There's actually a hidden reason why you never see Kate Middleton with her hair up.

It’s rare to see Catherine, Duchess of Cambridge, without her absurdly perfect hair tumbling down around her shoulders.

As much as we’d enjoy the sight of the 36-year-old royal rocking a scrunchie or a claw clip, you’d be lucky to even spot her sporting a dainty little hair elastic.

At the tennis? Hair out.

At the polo? Hair out.

Opening a bridge/watching parades/shaking hands with the unwashed masses? Hair out.

Even, believe it or not, while yachting. (Heck, even we lowly commoners know how sailing works. There’s wind, love. Lots of wind.)

But it turns out there's a reason the mother-of-three rarely pulls her hair straight back.

And it's not just because she has staff to coif it every morning, because it's naturally thick and lustrous and because she looks like a walking shampoo commercial.

(I mean, c'mon! This is not human!)

Instead, it's partly because the Duchess is concealing a large a scar that runs down the side of her head.

The scar sits behind her hair line and stretches roughly seven centimetres from just above her ear toward her forehead.

When alarm about the mark was raised several years ago, Kensington Palace told E! News that she had endured surgery when she was young: “The scar is related to a childhood operation,” it said in a statement.

What that involved remains unclear. Both the Palace and loved ones have kept quiet about what the procedure entailed - prompting rumours ranging from a cosmetic procedure to removal of a tumour.

However, John Scurr, a consultant surgeon at the Lister Hospital in London later told the Daily Mail it's unlikely to have been that drastic.

“I really doubt it was any serious medical condition," he said. "I would say it is as a result of an arteriovenous malformation – a birthmark – being removed.”