By Christian Jarrett

How accurately or not we are able to judge the size of our own bodies and specific body parts is an important topic in clinical psychology because a distorted body image is thought to play part in eating disorders, body dysmorphia and other related conditions. However, research has until now been limited in always involving one- or two-dimensional judgments, with volunteers asked to estimated the length of various body parts, for instance, or asked to judge which of various 2-dimensional visual depictions of their body is most accurate. In reality, of course, we don’t just have a sense of how our body looks in two dimensions from the outside but also how it feels from the inside, including how much space it occupies.

A new study published in Cortex is the first to examine how accurately people of healthy weight can estimate the volume of their entire body and specific body parts. Renata Sadibolova at Goldsmiths, University of London, and her colleagues write that “these findings … highlight the importance of studying the perceptual distortions ‘at the baseline’, i.e., in healthy population, given their potential to further elucidate the nature of perceptual distortions in clinical conditions.”

The researchers recruited 40 young men and women (average BMI 24) and asked them to wear a black smock to cover their bodies and to sit at a table. Here they were challenged to judge their bodies in one of two different ways. Half the participants made judgments of the length and volume of various parts of their body based on using their sense of the length and volume of their hand, such as estimating how many hand-lengths is their foot, their head, right arm, right leg, and entire body; and how many times could their hand hypothetically fit inside of those same body parts (thus giving an estimate of their sense of the volume of those parts).

The other half the participants made the same judgements but used as a reference a book (wrapped in brown paper to hide irrelevant details) and stick, chosen by the researchers to as closely as possible match the actual volume and length of each participant’s hand as determined using a water displacement technique. Participants in this group had to estimate how long their body parts (foot, head, arm, leg, torso, entire body) were in reference to the stick length, and to estimate the volume of those same parts using the bulk of the book as a reference point.

The researchers also went to great trouble to estimate the true length and volume of the participants’ various body parts. This included using water displacement techniques and tape measures to record the length and circumference of various body parts, and then – guided by research on the proportional dimensions of cadavers – extrapolating from these measures to obtain estimates of volume.

Overall the results revealed an intriguing picture in which participants tended to overestimate the length of their various body parts, but to underestimate their volume. Moreover, there was an apparent association with the surface area of body parts, such that the distortion (whether underestimation of volume or overestimation of length) was greater for parts with less surface area. These distortions may partly arise, the researchers suggested, because we are much less aware of signals arising from within the body compared from signals from the outside, thus leading our volume and length judgments to be skewed in contrasting ways by the amount of outward-facing skin on a given body part (an overcompensation in the case of length, and a lack of compensation for volume).

“The patterns of misperception across body parts thus gave rise to proportionally distorted body shapes, that are similar to a well-known depiction of a somatosensory homunculus and a tall bean-pole respectively,” the researchers said (see images above).

Note that there were some judgment differences based on whether the participants used their hand size as a reference point or the objects (the book or the stick), with some of the length overestimates being greater for those who used their hand as a unit of measurement. However, the general pattern of results was the same regardless of the measurement reference used.

The researchers concluded “Our findings add to a growing evidence that healthy adults do not have a ‘highly accurate–if not infallible’ representation of their body size as previously assumed, and that their perceptual errors may be determined by the role of body parts in external signal processing.”

—Distortions of perceived volume and length of body parts

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest