Results can start to be worthwhile early on. But the life-altering decisions people make over a two year period when guaranteed a subsidy that runs out at the end of those two years are not particularly likely to be the same as the life altering decisions people make over a two year period when guaranteed that subsidy for life.

(If you wanted to really understand all the dynamics, you'd probably want to wait more than a generation to understand how subsidy recipients' decisions to raise children are affected and whether those children make systematically different life choices.)