First demographic transition: change from low and stable population of pre- industrial societies (high fertility, high mortality) to growing population of industrial societies (slowly declining fertility, fast declining mortality) Modernization theory of development: industrialization fosters labour force participation and rationalism, which foster decline in fertility. Cultural lag: cultural norms maintain high fertility. Second demographic transition: change from growing population of industrial societies to high and stable (later: declining) population of post-industrial societies (low fertility, low mortality) Demographic change is the result of fertility, morality, and net migration. Declining fertility is the most important cause of population aging. Fertility declined from 7 births per woman in the 1850s to under 2 by the 1980s. Replacement fertility is the average number of children needed to replace on generation by the next (2. 1 children per woman) Proximate causes: fewer long-term relationship and marriages, older age at marriage, more divorces, cohabitation, and use of birth control.