Breast cancer and abortion

Orac's Respectful Insolence has a good post on the non-relationship between breast cancer and abortion:

"Although there are studies that claim a link between abortion and breast cancer, they are almost all weaker case control studies, which are prone to recall bias. It's been shown that healthy women are less likely to reveal that they have had an abortion to an interviewer, while women with cancer are more likely to do so, primarily because they are searching for causes of their cancer. (This is not unlike the problem with Generation Rescue's dubious vaccination phone survey.) Nearly all the better designed prospective studies have found no link. Indeed, now there are numerous studies that have failed to find a link between breast cancer and abortion. Given the preponderance of evidence, although it is still possible that there may be a link between abortion and breast cancer, it is highly unlikely that there is. In this, it is not unlike the state of evidence regarding vaccines and autism. Current evidence
does not support such a link, and there are enough studies to allow us to conclude that there probably is none. That's as good as it gets in epidemiological studies."

Note that a case control study takes a bunch of women with breast cancer and a matched (for age etc.) group of women without it and ask if they have ever had an abortion. As Orac says, there is a risk of recall bias. A prospective study follows a group of women through their lives so abortions are recorded before breast cancer has developed, preventing recall bias. Case control studies are a good way to generate hypotheses, and the hypothesis that abortions increase breast cancer risk was plausible, it is just that the evidence did not bear that out.