start.
How Melanin Determines Eye Color
The appearance of color in the eye is determined by the concentration of melanin within the iris. Melanin is produced by specialized cells called melanocytes. The iris has two main layers: the stroma (front) and the epithelium (back). Brown eyes result from a high concentration of melanin distributed throughout the stroma, which absorbs most light entering the eye. Conversely, blue eyes contain very little melanin in the stroma. When light enters, it scatters as it travels through the translucent tissue. This scattering phenomenon, known as the Tyndall effect, reflects the shorter, blue wavelengths back out, making the eyes appear blue.
The Timeline for Eye Color Development
A newborn’s melanocytes in the stroma are often inactive because they have spent nine months in the darkness of the womb. This initial lack of pigment means that light scattering dominates, resulting in the characteristic blue or gray birth color. Once the infant is exposed to light, the melanocytes begin to receive stimulation, initiating melanin production. This gradual activation explains why color change is observed over many months.
As melanin production increases, the pigment slowly accumulates in the stroma, leading to a visible darkening of the iris. If only a small amount of additional melanin is produced, the eyes may shift to green or hazel; a substantial increase results in brown eyes. The most noticeable shifts typically occur between six and twelve months of age, though color can continue to subtly change and fully stabilize up until a child is three years old.
Heredity and Final Eye Color
While the initial blue or gray color is temporary, the adult eye color is ultimately fixed by the child’s genetic makeup. Eye color is a polygenic trait, governed by the complex interaction of multiple genes. The two genes most significantly associated are OCA2 and HERC2, both located on chromosome 15.
OCA2 provides instructions for creating the P protein, which is involved in the maturation of melanosomes that produce and store melanin. HERC2 contains a regulatory region that controls the activity level of the OCA2 gene. A specific variation in HERC2 can decrease P protein production, leading to less melanin in the stroma and lighter eye colors. The final color results from the total amount of melanin the body is genetically programmed to deposit in the iris.