Men and women both find thinness in females attractive because they equate it with youth, fertility and a lower risk of diseases, a new study has found.
In the study, more than 1,300 people, both males and females, from the UK and nine other countries, rated the very thinnest female images with body mass index (BMI) of around 19 as most attractive.
Scientists from the University of Aberdeen have been working as part of an international collaboration co-ordinated by the Institute of Genetics and Developmental Biology at the Chinese Academy of Sciences in Beijing to try and discover why there is a link between body fatness and perceived physical attractiveness.
"What we wanted to investigate was the idea that when we look at someone and think they are physically attractive, are we actually making that assessment based on a hard-wired evolutionary understanding of their potential for future survival and reproductive ability?" said Professor John Speakman from the Chinese Academy of Sciences and the Institute of Biological and Environmental Sciences at the University of Aberdeen in Scotland.
To test their idea, the scientists built a mathematical model which combined the relationships between levels of obesity and the future risk of mortality from all causes, and the relationship between obesity and the future possibility of having children.
This model predicted that people would perceive females with a body mass index (BMI) of between 24 and 24.8 as being the most physically attractive.
The researchers tested the prediction on study participants. They 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.
When subjects were asked how old they thought the people in the images were, they judged that fatter people were older.
Age is itself a strong indicator of evolutionary fitness. When the age factor was included into the model the optimum fatness fell to a BMI somewhere between 17 and 20 - corresponding exactly to the images people found to be most attractive.
This suggests that we find thinness in females so attractive because we equate it with youth - a BMI of 17-20 corresponds to the average BMI of a young 18-20 year old with maximal fertility and minimal risk of future disease, researchers said.
"Although most people will not be surprised that extreme thinness was perceived as the most attractive body type, since this prevails so heavily in media, culture and fashion, the important advance is that now we have an evolutionary understanding of why this is the case," said Dr Lobke Vaanholt, who organised the University of Aberdeen component of the study.
