So what causes this change?
"It depends on their genetic inheritance," Desmond Tobin, a professor of dermatological science at University College Dublin, told Live Science in an email.
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The activity of hair follicles isn't necessarily fixed throughout childhood. As children grow, the biological signals controlling pigment production can change, too.
From blond to brown
It is likely the hormonal changes during puberty, such as estrogens and androgens that stimulate an increased production of eumelanin, that gradually turn blond hair darker over time, he said. "Those that remain blond typically have a genetically determined reduction in eumelanin pigment formation," Tobin added.
The most noticeable hair darkening seems to happen around puberty. (Image credit: Halfpoint Images via Getty Images)
One clue comes from a group of hormones called melanocortins. One of these, alpha-melanocyte-stimulating hormone (alpha-MSH), is involved in telling pigment-producing cells to make more melanin. In our skin, alpha-MSH is triggered by ultraviolet (UV) light. UV light can damage DNA in skin cells that, via a cellular signaling pathway, encourages the release of alpha-MSH to make the skin tan.
The melanocortin system shows that hormones can tell pigment-producing cells to make more eumelanin. What remains unclear is whether changes in this system are actually responsible for blond hair darkening during puberty.
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Branicki posits that in some children, the hormones could induce the production of melanin or the transport of pigment within the hair follicle. In other words, two children could start out with similarly blond hair but respond differently to the hormonal changes as their bodies develop. One might continue producing relatively little eumelanin and stay blond, while the other's follicles might gradually ramp up pigment production, causing new batches of hair to grow darker.
"Research on human genome variation is quite advanced, but not all aspects have been explained yet," Branicki said.
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