It's having enough intelligence(or education/awareness) to know that high cost of living would put more pressures on the parents and even the kids, if more kids were to be born, coupled with the existence of free will to take a decision/action (that is, not to have children) based on that awareness - which is causing the low birth rates.

A pattern that stuck me, across different countries and even different states within a country - the higher the education level generally is, the higher is the free will (or the desire to express it) and the capability to take independent actions to external pressures, the lower is the birth rate. Of course, this is putting it in simplistic terms.