http://www.theguardian.com/society/2014 ... ncan-smith

I'm not saying it isn't something to consider, but you know, IDS. And the Tories. Messing around with the housing benefit of the under-25s? The very people who are trying to make something of their lives in the worst conditions seen for decades? Or the bedroom tax? How about ATOS?

Oh, but not the pensioners, oh no. They vote for arseholes like IDS.

I quite agree with this (unlike some of their other policies).
The population would remain stable if two parents had two children. More than that and the population goes up. (Obviouly immigration/emigration have an effect too).
This isn't post WW2 when people needed the help as much, and the country needed the population increase back then too. It makes sense to limit this to two children now.