Many of us are shorter and heavier than we think we are, according to a study that compared people’s self-reported height and weight with their actual measurements.
James Hodge and his colleagues at the American Cancer Society in Atlanta, Georgia, looked at how well people know their own measurements.
Many studies investigating the relationship between body size and disease risk tend to rely on people’s self-reported height and weight. This can lead to unreliable data and flawed results. Instead, in this study, the researchers asked 2643 people in the general US population aged between 30 and 65 to give their height and weight. A certified biometric technician then took their actual measurements.
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On average, men said they were 0.48 centimetres taller and 1.54 kilograms lighter than they really were. Women, on average, said they were 0.16 centimetres taller and 0.88 kilograms lighter than their actual measurements.
Age effect
This may be because people tend to naturally shrink and gain weight as they age, and often don’t know their current measurements, says Ian Stephen at Macquarie University in Australia.
“When you’re 25, you learn, ‘Oh, I’m 182 centimetres and 70 kilograms.’ Then, by the time you’re 60, that’s no longer the case, but that’s still what you tell everyone,” he says.
Another reason may be that body measurements can vary between instruments or on different days. Also, “there’s an incentive for people to remember the measurement they’re most pleased with”, says Kevin Brooks at Macquarie University.
The study found that the shortest people were most likely to over-report their height and the heaviest people were most likely to under-report their weight. This may be due to a psychological phenomenon called the social desirability bias.
This effect motivates people to portray themselves in the most favourable light, says Brooks. “Being taller and lighter are generally seen as more ideal, so the further you are from that ideal, the larger your reporting error might be,” he says.
The social desirability bias may also explain why men over-reported their height more than women, since tallness tends to be more valued in men, says Brooks. Men may have under-reported their weight more than women because more of them were obese, meaning they may have felt more compelled to change the numbers slightly, he says.
PLoS One
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