Middle class life was supposed to bring a more relaxed atmosphere in which to enjoy having kids, but since the 'Leave it to Beaver' days this has not been the case. The increased stress of an uber-competitive society has had the effect of making people personal-agenda-based thinkers, lacking the time to relax or be altruistic enough to do the work involved in raising children.

Plus the obvious fact of pensions supplanting the need to have descendants in the paid workforce cushioning old age.

If the neolib push to shrink/deny pensions continues, one may well see more child-rearing in the (ex?)-middle class.

Another data point is the effect of extreme modernity diminishing libido, viz Japan.

'The history of public debt is full of irony. It rarely follows our ideas of order and justice.' Thomas Piketty