the multiple birth anomaly which is an exception rather than the rule aside, life is expensive, success is measured in wealth. and capitalist society fuels itself by perpetuating this mantra.
so, it is what it is - success is measured in wealth - taking that as the core argument, an uneducated person from a poor background, should have one child.
That uneducated person, does everything within his or her power to afford the best education for that child, the child then grows up to be well qualified, good university - not the best - his parents couldn't afford the best.
that educated person - now in firmly economic middle class meets similar they then have one child
that child best education 2 mc parents can afford - which is better than they had one of the top schools in the country
that person is now mixing with toffs - but of course cannot really be one
his child uses the same formula as above marries toff, toff kid
now all the very intelligent people have bred, there are no stupid people left to pay minimum wage, to run their respective empires
this makes the above people stupid
you need stupid people in a capitalist society - then you can exploit them and keep yourself rich