#2 is assuming UBI is a one-time cash injection to stimulate aggregate demand, similar to QE. I have a hard time seeing the path forward to the (substantially)

#2 is assuming UBI is a one-time cash injection to stimulate aggregate demand, similar to QE. I have a hard time seeing the path forward to the (substantially) higher taxation on the middle and upper classes needed to make UBI a reality with the current political and economic climate in the US. But this is subject to debate. You are correct that a redistribution of the tax burden, if feasible, would not lead to inflation. #3 - yes, in agrarian societies poorer people have more children to help out with labor-intensive work. I think the situation would play out different in a services-based economy like the US. The monetary aspect of UBI would not influence the wealthy, educated cohort's decision to have more children (usually capping out at 3). It would give lower-income individuals an incentive to have more children in order to claim more benefits, as is common for contemporary social services programs. If UBI was substantial enough to allow lower/middle income individuals to quit their jobs, they would also have much more time to have and raise children - not out of necessity, but out of ability. Agreed that the effect on population size is uncertain and needs an actual study.