Earlier quoted context omitted.
Evolutionary pressures aren't as simple as "if it lowers your chance of reproducing it's bad." You can read "The Selfish Gene" for more info. Consider a gene that promotes compassion, selflessness and sharing. While it may lower your individual fitness, groups with some individuals with this gene may fare better as a whole. Same as autism. For example, say autistic people see connections that other people don't. Havi…
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.
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.