Demographic transition theory is a thesis linking population patterns to a society's level of technological development, suggesting that as societies industrialise and modernize, they go through a predictable sequence of changes in birth and death rates. The theory typically outlines four or five stages. In the first stage (pre-industrial), both birth rates and death rates are high, resulting in a relatively stable and low population size. In the second stage (early industrialisation), death rates begin to decline due to improvements in sanitation, healthcare, and nutrition, while birth rates remain high, leading to rapid population growth. In the third stage (late industrialisation), birth rates also start to decline as societies become more urbanised, education levels rise (especially for women), and contraception becomes more widely available, resulting in a slowing of population growth. In the fourth stage (post-industrial), both birth rates and death rates are low, leading to a stable or even declining population size. Some versions of the theory include a fifth stage where death rates may slightly exceed birth rates, leading to further population decline. This theory provides a framework for understanding historical population changes and for making projections about future population trends based on a society's level of development.