In developed countries, the groups with the highest birthrates are religious groups.
Fertility varies greatly whether or not people are devout x or y, but it seems to be the case that religion provides some type of cultural insulation that shields people from whatever happens in developed countries across all ethnic groups when religion and religious structures dissapear

Religion also imprints a social formula (ie tenants with a reward/punishment conditioning system) that makes people predictable, which increases trust within the group (unpredictable = untrustworthy).

Combats nihilism and existential angst that comes from being self aware/sentient.

Not religious myself, but if you treated religious and non religious people like different organisms, would you say that religious organisms are more likely to reproduce and spread/assimilate than non religious groups?

If having a high IQ is better overall, why doesn't it correlate with higher birthrates? Would this count as part of the "great filter"?