NEW HAVEN, Conn. (AP) - Women who breast fed their babies for two years or longer reduced their risk of breast cancer by 50 percent, a study of women in rural China found.

The study by Yale University researchers bolsters other studies in China and in the United States, and finds that the benefits are similar whether the cancer strikes before or after menopause.

Researchers said they hope the study will change cultural attitudes and encourage more American women to breast-feed for longer periods of time.

On average, less than one-third of American women continue breast-feeding until their babies are 6 months old, government statistics show.

A tiny percentage of American women continue to nurse until the child is 2 years old, but in China and many other developing countries, breast-feeding for two years or more is the norm.

Researcher Tongzhang Zheng said that women cannot control many risk factors for breast cancer, such as family history and environmental factors.

"It's one factor people may be able to change," said Zheng, an associate professor of epidemiology and public health at the Yale School of Medicine.

The study, published in the American Journal of Epidemiology, looked at the medical history of 808 Chinese women in rural Shandong province from 1997 to 1999. Half of the women - who ranged in age from 30 to 80 - had had breast cancer, half had not.

The number of babies a woman breast-fed and her age at her first breast-feeding did not appear to be factors in breast cancer risk, Zheng said.

The study did not explore reasons why breast-feeding lowers the risk of breast cancer. One theory is that breast-feeding reduces exposure to estrogen and the regular female hormone cycles, Zheng said. Another theory is that fat-soluble carcinogens and pollutants are not stored as much in lactating breasts.

The study is compatible with other studies done over the years in other countries, said Dr. Ruth A. Lawrence, who teaches pediatrics at the University of Rochester.

"It's extremely valuable to do these studies and confirm studies that have been done earlier and in different populations," Lawrence said.

She said the study was particularly interesting because it showed a reduced risk for post-menopausal women, since other studies had focused on benefits for younger women.

Research has long shown that breast-feeding also is good for babies, providing better nutrition, strengthened immune systems and fewer illnesses than a formula diet.

The American Academy of Pediatrics recommends breast-feeding for the first year of life. UNICEF and the World Health Organization recommend breast-feeding at least until age 2, supplemented with other foods.

Still, a stigma persists in the United States that discourages breast-feeding.

Women who try to breast feed are subjected to leers, rude remarks and criticism, said Katherine A. Dettwyler, an associate professor of anthropology and nutrition at Texas A&M University who has studied long-term breast-feeding.

"As long as our culture views breasts as sex objects, it's going to be difficult for women to breast-feed and have real lives," Dettwyler said. "In most of the world, breasts are just for feeding babies."

La Leche League, a breast-feeding advocacy group, encourages mothers to breast-feed for as long as the mother and baby want.

Copyright 2001 The Associated Press. All rights reserved.