• ethics
• science

Debate over whether mammography screening saves lives has become front page news again in recent months. The opening salvo came in October 2001 from Gotzsche and Olsen, who argued in the Lancet that five of the seven randomised trials of screening mammography were of poor or flawed quality and that the remaining two fail to show that the benefits outweigh the risks.¹ In the US, scientific experts remain divided in their recommendations regarding the benefits of mammography for women in their 40s, although the practice has been strongly defended by political leaders.² This controversy, like many in the realm of disease prevention, raises an important question: when science is uncertain, what else drives an individual investigator’s interpretation of the evidence?

In the midst of a public debate over prevention recommendations, it often seems that even in the best of circumstances (that is, when there is a randomised clinical trial) there is little objective knowledge to be found. Indeed, prevention science is frequently charged with being subjective and value laden. The investigator’s scientific judgment may be polluted by influences other than the evidence itself. The list of suspected polluters is long, including commitment to a favourite theory, type of training (physician, epidemiologist, biologist), personal habits (smoking, diet), concern about the financial costs of screening, and attitudes towards balancing …