It is widely known that women tend to gain weight after giving birth, but now a large study has found evidence that even among childless women, those who live with a mate put on more pounds than those who live without one.

The differences, the scientists found, were stark.

After adjusting for other variables, the 10-year weight gain for an average 140-pound woman was 20 pounds if she had a baby and a partner, 15 if she had a partner but no baby, and only 11 pounds if she was childless with no partner. The number of women with a baby but no partner was too small to draw statistically significant conclusions.

There is no reason to believe that having a partner causes metabolic changes, so the weight gain among childless women with partners was almost surely caused by altered behavior. Moreover, there was a steady weight gain among all women over the 10 years of the study.

This does not explain the still larger weight gain in women who became pregnant. The lead author, Annette J. Dobson, a professor of biostatistics at the University of Queensland in Australia, suggested that physiological changes might be at work.