The reason for fast declining birthrates since 2007 in the US and among most nations globally seems to be the current ZIRP and low interest rates and Quantitative Easing programs which have the effect of inflating asset prices. The majority of assets are held by large institutions and post-child-bearing age populations. The flow through of these policies are asset prices rising significantly faster than incomes. For example, non-discretionary items like homes, rent, education, healthcare, insurance, childcare, etc. are skyrocketing versus wages.
For young adults, this means a far greater reliance on debt to educate themselves and a far greater portion of their subsequent income to service that debt. Greater reliance for young adults on debt to purchase a home or a greater portion of their income to pay their rent, provide healthcare, insure themselves, or provide childcare (as both parents are more typically working full time). The net result of these federal government and central bank policies to boost the stock market, home prices, and collapse interest paid on savings are collapsing birthrates and collapsing total births. This is diminishing present and future demand and quality of life for the young, poor, primarily non-white in perhaps the greatest transfer of wealth mankind has seen.
This is creating an economic doom loop whereas fast rising asset valuations push up the costs of living far faster than wage growth, pushing birthrates (and present and future demand) lower. The lower birthrates fall, the greater the reliance on asset boosting policies, and the majority of young, poor, and minorities with little or no assets are most harmed...wash, rinse, and repeat.