Parents tend to be just a bit biased about their children’s looks (not me though—my kids are objectively beautiful), but as it turns out, this type of self-deception is not as benign as one might think. According to recent research, many parents appear to suffer from a sort of denial concerning their kids’ weights, which poses a considerable obstacle to remediating childhood obesity by way of promoting healthy eating habits at home.

The latest of such studies was published last month in the American Journal of Human Biology, and conducted by a team of scientists at the University of Coimbra in Portugal.

Daniela Rodrigues and her collaborators, Aristides Machado-Rodrigues and Cristina Padez, recruited hundreds of parents and children for their research. All the participating children were between 6 and 10 years old and attended elementary school in Portugal.   

A total of 834 parents completed questionnaires that included a variety of questions, such as whether they thought that their children’s weight was a bit too little, a bit too much, way too much, or just fine.  In turn, the team collected the weights and heights of the 793 participating children, at their respective schools.

The results were in line with the researchers’ predictions, but nonetheless remarkable. Of the 33% parents who misperceived their children’s weight, 93% underestimated it. Moreover, parents who underestimated their kids’ weights were 10 to 20 times more likely to have an obese child.

Several factors were associated with the parental weight underestimation, including a higher BMI (body mass index) for the mothers, younger ages for the children, lower household income (for girls) and urban living (for boys). However, such associations did not explain why parents underestimated their children’s weights to begin with.

The researchers speculated that parents might “feel embarrassed to discuss the fact that their child is overweight/obese,” which in turn could make them less likely to make proactive efforts, such as seeking the advice of a health care professional. Alternatively, parental embarrassment might not be the main reason for the underestimates, if parents truly misperceive their children’s weight.

Supporting the latter possibility is the fact that parents underestimated the weight of younger children more than that of older children. A 2014 study found that mothers misjudge the heights of their youngest children as smaller than they are, by an average 7.5 cm. The explanation for the height misperception, the researchers proposed, could be adaptive: underestimating the height of the youngest child could lead to greater parental care and allocation of resources to the baby of the family.

It seems plausible that a similar adaptive effect might be at play, not just for one’s children’s heights, but also for their weights. If so, parents who misperceive their children (especially the youngest ones) as thinner than they are could engage in greater efforts to feed them, and thus unwittingly elevate their risk of obesity.      

Future research might disentangle the various possible contributors to this puzzling phenomenon of parental misjudgment. In the meantime, and where your kids’ weight is concerned, trust not your eyes, but the scale.