When we choose a partner for a relationship there may be many and complex reasons for our choice, but it has been known for many years that we tend to select people with characteristics similar to our own.
For instance, tall people tend to marry other tall people, and we tend to marry within our own social class, within our own educational class, and within our own race. Scientists call this assortative mating.
Aberdeen scientists have now revealed that we also choose partners with a similar body fatness to our own. This may be contributing to the obesity epidemic since it implies that overweight people will choose a partner who is also overweight, and so they will pass on to their children a double dose of the genes that have made them susceptible to being overweight or obese.
The study is published tomorrow, Wednesday August 8th, in the August 2007 issue of the prestigious American Journal of Clinical Nutrition, and was carried out by scientists based at the Rowett Research Institute and University of Aberdeen. They measured the body composition of forty-two couples using a sophisticated technique called dual-energy X-ray absorptiometry (DXA). Their results showed that the amount of body fat in one person was proportionately very similar to that of their partner's, suggesting that we assortatively mate for body fatness.
"The idea that two people in a relationship have similar amounts of body fat is not new since previous studies have shown a link between couples and their body mass indices (an indirect measure of fatness). It has also been suspected that BMI may be linked to other things that people choose their partner by, such as social class, or age," said Dr Diane Jackson from The Rowett Research Institute.
"In our study, we corrected the results for all these other factors and we measured body fat using DXA, which is a much more accurate and reliable method of assessment than BMI. We also showed that the assortative mating for body fatness was not linked to the length of time that a couple had lived together."
"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," said Professor John Speakman from The University of Aberdeen, who was also involved in the research with Dr Jackson.
For assortative mating for body fat to have contributed to the obesity epidemic it needs to have increased over time. The authors of the publication suggest that this has probably been the case.
"In the 1940s and 1950s people mostly got married in their early twentys before they were overweight or obese. So it would have been difficult for them to assortatively 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. 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," said Professor Speakman.