The model, created by scientists from the University of Aberdeen in Britain based on evolutionary theories, first predicted that men would perceive women with a body mass index (BMI) of between 24 and 24.8 as being the most physically attractive and healthy.
They tested the prediction on more than 1,300 people, both males and females, from Britain and nine other countries.
Participants were shown 21 image cards showing females with different levels of body fatness and were asked to reorder them from least to most attractive.
In all the populations, males and females rated physical attractiveness of the female images very similarly.
The very thinnest images with body mass index of around 19 were rated as most attractive.
As fatness increased above that value, the less attractive they were rated.
This, however, contrasted the predictions of the mathematical “fitness” model that there should be a peak in attractiveness around a BMI of 24 to 24.8.
When the age factor was included into the model, the optimum fatness fell to a BMI somewhere between 17 and 20.
“This suggests that we find thinness in females so attractive because we equate it with youth,” said lead researcher professor John Speakman from the Institute of Genetics and Developmental Biology.
A BMI of 17-20 corresponds to the average BMI of a young 18-20 years old with maximal fertility and minimal risk of future disease.
This was consistent across European, African and Asian test groups.
Fitness in evolutionary terms comprises two things: survival and the ability to reproduce.
One idea about how we rate physical attractiveness is based on the impact that different aspects of our bodies (like body fatness) have on evolutionary fitness.
“For example, we know that above a certain body fatness females have greater risks of chronic diseases like diabetes and heart disease and lower fertility which might make fatter subjects less attractive,” noted Speakman.
In the past, fatter people might have had greater abilities to survive famines, making fatness more attractive.
“This might suggest there is an optimum level of fatness that is maximally attractive which is somewhere in between,” Speakman noted.
Although most people will not be surprised that extreme thinness was perceived as the most attractive body type, “the important advance is that now we have an evolutionary understanding of why this is the case,” said Lobke Vaanholt from the University of Aberdeen.
The study, funded by the National Science Foundation of China and involvoing researchers from 10 institutions in the world, was published in the journal PeerJ.