People often choose partners with similar body fatness to their own, according to new research.
Researchers said people look for partners with similar fat levels
The Aberdeen study involving 42 couples said people often married within their own social class as well as taking looks, height and race into account.
However, they have now found those with about the same amount of fat are likely to be attracted to each other.
Scientists said this "assortive mating" could be contributing to a worldwide obesity epidemic.
This, they argue, is because children who have an overweight mother and father could be more susceptible to putting on weight.
Scientists based at Aberdeen's Rowett Research Institute and Aberdeen University measured the body composition of 42 couples.
The results found the amount of body fat in one person was proportionately very similar to that of their partner.
Prof John Speakman, of Aberdeen University, said "What is currently unclear is how these associations come about.
"Perhaps the social activities of the overweight and obese people coincide, making them more likely to meet partners who are also overweight and obese."
He said assortive mating for body fat was relatively new as in the 1940s and 50s people got married in their early 20s, often before they were overweight or obese.
"It would have been difficult for them to assortively mate for body fatness because it would be impossible to distinguish somebody who was thin from somebody who was thin but going to become fat," Prof Speakman said.
"Nowadays, we choose partners and have children much later, but if we are going to become obese, on average we do so much younger.
"This makes it possible for potential partners to select each other on the basis of body fatness."
The research is published in the American Journal of Clinical Nutrition on Wednesday.