Why we still don’t know if there’s any link between ‘forever chemicals’ and female fertility

People are having fewer babies. There’s no argument about that: this drop in the fertility rate is happening in lower-, middle-, and high-income countries, and has been since around the 70s. The big question is why. To what extent is the decline in fertility due to societal changes, economic circumstances, and cultural shifts that make it less desirable or practical to have kids?