Women from European descent are twice as likely to be naturally blonde as men, according to new research.
The largest ever genetic study on pigmentation looked at nearly 300,000 people of European descent.
It found 124 new genes that play a major role in determining human hair colour variation.
The report's authors say they are not sure why there are so many more blonde women than men, calling it an "intriguing mystery".
The study which is published in Nature Genetics, builds on previous genetic studies that had only identified a dozen or so hair colour genes.
The data was supplied by the UK Biobank, American DNA testing company 23andMe and the International Visible Trait Genetics Consortium and their study partners in the Netherlands, Australia and Italy.
They chose people of European descent because of their variety in hair colours.
They found men were three times as likely as women to have black hair.
Prof Tim Spector, joint lead author, from Kings College London, told the BBC they were not expecting to find so many more blonde women than men.
"It's a mystery and it's intriguing because it wasn't what we were looking for. We thought it was a bias but it wouldn't go away and it's found in every sub-group of every population we saw," he said.
"It's a curious mystery because it's a very big effect - to see two and threefold effects both in a whole variety of American populations and European ones was quite amazing."