In the latest analysis of worldwide data from the World Health Organization, researchers led by Cecile Pizot from the International Research institute in Lyon, France, found that in 39 of the 47 countries studied, breast cancer death rates have declined from the 1980s to 2013. The report was presented at the San Antonio Breast Cancer Symposium of the American Association for Cancer Research. In the U.S., for example, mortality dropped by 42% in that time. Encouragingly, mortality dropped more dramatically for women under age 50, but it’s not clear whether screening or treatments, or a combination of both, are responsible.

While it may make sense to assume that screening with mammography is contributing to early detection of cancer among younger women, Pizot notes that both New Zealand and Belgium had similar breast cancer mortality rates in the 1980s, and experienced similar declines in deaths. That’s despite the fact that New Zealand introduced widespread mammography screening in 1988 and Belgium didn’t do so until 2005. “The decline in mortality was the same irrespective of the introduction of mammography,” she says. “So our hypothesis is that screening mammography is not responsible for the decline in mortality.”

Other factors, including use of certain drug or other cancer treatments, may be at work. Pizot says that different health care systems, and differing strategies for managing cancer are also important to consider.

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