Even the slimmest women have, on occasion, stood in front of the mirror and asked: "Does this dress make me look fat?" But according to a new study, long-suffering husbands and friends called upon for reassurance may be dealing with more than just irrational anxiety.

Scientists have discovered that the body image a person projects in their own brain is “massively distorted” and can be up to two thirds wider than it is in reality. The brain’s own “body model” is also around a third shorter than the body actually is, according to the study at University College London.

Researchers believe the findings could explain why slim women look in the mirror and see themselves as fat. They may also help explain the cause of some eating disorders.

The effect could be more pronounced in women as they tend to be more sensitive to the appearance of particular parts of their bodies, the researchers believe.

Dr Michael Longo, the neuroscientist who led the research, said: “These findings may well be relevant to psychiatric conditions involving body image such as anorexia, as there may be a general bias towards perceiving the body to be wider than it is.”

“Some people look in the mirror and receive information which tells them they are not fat, but they still can’t use that to over-ride their distorted body model and make themselves believe it.”

The researchers conducted the study by asking participants to place the palm of their left hand down under a board and then to judge the locations of 10 “landmarks” such as their knuckles and fingertips.

Their estimates were then used to create a “map” of the brain’s image of the hand, which was compared with its actual size and shape.

The distance between the index finger and the thumb was estimated to be on average 69pc larger than the actual size of the participants’ hands.

The length of the fingers was judged to be an average 27.9pc shorter than their actual length.

Scientists believe the distortion stems from the number of sensory signals being sent to the brain from different parts of the skin.

The brain's warped "model" of the hand could be extrapolated to the rest of the body, especially those which have “high tactile sensitivity”, Dr Longo said.

The oversized body model could be particularly exaggerated in women who are anxious that parts of their bodies, such as their thighs or stomach, are too big, Dr Longo said.

“It’s certainly possible that there may be situations in which these implicit distorted perceptions that we have observed can come to dominate the brain image and rise into the consciousness, where they could explicitly affect the way a person views their body,” he said.

Curiously, participants shown images of various hand “templates” were generally able to match their own hand to another of a similar size.

Researchers believe this shows that people have an accurate visual image of their own body but are still unable to use that information to over-ride the “brain model” which tells them they are larger.

“It has been known for a long time that people have a distorted image of their body, with areas with a high tactile sensitivity enormously exaggerated. But previously, it has simply been assumed that people could use an accurate representation of their bodies for their position sense,” Dr Longo said.

“This study showed that they weren’t able to do that, which raises questions about how capable we are of over-riding our distorted brain models with an accurate visual representation of ourselves.”

The study is published in the journal Proceedings of the National Academy of Sciences.

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