I heard an article on NPR that talked about the consequences of China's Only One Child policy that has been in place for the last 30 years. In other words, more than a generation.
The effect that really chilled me was that not only do today's kids not have any brothers or sisters, they also don't have any first or second, and maybe even third cousins.
I like my cousins! I've even gotten to live close enough to a couple of them to have a relationship, though I have neglected most of them for all too long. But my life definitely would have been poorer without them.