The demographic transition is one of the most interesting puzzles in human evolutionary biology, in my opinion. Several hypothese have been put forward, but in this new paper the authors suggest one that I had never heard or thought of: that in modern societies, interactions with kin are less frequent, and therefore there is less pressure from kin encouraging you to have kids, given that kin have an interest in you having kids... I guess I buy it, to a certain extent. There's got to be several good and easy ways to test this, other than their "role play studies" - whatever that is (...to be fair, I didn't read the paper).
Influences on communication about reproduction: the cultural evolution of low fertility
Lesley Newson, Tom Postmes, S.E.G. Lea, Paul Webley, Peter J. Richerson, Richard Mcelreath
Evolution and Human Behavior Volume 28, Issue 3, Pages 199-210 (May 2007)
Abstract: The cultural norms of traditional societies encourage behavior that is consistent with maximizing reproductive success but those of modern post-demographic transition societies do not. Newson et al (2005) proposed that this might be because interaction between kin is relatively less frequent in modern social networks. Assuming that people's evaluations of reproductive decisions are influenced by a desire to increase their inclusive fitness, they will be inclined to prefer their kin to make fitness-enhancing choices. Such a preference will encourage the emergence of pronatal cultural norms if social networks are dense with kin. Less pronatal norms will emerge if contact between kin makes up a small proportion of social interactions. This article reports evidence based on role-play studies that supports the assumption of the kin influence hypothesis that evaluations of reproductive decisions are influenced by a desire to increase inclusive fitness. It also presents a cultural evolutionary model demonstrating the long-term effect of declining kin interaction if people are more likely to encourage fitness-enhancing choices when interacting with their kin than with nonrelatives.