My pet theory is that people don't spend much time around babies and young children when they're growing up so have less comfort around the idea of themselves b

My pet theory is that people don't spend much time around babies and young children when they're growing up so have less comfort around the idea of themselves being parents. This tracks with the "wealthier" pattern also because as people become wealthier, they're able to become more socially isolated. My guess is that in pre-modern times, a lot of child care would happen by various neighbors so everyone would have some experience being around small kids. Now childcare is largely a business where specific people spend everyday, all day caring for kids (daycare, pre-K, kindergarten, etc).