There could be something to the horoscope thing. Someone born in winter will have slightly different initial conditions than someone born in summer. Seasonal variations could affect early years.

That's not even getting into how month-of-birth could affect how a child fits into the school system. Some months will be on average older than their classmates, some younger.

I think the second effect is probably stronger in the modern (air-conditioned, artificially-lit, state-schooled) era.

Well.. there could be. But there doesn't seem to be. People can't hit the horoscope sign even roughly. They can't do it better than change for all of the year.

Funnily enough, looking into it, there are apparently some measurable differences between winter and summer births.

From the abstract: "Winter/spring birth, while associated with an increased risk of schizophrenia, is generally associated with superior outcomes with respect to physical and cognitive development."

Source: Schizophrenia Research 81 (2006) 91–100. "Season of birth is associated with anthropometric and neurocognitive outcomes during infancy and childhood in a general population birth cohort". https://dash.harvard.edu/bitstream/handle/1/3743679/sdarticl...