Rouillon should know: Since 1997, he has been studying the breasts of 330 volunteers ages 18 to 35 using a slide ruler and a caliper to record the changes in breast position as the women aged, CBSNews.com reports.
His meticulous research revealed that women could experience about a 1/4-inch (7-millimeter) lift in the nipples each year they didn't wear a bra, news site Counsel & Heal reports.
The researchers involved in the study suggested that bras — which Rouillon now calls "a false necessity"— discourage the growth of supporting breast tissue, leaving the breasts to sag more quickly.
Other research has challenged the conventional wisdom that breast-feeding causes breast ptosis (sagging). A 2007 study of 132 women revealed that breast-feeding had no effect on the degree of breast sagging.
What did affect breast ptosis, that research revealed, was age, smoking status and the number of times a woman had become pregnant.
Because Rouillon's study focused on younger women, he cautioned that the results can't be generalized to all women.
"It would be dangerous to advise all women to stop wearing their soutien-gorge [bra] as the women involved were not a representative sample of the population," he said, as quoted in French news site The Connexion.
"It would be of no benefit to a 45-year-old mother to stop wearing a bra," Rouillon said.
One of the participants in the study extolled the virtues of going bra-free: "There are multiple benefits: I breathe more easily, I carry myself better and I have less back pain," she told news site France Info.