Why Children's Hair Mysteriously Darkens

Q: When I was born, I had blond hair, but now it is dark brown. What caused this change?
Eric, Farmington, Missouri, USA

A: "Light hair, as a rule, has a tendency to darken during childhood, says geneticist Rick Sturm, principal research fellow at the University of Queensland in Australia.

Hair darkens because genes that control hair color switch on.

The more pigment produced in the hair follicle, the darker the hair. Much pigment results in dark brown hair; little pigment causes blond hair.

Sometime during your childhood, the genes* controlling hair color changed, and told the pigment protein to produce much pigment. Your hair consequently gradually turned to dark brown. It's an 'on' / 'off' order, like switching on a light.

We don't, however, have a clue why the genes changed. They just do for many children.

We strongly suspect genes control the timing of the change. In 1975, two pediatricians (Adam P. Matheny Jr. and Anne Brown Dolan of the University of Louisville School of Medicine) examined the hair color of identical twins (169 female twins and 161 male) routinely from the time the twins were six months old until they were six years old.

They found marked changes in the hair color for both sexes. Moreover, they discovered a "high rate" of agreement at every age "in spite of the general change in hair color." Both twins' hair color changed, beginning at about the same age and continuing at nearly the same rate. They, therefore, concluded that genes influenced the timing of color changes.

In addition to hair darkening during childhood, "there are changes [causing hair to darken] also apparent at puberty that would likely be hormone related," says Sturm. The darkening can continue past puberty into the 30's and 40's.

By the way, the hair of some herd animals (for example, giraffes) darkens, but not with age. "The top male's hair darkens when he takes the alpha position," says Sturm, who suspects hormonal changes (maybe testosterone). "The specific example I have seen was a group of giraffes where the alpha male had noticeably darker spots. The keepers told me that this changed over time as the alpha male position was handed on. And it happens in other animals."

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*Genes controlling hair color include: tyrosinase,TYRP1, MATP, SLC24A5, SILV etcS and OCA2

Further Reading:

Sex and genetic differences in hair color changes during early childhood, University of Louisville School of Medicine, 1975.

Hair color, Wikipedia

Understanding genetics, hair color, Stanford University