Who, What, Why: Can a bang on the head cause a bald spot?

US Republican presidential candidate Scott Walker says the hairless patch on his head comes from a scar he received from bumping his head while fixing a kitchen sink years ago. Can this be true?

Scott Walker seems to be rather sensitive about the state of the hair on his head.

"I wish I could grow a little bit more hair on my scar up there," he told the Washington Post the other week, relaying the story of the kitchen sink.

The Wisconsin governor also brought up his bald patch in a conversation with a local paper's cartoonist last year, saying he went to the doctor at the behest of his wife and was told he wouldn't be growing hair in that area any more as a result of the accident.

The comments prompted some ridicule at the time, with left-leaning One Wisconsin Now accusing him of making "hair-raising excuses" and "bald-faced distortions".

Walker's "shiny bald spot" also boasts its own Facebook page, with more than 2,700 likes.

But can a bang on the head really cause a bald spot?

Spencer Kobren, founder of American Hair Loss Association and host of US radio The Bald Truth, says it's very rare for a permanent bald spot to be caused by a head trauma, but it can happen.

"It would have to be a pretty significant trauma - a severe blow to the head," he says.

"When it happens, hair is shocked out of the system. Hair will normally grow back, but if the trauma is really severe it's like damaging the manufacturing plant - it can cause scarring of the underlying hair follicles and create lasting damage."

It's more common for hair to be shed temporarily as a result of intense physical trauma, than to disappear permanently, Kobren says.

"In 99% of cases hair will grow back. I've worked in the field for 20 years and only come across three cases caused by trauma - and only one of those was permanent."

Looking at pictures of Walker's scalp, Kobren notes that his bald spot is extremely distinct.

"It's almost void of hair completely, so it is possible he did have accident at some point in his life. The only way to definitively determine if that was the case would be to do a biopsy.

"But looking at the density of his hair and his hair line I can see some recession - especially in the right temple area. That is a common sign of male-pattern baldness which is probably progressing naturally," he says.

Dr William Yates, who describes himself as a hair loss medical doctor, agrees.

"The evidence suggests his hair is thinning at the front, which is probably down to his genes. There are classic signs of recession and hair miniaturisation," he says.

Yates agrees that traumatic incidents can stress the body so much that it leads to sudden hair loss, but he says even being hit on the head by a shovel, or falling off a fence and landing on your head, would be unlikely to "shock" hair out. You would also expect to see scar tissue, he says.

"It would be more like when Michael Jackson had third degree burns to his scalp. Also, why would the body just pick that circle? I'm betting Walker did have some level of trauma if he said he did, but either way, he'd have a hole there now anyway."

Most men will have some signs of hair loss by their late thirties, and around half of all men are affected by male-pattern baldness by the age of 50.

Scott Walker is 47 years old.

So why is he seemingly at such pains to point out his hair loss is the result of an accident?

Kobren says there is still a stigma associated with baldness.

"The psychology behind hair loss is something that most people can never really fully comprehend. Some people don't want to admit it - they might not want to appear genetically weak or are worried about being the brunt of jokes.

"A balding man might think it will hurt his political career. The last US president that was bald was Gerald Ford, and that was because Nixon resigned. Before that it was Lyndon Johnson, and again he wasn't elected. The world, America and society looks at men with full heads of hair as healthier, stronger and more virile, and some people probably equate that to a better leadership in life," he says.

US voters do seem to have a preference for the thick mane of a John F Kennedy or a Ronald Reagan. There have only been about five follically challenged presidents in US history.

The Republican candidacy is a crowded field. Walker might simply be doing everything he can to make the cut.

Subscribe to the BBC News Magazine's email newsletter to get articles sent to your inbox.