"All these twins did not breast-feed without being pregnant and pregnancy has a negative effect on breast appearance," he said. "My explanation is that women who breast fed have a different hormonal milieu -- sort of like internal hormone replacement. So even though those were disadvantages, they gained some benefit."
Soltanian, who does reconstructive surgeries for women after breast cancer, said this twin research could be expanded to longitudinal studies that look for environmental influences when one twin has cancer and the other doesn't.
As for the study's importance, he said, "It's obvious to me that breast appearance and breast health as a whole are a major part of female health."
But according to Judy Kuriansky, a sex therapist from Columbia University, there is no discounting the psychological importance of breast appearance to women.
Breasts carry such anthropological importance, she said. "Like the hip size, it's about being fertile."
"Women are obsessed with their breasts at all ages, because men are obsessed with their breasts," she said. "No matter how liberated women feel about their bodies, their breast size is so tied up in their sexuality and self-image."