Demographic transition considers both birth rate and death rate and refused to the transition from high birth and death rates to low birth and death rates which a country develops when it moves from a pre-industrial to an industrial system of economy. This is demonstrated through a demographic transition model or DTM. The theory behind this is based on an interpretation of demographic history developed in 1929 by demographer Warren Thompson. It was Thompson who observed transitions in both the birth and death rates in industrialized societies over a course of 200 years. Accordingly, most developed countries are in stage 3 or 4 of the model. Most developing countries have reached stage 2 or 3. The exceptions are chiefly in poor sub-Saharan Africa as well as some Middle Eastern countries. These poor countries are typically affected because of government policy as well as civil strife.