Volunteering has its own reward - it can help you live longer, research from the University of Exeter has shown.

A review of studies into the long-term effects of charity work showed that people who volunteered had their risk of dying reduced by more than a fifth.

Volunteers were also less likely to suffer from depression, and had higher levels of life satisfaction and well-being.

But there was no obvious link to any physical effects.

Scientists are unclear about what lies behind the benefits of volunteering.

Study leader Dr Suzanne Richards said: "Our systematic review shows that volunteering is associated with improvements in mental health, but more work is needed to establish whether volunteering is actually the cause.

"It is still unclear whether biological and cultural factors and social resources that are often associated with better health and survival are also associated with a willingness to volunteer in the first place.

"The challenge now is to encourage people from more diverse backgrounds to take up volunteering, and then to measure whether improvements arise for them."

The research, published in the online journal BMC Public Health, pooled a large amount of data from 40 scientific papers.

Combined results from five large longitudinal cohort studies which looked for patterns emerging over long periods of time showed a 22 per cent reduced mortality risk in volunteers.

Worldwide, the proportion of adult populations who volunteer varies from 22.2 per cent in Europe to 27 per cent in the US and 36 per cent in Australia.

Volunteers typically cite altruistic motives, such as "giving something back" to their community or supporting an organisation that has helped them.

Some people volunteer for more selfish reasons, such as gaining work experience or finding new friends.

In 2010, the Government launched its Building the Big Society policy which encouraged people to participate in their local communities and volunteer more.

But evidence suggests that if volunteering gives nothing back to the volunteer its positive effect is limited.

Too much volunteering can also become a burden, creating other problems.

The researchers wrote in their paper: "Although people tend to volunteer for altruistic reasons, if reciprocity is not experienced, then the positive impact of volunteering on quality of life is negated."