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Forget the mid-life crisis, men approaching any milestone birthday are more likely to cheat, according to new research.

Males who have an age ending in nine - 29, 39, 49 - were much more likely to register for an extramarital dating site than men of other ages.

Over a series of studies, experts found that these so-called '9-enders' were also particularly prone to behaviour 'that suggests a search for, or crisis in meaning'.

Researchers found that men who have an age ending in nine - 29, 39, 49 - were much more likely to register for an extramarital dating site than men of other ages. Over a series of studies, the experts found that these so-called '9-enders' were particularly prone to behaviour 'that suggests a search for, or crisis in meaning'

'People audit the meaningfulness of their lives as they approach a new decade in chronological age,' said Adam Alter, a professor at New York University, and lead author of the research paper.

'Once they do audit the meaningfulness of their lives, people tend to reach one of two conclusions: either they conclude happily that their lives are indeed meaningful or they decide that their lives lack meaning in at least one important domain.'

If they feel something is missing, many 9-enders will respond positively, such as training to run a marathon, continued Professor Alter.

'In contrast to this adaptive behaviour, some people might struggle to come to terms with the conclusion that their lives lack meaning,' he said. 

'They might seek a socially damaging extramarital affair, or in extreme cases, to end their lives altogether.'

For the first part of their study, the researchers analysed the responses of 42,000 adults who completed the World Values Survey - a large-scale questionnaire completed by people from more than 100 countries between 2010 and 2014.

They found that 9-enders reported thinking and questioning the meaning of life more than other respondents.

Subsequently, the researchers obtained data from an online dating site that targets people who are already in a relationship and found that there were far more 9-ender men - 18 per cent more - than would be expected by chance.

Further analysis found a similar, though less-pronounced, pattern of results for women.

Using government data, the researchers then examined the number of suicides per 100,000 individuals aged between 25 and 64 across the U.S. from 2000 to 2011.

The researchers also found that many people entering a new decade tend to engage in 'productive meaning-seeking behaviours', such as taking up a new sport. They randomly chose 500 first-time participants in one of five major marathons (stock image). Of that sample, 74 were 9-enders – 'an overrepresentation of 48 per cent'

They found a small, but statistically significant, increase in the number of 9-enders committing suicide compared to other ages.

The researchers also found, however, that many people entering a new decade tend to engage in 'productive meaning-seeking behaviours', such as taking up a new sport.

They randomly chose 500 first-time participants in one of five major marathons in the US. Of that sample, 74 were 9-enders – 'an overrepresentation of 48 per cent'.

They also analysed another set of marathon runners' race times and found that people ran an average 2.3 per cent faster in their 9-end year than the two years before or after.

This suggested that they were more motivated and trained harder as they approached their milestone birthday, the researchers said.

The research paper was published in the journal Proceedings of the National Academy of Sciences.