The graph below is taken from the wikipedia page on the epidemiologic transition. Wikipedia's job: cool pictures, my job: explain what the heck they mean!
Phase 1 refers to pre-industrial society when both birth and death rates were very high, but both about equal. Couples had more children than today's standard, but lack of safety in the workplace, war, and infectious disease kept the population stable.
Phase 2: death rates fall because of advancement in public health (water sanitation, vaccination, etc) and medicine. Birth rates remain relatively high which causes the population to increase. This represents the situation in many developing countries today.
Phase 3: Death rates continue to decrease at a steady rate, but birth rates drop dramatically because of access to contraception, urbanization, and increases in household income.
Phase 4: Low birth rates and low death rates, both remain relatively equal. This represents the situation in some developed countries. (ie/ Japan). The population growth begins to level off as a result of ~equal birth and death rates.
Phase 5: sub-replacement fertility. This is the case here in Canada, where birth rates are below death rates. This is attributed to first and second-wave feminism and the associated increase in the rights of women (such as in the workplace), further increases in the use and variety of contraception, and societal pressures. With a shift to chronic diseases as the #1 killers, the death rate remains stable dispite a marked decrease in infectious disease. In theory, population growth declines, however, due to higher levels of immigration from developing countries, their population growth continues to rise (not shown here).
This theory was originally posited by Abdel R Omran in 1971.And now you know! :) Are there any other reasons for the trends that might be observed? Are the trends a predictor of birth/death rates to come...and what does stage 6 look like?