People are thinning themselves out by not having children because the cost of living is too high to have children in developed nations

 In undeveloped nations people have children so they don't all die by 40 or 50. This is a different strategy because in undeveloped nations there often is no health care or Social Security. So, without children death is pretty certain from 40 to 70 years old. You need them to help support you into old age in undeveloped nations.