Here's a theory to explain this. Older couples birth more daughters so as a result of their maturity and life experiences, they are less worried. For couples

Here's a theory to explain this. Older couples birth more daughters so as a result of their maturity and life experiences, they are less worried. For couples with both daughters and sons, it might be due to biological differences in progress in maturity and resilience in kids. Girls are more mature at age 10-11 than boys and the trends remain the same till early adulthood.