what do you mean in recent? and by decline do you mean not marriyng, divorce rates or birth rates? but i would say 19th century, tho there is very little data from before. at beggining it was limited to white collar middle class, which allready has very low birth rate and high divorce rate (look up some semi known people from this time in britain, germany, france...), many of them were divorced or married divorced women.  as for reasons, i think its multiple factors. part of it its an agenda being pushed by kikes and libs, but also material conditions - changing way of live, higher population density, urbanization... i think that /pol/ overestimate the appeal of traditional family and traditional life for many people and just consider those that dont want it brainwashed, while many people actually like many aspects of liberal/modern life.
Pirate