PARIS—You know this chronic scalp condition has reared its ugly head when the white flakes of dead skin have spread from your head to the neck and shoulders and have become a major turnoff.

What in the world causes this most commercially exploited scalp condition?

Dandruff is not caused by a fungus, as many believe, but by an imbalance between two competing bacteria that naturally colonize the human scalp, according to a study released on Thursday.

Yin and yang

Think of Propionibacterium and Staphylococcus as the yin and yang of healthy hair: When they hold each other in check, so, too, is dandruff held at bay.

But when one dominates the other, the telltale white flecks that settle on shoulders like snowdust begin to proliferate, a team of Chinese and Japanese researchers reported.

Dandruff is the most common scalp disorder on the planet, a tonsorial bane for about half the world’s population.

Not microorganisms

But experts have differed sharply over the years on its causes and possible cures, though consensus has formed of late around one culprit in particular.

“Microorganisms on the scalp—especially fungi—have been predominately thought to be the main cause of the development of dandruff,” they noted in the study, published in the Nature Group’s Scientific Reports.

Suspecting this might be wrong, a team anchored by Zhang Menghui of Shanghai Jiao Tong University isolated a large number of variables in the ecosystem of the human scalp to see how they interact.

Villainous fungus

In 59 Chinese volunteers aged 18 to 60, they measured levels of water, several dozen bacteria, sebum—an oily secretion that proliferates in adolescence and early adulthood—and the allegedly villainous fungus, called Malassezia.

Participants washed their hair 48 hours in advance of the tests, and dandruff samples were taken from eight different sections of the scalp.

Balance

“Overall, fungi did not exhibit an important role in the severity of dandruff,” they concluded.

“The relationship between the bacteria and the dandruff was significantly stronger.”

But the scientists also noticed that prevalence of the pesky white particles was higher when the ratio of two dominant bacteria—which normally cancel each other out—was altered.

“This study suggests that adjusting the balance of the bacteria on the scalp … might be a potential solution to lessen dandruff,” they concluded. TVJ