Decisions about breast cancer screening just got tougher for women in their forties. On Monday, the U.S. Preventive Services Taskforce (USPSTF), a government-sponsored panel of clinicians and preventive medicine experts, released new guidelines that for the first time recommend against annual mammography for "normal risk" women in that age group.

Instead, it recommends they have the test every two years. Sounds pretty straightforward, right? The problem is that the recommendation conflicts with those of almost every other major U.S. health organization. They include the the American Cancer Society, which is strongly critical of the USPSTF. Why the disagreement?