March 28, 2011

Do people lie to pollsters about their physical characteristics?

Q: People fudge details of their appearance for personal ads and dating websites. Are they honest about their physical characteristics in surveys?

Apparently not. Whether they are consciously lying is unclear — they may be trying to fool themselves as well as survey researchers — but it appears that many people underestimate their weight when answering questions over the phone. Multiple studies by health researchers have found, for example, that the average respondent to a telephone survey reports a lower weight than the average respondent to a face-to-face survey in which the person is weighed on a scale. The average phone respondent also reports being taller than the average person who participates in a study involving direct measurements of height. Using the standard Body Mass Index (BMI) classification, phone respondents are less likely to be categorized as obese or overweight than are people measured in person.

The Pew Research Center described such underestimates and overestimates of physical characteristics in its April 2006 study of weight problems. The average woman in our phone survey reported a weight five pounds lower than the average woman’s directly measured weight in a U.S. government health study. The average man in our phone survey claimed to be two inches taller than the average man directly measured in the government study. Consequently, when we fed our respondents’ information into a BMI calculator, we found that only 19% were classified as obese, while 31% of respondents in the government study were classified as obese.

The prevalence and consequences of obesity are important public health issues. Because of the discrepancies between self-reports and direct measures of weight and height, governments often choose to study such characteristics directly rather than relying on self-reports, which are less expensive to collect.

You can find more information about these measurement differences in this review article, although a subscription or a one-time fee is required in order to access it.

Conrad Hackett, Demographer, Pew Forum on Religion & Public Life