Scientists at Ohio State University arrived at the figure after studying over 1,000 obituaries from across the U.S. The team also factored in whether the person was married and their sex, which both can affect how long a person will live. 

Laura Wallace, lead author of the study and a doctoral student in psychology at Ohio State University, said in a statement: “religious affiliation had nearly as strong an effect on longevity as gender does, which is a matter of years of life.”

To investigate whether religious beliefs cold affect the age a person dies, the team first assessed over 500 obituaries in the Des Moines Register, a newspaper in Iowa, from between January and February 2012. They documented the age, sex, marital status, social and volunteer activities listed in the piece, as well as the religious affiliation of the deceased.

The data revealed that those whose obituaries included a religious affiliation lived 9.45 years longer than those who did not - which dropped to 6.48 when gender and marital status were factored in.

A second study involved over 1,000 obituaries from 42 major U.S. cities, published on newspaper websites in the year following August 2010. That analysis showed people whose obituaries included their religious affiliation lived an average of 5.64 years longer than those who didn’t. The average dropped to 3.82 years when gender and marital statues were taken into account.

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Building on previous studies which suggest that volunteering and social events can extend lifespan—both activities which are integral to many religious groups—the researchers combined their new data to unpick whether these explained the spike in longevity.

While they did play a part, the researchers behind the study published in the journal Social Psychological and Personality Science believe they weren't the only factors. 

Read more: Live Longer: Walking Faster Can Help, According to a New Study

Lifestyle guidelines, such as abstaining from drinking alcohol or taking drugs, could explain the boost, as well as practices which ease stress, such as praying or meditating.

“There’s still a lot of the benefit of religious affiliation that this can’t explain,” said Wallace.

Furthermore, the relationship between religion and a person’s lifespan might also depend on the average religiosity of the city in which they live. 

"The positive health effects of religion spill over to the non-religious in some specific situations," Wallace said. “"The spillover effect only occurs in highly religious cities that aren’t too concerned about everyone conforming to the same norms. In those areas, non-religious people tend to live as long as do religious people."

The researchers acknowledged their study was limited by the fact it could not control race and lifestyle choices, which are important factors for longevity. 

This is the latest research to point to religion having life-boosting effects. In 2016, a study published in the journal JAMA Internal Medicine suggested that regularly attending religious services can increase lifespan.

The team at the Harvard T.H. Chan School of Public Health analyzed data on around 75,000 middle-aged female nurses in the U.S., who were asked every four years between 1992 and 2012 about their, including whether they attended a religious service regularly.

Those visiting church at least once a week were found to have 33% lower risk of death than those who never went.