All relationships are different, and people can change and improve their own behavior. However, a piece of research has found evidence that backs up the old "once a cheater, always a cheater" rule when it comes to staying faithful in relationships.
Researchers from the University of Denver wanted to look at whether infidelity in a previous relationship was a risk factor for infidelity in the next relationships. To do this, they looked at 484 people in mixed-gender romantic relationships, and asked them about their sexual activities outside of their current relationship (whether they had been cheating or not) as well as whether they were suspicious their partners were cheating.
They then followed these people through from this relationship to their next relationship, to measure whether people who said they'd cheated in the first relationship went on to cheat in the next one.
The study, which followed them over a five-year period, found that people who cheated in their first relationship were three times more likely to cheat on their next partner than those who stayed faithful.
The study also found that those who suspected their first partner of cheating were four times more likely to have suspicions that their next partner was cheating.
Strangely, people who knew for certain that their first partner had cheated, rather than just suspecting it, were twice as likely to report that their subsequent partner had also cheated.
The authors wrote that the study, aptly titled "Once a Cheater, Always a Cheater? Serial Infidelity Across Subsequent Relationships", showed that previous cheating was an important risk factor for infidelity in the next relationship. Basically, if they have cheated before, there's a much higher chance that they'll do it again.
The psychologists controlled for demographic risk factors, and for gender and marital status. So if they cheated in their first relationship with someone they weren't married to, they were still more likely to cheat in the second relationship even if they were married this time.
The authors acknowledged that the sample size was small, and that further study needed to be done, including research on other types of relationships than purely mixed-gender relationships. However, they hope that the study could lead to novel interventions to prevent serial infidelities in relationships.
The study is published in Archives of Sexual Behavior.
An earlier version of this article was published in August 2017.