I can think of several arguments for or against this, but the two important (and somewhat independent) questions to ask first are "is our target population larg

I can think of several arguments for or against this, but the two important (and somewhat independent) questions to ask first are "is our target population larger than present?" and "what do we want our age distribution to look like?". making childfree adults pay more taxes is essentially a subsidy towards having children. in general when you apply a subsidy to something, you get more of it. in the west, we already face significant sacrifices to our standard of living over the next couple decades if we want to even approach a sustainable usage of our planet's resources. if we actually grow the population, we have to make even larger sacrifices. from this perspective, your tax proposal seems like a bad idea. on the other hand, we also have a looming demographic crisis as the ratio of retired to working adults increases. if nothing changes, the working population will bear a much greater burden in the future to support retirees, or we will have to drastically cut retirement benefits. from this perspective, it does seem beneficial to increase the incentive to have children (or at least blunt the disincentive a bit more). yet another perspective: childless people (in the US, at least) already pay more taxes than parents. they are ineligible to take the per-child tax deduction that you probably take every year. they also pay for services like schools which their (nonexistent) children will never consume or benefit from. there is also a negative correlation between number of children and income. because of this correlation, people with few or no children are more likely to already pay more into the system than they receive in services/benefits. they are also more likely to have adequate savings for retirement and depend less heavily on government retirement benefits. I think in an ideal world, some well-meaning experts would convene and establish a target population and age distribution for a country (or the world, but I'm trying to keep at least one foot grounded in reality). then if we're on track to meet the target, no special benefit for parents. if we're below target, a proportional tax advantage for parents. if above, a proportional penalty tax. the the tax benefit/penalty would be locked in for the next twenty years so that parents wouldn't have the rug pulled out from under them. actually maybe the penalty could be removed, seems unnecessarily punitive otherwise.