A University of Otago study suggests that religious people may receive more help from their community raising their children, which in turn allows them to have more children.
Dr John Shaver of Otago's Religion Programme said while scholars have predicted that religion would fade away for a long time, it's not disappearing as quickly as they anticipated.
"While religion has been declining in New Zealand for decades, our findings point to a countervailing trend, one that is driven by the co-operative breeding dynamics of religious communities," Dr Shaver said.
"Co-operative help to mothers is one of the reasons for our success as a species. In modern environments mothers receive far less help than in our recent past.
"Less help drives down fertility levels in modern environments. However, religious mothers have more help, and more children than secular mothers."
The study was published in the journal Evolution and Human Behaviour, and researchers analysed data from 12,980 people who enrolled in the New Zealand Attitudes and Values Study.
"We found that religious people have more children, and that non-reproductive (those who don't currently have their own children) religious people tend to look after children who are not their own more frequently than non-reproductive secular people," Dr Shaver said.
"Our findings point to why religion is not disappearing as quickly as many have predicted – religious people are able to have more children because they get more help with childcare than secular people.
"We hope that our research draws attention to the impact of a person's religious behaviour on core biological and sociological processes."