Couples happy in married life are more likely to put on weight than their unhappy counterparts, according to a new study.
Couples who were less satisfied in their relationship tended to stay trim, as well as having more thoughts of leaving their partner.
Scientists say this suggests the primary motivation for staying slim is the desire to attract another mate.
Researchers at Southern Methodist University in the US carried out a survey of 169 newly married couples.
Twice a year, for four years, the participants reported their marital satisfaction and their height and weight.
For each unit rise in marriage satisfaction, both men and women gained one tenth of a BMI unit - a measure whether you're a healthy weight for your height - every six months.
This worked out at around a pound a year for a 5ft 4ins tall women weighing in at eight-and-a-half stone.
Lead researcher, Andrea L. Meltzer, said that while weight gain may seem small in the short-term, over a long period of time it could have a serious effect on health.
She said: "If you take one of those happy marriages that go on for 20, 30, or 40 years, it could potentially become unhealthy."
The study concludes that happy couples should focus on the health benefits of staying slim rather than the benefits to their appearance.
Meltzer added: "We know that weight gain can be associated with a variety of negative health consequences, for example diabetes and cardiovascular disease.
"By focusing more on weight in terms of health implications as opposed to appearance implications, satisfied couples may be able to avoid potentially unhealthy weight gain over time in their marriages."
Staying at a healthy BMI by eating well and exercising regularly is just one way you could lower your health insurance and life insurance premiums.
Calculate your BMI
You can calculate your BMI by dividing your weight in pounds by your height in inches squared.
Calculate your BMI using the calculator on the NHS website.