Tuesday, 8 March 2016

The bigger you get, the harder it is to tell whether you've gained or lost weight

When your waistband feels tighter than usual, or the scales say you've put on a few pounds, it's easy to blame the news on clothes shrinkage or an uneven carpet, especially if your body looks just the same in the mirror. And that lack of visual evidence for weight gain (or loss) is especially a problem for more obese people, according to a new paper in the British Journal of Health Psychology.

The researchers asked female participants to estimate the weight of 120 differently sized women (their weights ranged from 28.2 to 104.9 kg; roughly 4.5 to 16.5 stones). The heavier the women in the photos were, the more the participants tended to underestimate their weight – on average, "an observer who judges the weight of a 100 kg woman will underestimate her weight by ~10 kg" the researchers said.

In a second study, participants had to judge whether pairs of real or CGI women had the same or a different BMI (body mass index). When the women in the pictures had a higher BMI, the difference in their respective BMIs had to be greater for participants to notice a difference (this is actually an example of a basic perceptual phenomenon known as Weber's law).

"Our results clearly point to the potential for perceptual factors contributing to problems with detecting obesity and weight increase," concluded the researchers led by Katri Cornelissen at Northumbria University. As people get heavier they will find it more difficult to detect extra weight gain, and conversely they will also struggle to detect when they have lost weight, which may undermine dieting efforts.

--Visual biases in judging body weight

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

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