We're going to have a system where the middle classes are discouraged from breeding because it's jolly expensive.
But for those on benefits, there is every incentive. Well, that's not very sensible.
But the truth - of course - is that people do, at the margin, respond to incentives. If you raise the cost of having children, people will have fewer of them.
For example, this study of the introduction of working tax credits - which increased work incentives for single parents - found that the move "led to a significant reduction in single mothers’ subsequent fertility."
This paper finds that:
Financial incentives play a notable role in determining fertility decisions in France...Adding to the existing tax-benefit system a child subsidy of 150 euros per month...would raise total fertility by about 0.3 points.
And there's also evidence here (pdf) from Israel.
It is, therefore, clearly true that withdrawing child benefit from top earners - forget that "middle class" crap - will discourage them from "breeding."
In this sense, Mr Flight is quite right. Where we might disagree with him, naturally, is in the class hatred indicated by that last sentence. But then,who would expect anything else from a Tory?