According to relationship experts, the two main causes that lead people to cheat are boredom and the need for emotional support.
Pepper Schwartz, professor of sociology at the University of Washington, said that the first category applies to people who are bored and not particularly happy with their sex lives, even if the sex isn't that bad.
He told Live Science: "They've been in a relationship for a long time, and they long for the excitement of something new.”
According to Eric Anderson, chief science officer at AshleyMadison.com, this is reflected on the site.
He explained that most women who joined it said they weren't unhappy with their partner, and they didn't want a divorce.
All they wanted was a bit more excitement.
However people in the second category were in unhappy relationships - and looking either for emotional support, for an escape, or for revenge on their partner.
"They don't want to leave for various reasons — children, money, you name it — but they're just sort of aching for something that's emotionally reinforcing," Schwartz said.
"And a lot of people use sex for that and pursue another relationship."
CynoSure Prime recently collected a series of passwords used by Ashley Madison account holders, which seemingly confirm the idea that many who used the site were having second thoughts.
They included:
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