Population stability is achieved when each woman bears an average of 2.1 kids over the course of her lifetime—one for her, one for her male partner, and a little overage to make up to childhood deaths. Today, there are sixty countries in which TFRs are below 2.1. For example, the European Union's TFR is 1.5 and no EU member state has a TFR at replacement or above. Even high population developing countries have seen steep declines in fertility. Since 1970, China's TFR fell from 5.8 to 1.6; India's from 5.8 to 2.9; Indonesia from 5.6 to 2.4; Japan's from 2.0 to 1.3; Mexico's from 6.8 to 2.4; Brazil's from 5.4 to 2.3; and South Africa's from 5.9 to 2.7. The U.S. TFR dropped from 2.55 in 1970 to around 2.1 today, largely because of the influx of higher fertility immigrants. However, the fertility of second generation Americans drops to the level of longer established Americans.This is the explanation:
So, modernity essentially transforms children from capital goods that produce family income into consumption items to be enjoyed for their own sakes, more akin to sculptures, paintings, or theatre. But that's just the problem—according to happiness researchers, people don't really enjoy rearing children.