[therealforsen]

Why would an individuals fertility be increased because the fertility of his group is increased? The gene would not propagate unless he has more children than his peers. There is no evidence of evolution by natural selection to begin with, though.

[bsder]

If the group with the trait has on average a slightly higher reproduction rate than a group without, that is sufficient.

This doesn't mean that any particular individual will have a higher rate (although most will).

One hypothesis for the spread of Christianity in the Roman Empire is that Christianity provided social services that improved reproductive rates by a couple of percentage points. Maintain that over a particularly stressful upheaval in the social order and you don't need many generations to significantly outbreed your rivals.

[robotresearcher]

> Christianity provided social services that improved reproductive rates

Social services, and perhaps this might have some impact:

"In 1968, Pope Paul VI issued his landmark encyclical letter Humanae Vitae (Latin, “Human Life”), which reemphasized the Church’s constant teaching that it is always intrinsically wrong to use contraception to prevent new human beings from coming into existence."

https://www.catholic.com/tract/birth-control

[lazyasciiart]

The gene can be propagated by individuals who did not express it - it could be sex-linked, it could be recessive, it could require two or three conditions to be met, etc. (Also, propagation doesn't require 'more children than his peers', it just requires children that go on to reproduce).