Part of the Atheist Demographics Series
If now asked to define the essential difference between the first and second demographic transitions, I would simply say that while the first, the traditional demographic transition, was a long term consequence of the decline in mortality, the second transition should be interpreted as a consequence of fertility declining way below the levels long thought plausible[...]
It is a truism to observe that just as social change was the underlying cause of the first demographic transition, it is the underlying cause of the second. Social change has three distinct dimensions and comes in various guises. The dimensions are structural, cultural, and technological in nature. Depending on the perspective chosen, social change is sometimes equated with ‘modernization’, sometimes with westernization’ or ‘technological civilization’. Nowadays the term ‘post-modernization’ is also frequently encountered; it is then used to describe a form of social change considered specific for the present era.
There is no doubt at all, that all three dimensions of social change have played a role, and will continue to play a role, in the important demographic shifts just reviewed[...] I have no quarrel at all with scholars who point at such structural shifts when they search for an explanation of the population trends observed since the mid-sixties. But, with Ariès I feel that there is more on people’s mind than concern about economic wellbeing, income, and material resources. Etienne van de Walle, expressed these sentiments very well when he argued that what we see in demographic behaviour is a ‘translation’ of cultural representations. Decisive, so it would
seem to me, is what people want out of life. What is the end state they aspire to? How do they look upon themselves as human beings? Do they live life in order to gain an ever blissful and happy afterlife? Or do they only recognize the one life as it is to be lived here and now?
The full introduction to the crucial concept of Second Demographic Transition below, as introduced by one of its first proponents, Dirk J. van de Kaa
The Idea of a Second Demographic Transition in Industrialized Countries - Dirk J. van de Kaa