Perhaps Coale's most major scientific contribution was to the understanding of the demographic transition. The Demographic Transition, as stated by Coale, occurs when a country develops a strong economy, and within the society, low fertility and mortality will start to reflect based upon the economic standpoint. Coale was the pioneer of the European Fertility Project, which examined the decline in marital fertility in Europe. The European Fertility Project's goal was to correlate the fertility rates within married couples with the rate of infant mortality. Coale established three pre-conditions to fertility decline. The first is "within the conscious of choice." In other words, it is up to the individual and within their own decision whether or not to have children. The second is that if a society sees not having children as advantageous, then fertility will decline. The third pre-condition is to have contraception methods ready. It is important to emphasize that a society will start to show signs of fertility decline if these three pre-conditions are met.