The research suggests that individuals look for affairs when they are on the verge of entering a new decade.
According to the study conducted by an Irish dating website, the most dangerous age of all for cheating is 39 as people get restless before approaching the milestone of 40.
Other years in which people are likely to stray are 19, 29, 49 and 59, the study found.
The study of 1,000 profiles on the dating website IllicitEncounters.Com found that there were 18 per cent more members whose age ended in 9 than for other years in the decade.
"Beware of the figure 9 - it is the danger age as far affairs are concerned. This is applicable to men and women. Both go looking for affairs when they are on the verge of entering a new decade," said Christian Grant, spokesperson for the website.