The Science Behind Why Blond Hair Often Darkens During Childhood

It is a common observation that children born with platinum blond hair often transition to brunette tones as they mature, with some locks eventually turning dark brown or black. This transformation is driven by a complex interaction between genetics, pigment-producing cells, and hormones that shift as a child grows.

Hair color is determined by melanin, a pigment created by melanocytes within hair follicles. There are two primary forms: eumelanin, which yields brown and black hues, and pheomelanin, responsible for yellow and red tones. Generally, higher levels of eumelanin result in darker hair, a phenomenon also seen during pregnancy when elevated estrogen and progesterone stimulate increased pigment production.

According to Desmond Tobin, a professor of dermatological science at University College Dublin, the most significant darkening typically occurs around puberty, generally between the ages of 10 and 13. Hormonal changes during this period, particularly involving estrogens and androgens, appear to boost eumelanin production. “Those that remain blond typically have a genetically determined reduction in eumelanin pigment formation,” Tobin explained.

While the timing correlates with puberty, the precise biological pathway is not yet fully understood. Scientists suspect melanocortins, such as alpha-melanocyte-stimulating hormone (alpha-MSH), may play a role. In the skin, UV light triggers alpha-MSH release to produce a tan, but it remains unproven whether similar hormonal spikes cause hair darkening.

Wojciech Branicki, a researcher at Jagiellonian University in Poland, noted that current science has not identified the specific genes controlling this process. He suggests that hormonal influences may vary based on an individual’s genetic background. Consequently, two children with similar blond hair might respond differently to pubertal changes; one may continue producing low levels of eumelanin, while the other’s follicles ramp up pigment synthesis, leading to darker hair.

“Research on human genome variation is quite advanced, but not all aspects have been explained yet,” Branicki said, highlighting the need for further study into how genes interact with hormonal fluctuations during development.