no evidence to suggest that abortion would result in a higher incidence of carcinogenesis. A more thorough examination of the phenomenon was conducted in 1982, confirming the results. A later study in 1987 further explained their previous findings. After differentiation of the mammary gland resulting from a full-term pregnancy of the rat, the rate of cell division decreases and the cell cycle length increases, allowing more time for DNA repair. Despite the fact that the Russos' studies found similar risk rates between virgin and pregnancy interrupted rats, their research would be used to support the contention that abortion created a greater risk of breast cancer for the next twenty years. However, because rats do not exhibit naturally occurring breast cancer, the extrapolation of these results to human abortion and breast cancer is viewed as dubious. Epidemiological evidence The results of prospective cohort studies on the relationship between abortion and breast cancer have been consistently negative. Such studies are considered more reliable than retrospective studies and case-control studies. The positive association between abortion and breast cancer risk observed in case-control studies may be accounted for by recall bias. In 1996, Brind et al. published a meta-analysis of 23 studies which reported a positive association existed between induced abortion and breast cancer risk. The authors estimated the relative risk of breast cancer among women who had had an induced abortion to be 1.3, compared to women who had not had an abortion. It was criticized by other researchers for multiple reasons, including allegations that it failed to account for publication bias (positive studies tend to be more likely to be published). The meta-analysis was also criticized because the studies it included were almost all case-control studies, which are susceptible to recall bias, and for which it is difficult to select an appropriate control group. In 1997, Wingo et al. reviewed 32 studies on the abortion-breast cancer relationship and concluded that the results of studies on this subject were too inconsistent to allow for definitive conclusions, for either induced or spontaneous abortions. A 2004 analysis of data from 53 studies involving 83,000 women with breast cancer reported no increased risk among women who had had either an induced or spontaneous abortion. The relative risk of breast cancer for women who had a spontaneous abortion in this analysis was 0.98, and that for induced abortion was 0.93. A 2015 systematic review and meta-analysis of