A reasonable question: We could use prohibitive means... demand monies for extra births, increase costs for larger families, prohibit certain activities to larger families... I am not in favour of these. I genuinely think education is really the only way forward. Education for women in developing countries and education about carbon footprint and population impact on climate in developed. There could be a social stigma option as well... much akin to how Sweden has a stigma on flights... but stigmatisation isn't fashionable these days.