With all of the New Year's diet ads claiming you can lose dozens of pounds in seemingly as many days, you probably are not alone if you looked in the mirror this morning and saw a less than ideal body. Or maybe you just picked up a new magazine in which already thin models have their remaining flesh scavenged by Photoshop to make them appear even slimmer. With all of these unrealistic promises and images, it can be hard to gain an accurate sense of one's own body. But the disjunction for some people might go deeper than manipulated photos.

A new study shows that the way people perceive their external appearance is likely linked to how they experience their bodies internally. Researchers found that people who had greater difficulties sensing their own internal bodily states were also more likely to be fooled into believing a rubber hand was part of their own bodies. These results, published online in the January 5 issue of Proceedings of the Royal Society B: Biological Sciences, may one day help scientists understand how body image can become so distorted in disorders like body dysmorphia and anorexia nervosa, says lead author Manos Tsakiris of Royal Holloway, University of London.

"The sense of self is built up from a representation of internal states," says Hugo Critchley, a professor of psychiatry at the University of Sussex in England who was not involved with the study. "This paper is showing that sensitivity of individuals to their internal state predicts the strength of their self-representation."

Most of the time, the image someone has of their body is pretty close to its external appearance. You may see your thighs as slightly bigger than they actually are, or your arm muscles as slightly smaller, but the discrepancy is usually minimal. In some mental disorders, however, body image can become dramatically distorted. Those who suffer from body dysmorphic disorder think that parts of their bodies are malformed or grotesque, even when these supposed flaws are not noticeable to others. In eating disorders such as anorexia nervosa patients continue to think they need to lose weight even as their bodies waste away.

Crucial to the formation of body image—pathological and otherwise—is the integration of external and internal cues. What we see in the mirror and what we feel against our skin melds with our own internal awareness of our bodies to create an overarching body image. Scientists have historically focused on how external factors like magazines and fashion models affect the creation of an accurate body image. Tsakiris and his colleagues, however, hypothesized that a person's internal awareness of his or her body, known as interoceptive awareness, was also related to the creation of an accurate body image.

So the researchers tested interoceptive awareness in a group of 46 female university students by asking them to count their heartbeats without taking their own pulses. The researchers also recorded the subjects' actual heartbeats. The manual counts were then compared with the recorded beats. For the analysis Tsakiris and his colleagues divided the students into two different groups: those with high interoceptive awareness (whose counted heartbeats were, on average, 80 percent accurate) and those with low interoceptive awareness (less than 50 percent accuracy).

After determining how well the students sensed their bodies from the inside the researchers then measured how well the students perceived their bodies externally. To do this, the researchers used a simple trick called the rubber hand illusion: A person places her hand on a table where it is hidden from view in a box or under a smock. A rubber hand is placed next to the hidden hand but within plain sight. The researcher then gently strokes both the hidden and rubber hands with a paintbrush. After about 30 seconds of stroking a person often begins to think that the rubber hand is actually their own hand. As the illusion persists the temperature of the hidden hand drops, indicating that the body is "forgetting" about that hand and adopting the rubber hand as the real thing.

Those female students with low interoceptive awareness were more likely to endorse the statement "I felt the rubber hand was my hand." The hand temperature in this group also dropped by 0.75 degree Celsius, whereas hand temperature remained relatively unchanged in the high interoceptive awareness group. Being more easily fooled by this illusion, the authors said, indicated that a person's body image was more malleable.

"People with low interoceptive awareness might have a less strict distinction between what is 'my body' and what is the external world," Tsakiris says. "They might be ruled more by vision, rather than by internal sensation." Previous studies have also found that people with anorexia have an impaired ability to sense internal cues, making the results of this new work useful for understanding and potentially treating severe body-image disturbance.

To be sure, the study used a small sample size and has not yet proved causation, but "if we can train people to sense their interoceptive states," Tsakiris says, "it might make a change in their body image. An interesting avenue for future research would be to see if improving interoceptive awareness impacts different areas of these disorders."