Psychologist and sex expert Dr Glenn Wilson, who has studied the effect of wearing sunglasses, says they make the wearer feel more confident and attractive to the opposite sex.

"Sunglasses give us a degree of anonymity so that shy and inhibited people can do things that they might not otherwise have done - topless sunbathing is a very good example," he said.

"It is partly because you feel in control because you can see more than you are seen so that gives you that cool that is often associated with sunglasses."

Dr Wilson, a psychologist at the University of London whose research is based on a series of single sex focus groups, said sunglasses had become an important fashion item.

"People wear sunglasses not just because the light is too light, but because they feel they look good in them and other people seem to agree," he said.

"But you can take that to extremes and, if it seems inappropriate or a bit posey, then it is a bit of a turn off."

Dirty Harry effect

Reservoir Dogs used sunglasses to create a menacing atmosphere

Dr Wilson said men liked wearing sunglasses because they gave them a sinister overtone - similar to Dennis Hopper in Easy Rider, or Clint Eastwood in Dirty Harry.

"It's a threat that comes from the unpredictability of their actions.

"You cannot see their eyes so you don't know what they are going to do next. I think the Krays wore sunglasses in their heyday.

"It's a big turn on to women that kind of power in men. A lot of women like that sort of thing. It is the dominance or control that is emitted by such men."

Dr Glenn Wilson: "Sunglasses help people lose inhibitions"

Dr Wilson said it was important that a man also knew when to take his sunglasses off.

"He must know when to take down the barrier, and say, this is me, let's connect.

"If you greet somebody on your doorstep wearing sunglasses or leave them on for a photo it looks a bit evasive.