How much weight loss or gain makes a person appear attractive to others?
The researchers are led by Prof. Nicholas Rule, of the University of Toronto in Canada, and they publish their findings in the journal Social Psychological & Personality Science.
He and postdoctoral fellow Daniel Re specifically investigated facial adiposity, which is the perception of weight in the face, because it accurately indicates a person's body mass index (BMI).
"It is a robust indicator of one's health," says Prof. Rule. "Increased facial adiposity is associated with a compromised immune system, poor cardiovascular function, frequent respiratory infections and mortality."
He adds that "even a small decrease can improve one's health."
As such, the researchers created a collection of photos digitally, which included male and female faces between the ages of 20-40 years old.
Each of the photos contained subjects with neutral expressions, their hair pulled back and no facial ornamentation.
Women need to lose slightly less weight than men to appear 'attractive'
After altering each image to create a range of images that included gradually increasing weights, the researchers asked participants in the study to compare randomly drawn pairs of faces and to select the one that appeared heavier to them.
From their results, the team found that a change in BMI of approximately 1.33 kg/m2 (2.93 lbs/m2) is the magic number at which changes are noticeable.
Re explains that they tallied the weight change thresholds in relation to BMI, rather than kilograms or pounds, "so that people of all weights and heights can apply it to themselves according to their individual stature."
After their first investigation, the researchers then looked into the threshold at which alterations in a person's facial adiposity triggered a change in perceived attractiveness.
They found that the magic decrease in weight at which the faces appeared more attractive to the study participants was 2.38 kg/m2 (5.24 lbs/m2) for women and 2.59 kg/m2 (5.7 lbs/m2) for men.
For women and men of average height, this translates to about 6.3 and 8.2 kg (13.9 and 18.1 lbs), respectively.
Commenting on their findings, Prof. Rule says:
"Women and men of average height need to gain or lose about 3.5 and 4 kg, or about 8 and 9 lbs, respectively, for anyone to see it in their face, but they need to lose about twice as much for anyone to find them more attractive."
He says this difference between men and women indicates that the facial attractiveness of women may be more susceptible to changes in weight, meaning that "women attempting to lose weight need to shed slightly fewer pounds than men for people to find them more attractive."
The researchers conclude their study by noting that their findings "contribute to a greater basic understanding of the precision and limits of social perception and may provide information of value to medical practitioners and individuals seeking to manage changes in weight."
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