UK scientists have identified the part of the brain that determines whether a person perceives themselves as fat.
The parietal lobe processes sensory information from body parts
The area is called the posterior parietal cortex which sits at the side of the head, just above the ear.
The University College London Institute of Neurology team believe the findings might explain why some people feel fat, even when they are thin.
Their findings in PLOS Biology are based on brain scans of volunteers experiencing a body size illusion.
To achieve the so-called "Pinocchio" illusion, the 17 participants each had a vibrating device placed on their wrist to stimulate the tendon and create the sensation that the joint was bending, even though it was not.
With their hand touching their waist, the volunteers felt their wrists bending into their body, creating the illusion that their waists were shrinking.
During the experiment, all of the volunteers reported that they felt as though their waist had shrunk by up to 28%.
The brain scans revealed increased activity in the posterior parietal cortex at the same time.
This brain region helps interpret the sensory information coming from different parts of the body.
The volunteers who reported the strongest shrinking sensation also showed the strongest activity in this area of the brain.
Lead researcher Dr Henrick Ehrsson said: "Unlike more elementary bodily senses such as limb movement, touch and pain, there are no specialized receptors in the body that send information to the brain about the size and shape of body parts.
"Instead, the brain appears to create a map of the body by integrating signals from the relevant body parts such as skin, joints and muscles, along with visual cues."
He said other studies have shown that people with injuries in the parietal cortex area of the brain experience the feeling that the size and shape of their body parts have changed.
For example, people who suffer from migraine with aura can sometimes experience a phenomenon called the "Alice in Wonderland syndrome", where they feel that various body parts are shrinking.
This might also be linked to the parietal cortex, said Dr Ehrsson.
"People with anorexia and body dysmorphic disorder who have problems with judging the size of their body might similarly have a distorted representation of their body image in the parietal cortex.
"These are areas which would be worth exploring in future research," he said.
A spokesman from the Eating Disorders Association said: "Anything that helps us understand these issues is welcome."