I'll throw my hat in the ring. I don't think that this is truly the primary mechanism. Maybe a contributing one, but definitely not the predominant cause. Let'

I'll throw my hat in the ring. I don't think that this is truly the primary mechanism. Maybe a contributing one, but definitely not the predominant cause. Let's say that the average woman wants to have children every 3 years. This is probably a gross oversimplification, but it feels about right; most people I know who have multiple kids or siblings have a 2-4 year age gap. In the days of yore (for some definition of "yore") women would typically have children from the age of 19-20 until 40-ish. That gives you about 20 years to have kids, resulting in families with ~7 children. Again, this feels about right; go back a few generations in my family tree and you see families of 6-8 kids. If spending time at school is the dominant mechanism then women would typically have 5-6 fewer years in which to conceive, and you'd expect families to have ~2 fewer children, making the average ~5. But it's, what, 2.3 children and dropping? A quick google search for fertility rate showed that the last significant peak in the US was in 1957 (after a slump around the world wars and great depression). A second quick google search for "what happened with women in the 60s" immediately starts discussing the women's liberation movement, with increased rates of women employment and education. A third google search for "when did birth control come out" gave the year 1960 for the use of Enovid as a birth control mechanism, but an actual release date of 1957 for cramps. If I was going to point fingers at an "obvious" reason for a decline in fertility, it would be that women finally had the choice as to whether or not they would have kids, and they had the means to effect that choice.