Body perception index: benefits, pitfalls, ideas

Abstract

Estimates of body size are often expressed as a ratio of actual size [body perception index or BPI=(estimated size/actual size)×100%]. In this article, we examine the possibility that overestimation of body size in patients with anorexia nervosa, as measured by the BPI, is due to their smaller body size rather than to their being anorexic. Using 50 mean body sizes derived from seven studies we investigated whether the error of estimation is a constant proportion of the body size to be estimated, as the use of the BPI assumes. A negative linear relation between BPI and actual body size was found, confirming that smaller size is associated with greater overestimation. However, although both groups showed a strong tendency to overestimate smaller sizes, anorexic subjects showed even greater overestimation than controls. Hence, overestimation of body size in AN can only partially be accounted for by the smaller body size of anorexic patients. Recommendations for future use of the BPI are put forward.

Introduction

On average, patients with anorexia nervosa (AN) estimate their body size to be greater than it actually is. This was recently established in a statistical meta-analysis of 33 studies comparing body size estimates from patients with anorexia nervosa (AN) to controls without an eating disorder [1]. This finding implies that these patients have a visual mental image of their own body as fatter than it really is, which is part of the more general disturbance of body image—a diagnostic marker of AN [2].

An alternative explanation for the overestimation of body size by patients with AN, however, might be that it is an artifact of their being thin rather than a consequence of their being anorexic. According to this explanation, the habit of displaying the body size estimate as a fraction of actual size puts anorexic subjects—who, by definition, have smaller actual sizes than normal-weight controls—at a disadvantage. The ratio of estimated size to actual size (×100%) is known as the body perception index (BPI; see Slade and Russell [3]). The BPI is typically used in so-called body part studies, which are directed at estimation of actual body widths.

Using the BPI, Ben-Tovim et al. [4]made the curious discovery that mean BPI was lower in anorectics than in controls. This, in combination with the fact that the controls in their study had smaller body widths than the AN patients, prompted them to investigate the relationship between BPI and actual width. They found a significant tendency for BPI to increase with actual width: the smaller the body part, the greater its percentage overestimation. When this tendency was taken into account, no statistical difference emerged between the anorexic and control group. Thompson [5]subsequently showed that a significant difference between anorexic subjects and controls, apparent when the BPI was used in the traditional manner, became insignificant when the actual measures of the controls were used as the denominator for the anorexic group in the absence of data on their own premorbid body sizes.

On the basis of the results obtained in a follow-up study to further investigate this phenomenon, Ben-Tovim and Crisp [6]claimed that estimates are random in relation to actual body sizes. Consequently, the smaller body sizes that are likely to be found in individuals with AN will increase the apparent size of perceptual error, as a function of dividing a random number by a relatively smaller number.

The above findings urge a more cautious approach to applying the BPI in populations differing in weight and size. On the basis of the conclusion from the meta-analysis that anorexic patients, on average, overestimate their body sizes as compared to normal controls, we now investigate whether anorectics overestimate because they are anorexic, or merely because of their smaller actual sizes. On the basis of the results, we wish to formulate specific recommendations for future applications of body size estimation methods.

It is appropriate first to highlight several assumptions that seem to have been made in applying the BPI, starting from the broader perspective of classical psychophysics. We will restate Ben-Tovim and Crisp's [6]claim that people with AN show stronger body size overestimation than the average-sized controls they tend to be compared to because of smaller actual body size, starting from Weber's law. Weber's law has generally been used in psychophysics to express the relation between the perceived and actual value of a stimulus, and seems to apply to this case. If person A's BPI is higher than person B's, this implies either that A's error of estimation in terms of centimeters is greater than B's, or that A's actual body size is smaller.¹ Comparing BPIs with each other when the denominators (i.e., actual body sizes) are unequal would seem to be justified by the additional assumption that smaller actual sizes are accompanied by smaller errors of estimation—or vice versa, that greater actual sizes may lead to proportionately greater errors. The assumption that error/actual size=k where k is a constant, is equivalent to Weber's law that δS=kS or δS/S=k: that is, the standard intensity of a stimulus (S) must be increased by a constant fraction (δS) to be distinguished (see Baird and Noma [8]). Correspondingly, in body size estimation studies an increase in body size is assumed to be accompanied by a proportional increase in estimation error.

Hence, in the present study we first investigate whether, in accordance with Weber's law, the BPI is constant over a range of body sizes, or whether there is an (artifactual) correlation with actual size. If, as Ben-Tovim and Crisp [6]put it, body size estimates are randomly related to actual sizes, the function relating BPI to actual size will be a negative linear one rather than a constant. Second, we investigate whether the difference in body size estimates between anorexic patients and controls remains after controlling for the influence of actual size on BPI. This study focuses on size estimation data cumulated over studies, using the method of statistical meta-analysis.