As we put on weight and our bodies become larger, it becomes increasingly harder for us — and for others — to tell visually if we’ve gained or lost weight, according to a recent study published in the British Journal of Health Psychology.

The study also found that we tend to significantly underestimate the weight of people who are very heavy.

This perceptual bias undermines our efforts to maintain a healthy weight, say the study’s authors.

“It is common knowledge that obesity levels in the West are rapidly rising and that people fail to recognize weight gain,” they write. “What has not been widely recognized before is that there are sound perceptual reasons for this failure.”

A two-part study

The study, which was conducted by researchers at Northumbria University and Newcastle University in the U.K., involved two experiments. In the first experiment, 29 women (staff and students at the British universities) were asked to estimate the weight of 120 women shown to them in photographs. The women in the photos had varying body sizes, and weighed from 62 to 230 pounds.

The researchers found that when the weight of the women in the photographs was below the population average for white women in the United Kingdom (about 154 pounds), the study’s participants tended to overestimate how much they weighed. The reverse was also true: When the weight of the women in the photographs was above that average, the participants tended to underestimate it — by as much as 10 percent for women at the higher end of the weight range.

In the second study, 28 different women were asked to determine if women shown in a photograph had the same or different body mass as that shown in a paired 3D computer-generated image of a female body. The researchers found that when the women in the photographs had a high body mass, the difference between it and the one in the paired computer-generated image had to be greater for the study’s participants to spot a difference.

These findings support two visual perception biases — contraction bias and Weber’s law — which have been previously ignored by people studying obesity, according to the current study’s authors.

“Contraction bias predicts that the weight of obese bodies will be underestimated and the degree of underestimation will increase as body mass index (BMI) increases,” they explain. “Weber’s law predicts that change in the body size will become progressively harder to detect as their BMI increases.”

Limitations and implications

This study had several limitations. Most notably, it involved only a small number of participants, and those participants were all women who either attended or worked at a British university. Other population groups might produce different findings.

Still, the findings offer a warning to people who are attempting to lose weight — or who simply want to maintain a healthy weight: You can’t rely just on visual cues.

“As people’s weight increases an observer will increasingly underestimate their body size,” the study’s authors write. “This may explain the discrepancy in the proportion of patients being reported as being overweight or obese relative to the proportion in the general population. This may also be a reason why parents do not seem to recognize their children are overweight and that they are getting heavier.”

If you want to maintain a healthy weight, you’ll need some objective data — and that means stepping on the bathroom scale regularly.

FMI: Unfortunately, the full study is behind a paywall, but you’ll find an abstract on the British Journal of Health Psychology website.