Why Blonde Hair Turns Brown During Childhood—and What Puberty Has to Do With It

If your blonde preteen seems to be turning brunette, you are not imagining it. In some children, hair that starts out platinum blonde gradually darkens as they grow, eventually becoming light brown, dark brown or even black.

So what causes blonde hair to darken?

Hair color is controlled by genes and melanin

“It’s due to their genetics,” Desmond Tobin, a professor of dermatology at University College Dublin, told Live Science in an email.

Hair color is determined by a complex interaction between genes, pigment-producing cells and, later in childhood, hormones. Scientists understand many of the genes related to hair color, but they are still trying to determine why some blonde children’s hair darkens while other children remain blonde throughout their lives.

Hair color comes from a pigment called melanin, which is produced by specialized cells called melanocytes inside hair follicles. There are two main types of melanin:

• Eumelanin produces brown and black shades.
• Pheomelanin contributes yellow and red tones.

The amount and type of melanin produced by a hair follicle largely determine the color of the hair that grows from it. Generally, the more eumelanin there is, the darker the hair will be.

Hair follicle activity is not necessarily fixed throughout childhood. As children grow, the biological signals that control pigment production may also change.

Why blonde hair often turns brown during puberty

For many blonde children, hair darkening becomes most noticeable around puberty, usually between the ages of 10 and 13, Tobin said.

Hormonal changes during puberty may stimulate the production of more eumelanin, causing blonde hair to gradually become darker. People who remain blonde usually have a genetically determined reduction in eumelanin pigmentation, Tobin said.

Hormones can affect hair pigmentation in other situations, too. For example, high levels of estrogen and progesterone during pregnancy may promote hair darkening because they can increase eumelanin production.

Scientists still do not know exactly which hormones are involved

One possible clue comes from a group of hormones called melanocortins. One of them, alpha-melanocyte-stimulating hormone, or α-MSH, helps instruct pigment-producing cells to make more melanin.

In skin, ultraviolet light can trigger α-MSH. UV radiation damages the DNA of skin cells, prompting cell-signaling pathways that release α-MSH and contribute to tanning.

Hair pigmentation is less clear. Scientists have not shown that increases in α-MSH, testosterone, estrogen or any other specific hormone are responsible for blonde hair darkening during puberty.

The melanocortin system may encourage pigment-producing cells to make more eumelanin, but researchers do not yet know whether changes in this system actually cause blonde hair to darken during puberty.

Why some children stay blonde

“The genes that control this process have not yet been identified,” Wojciech Branicki, a researcher studying the genetics of human pigmentation at Poland’s Jagiellonian University, told Live Science via email.

Branicki suggested that hormones may affect children’s hair follicles differently depending on their genetic background. Researchers still do not know which genes are involved or how they interact with the hormonal changes of puberty.

In some children, hormones may trigger more melanin production or change how pigment is transported within the hair follicle. As a result, two children who start with similarly blonde hair may respond differently as their bodies develop.

One child’s follicles may continue to produce relatively small amounts of eumelanin, allowing the hair to remain blonde. Another child’s follicles may gradually increase pigment production, causing new strands of hair to grow darker.

“Although considerable progress has been made in the study of human genomic variation, not all aspects are yet understood,” Branicki said.