The question of "why?" is familiar to anyone who's found out their partner has been unfaithful. And it's been posed many times to psychologist Martyn Stewart, head of psychology at Doha College in Qatar.

"I was doing some general research into relationship issues that people would like advice on," Stewart told DW. "As I was doing the research, I was speaking to women and all I kept hearing was, 'We just want to know why men cheat!' So I decided that's what I'm going to do some research on."

Excuses, excuses

Stewart questioned 500 men in the UK during his study, to try to find some answers. First, he asked the men themselves. 45% of those he spoke to stated that they'd cheated at some point in their lives, and 27% said they'd cheated on their current partner. Most of his interview partners, the researcher said, gave excuses for their behavior.

"They would say, for example, 'Oh, it was put on a plate for me' or 'My girlfriend is not giving me enough sex' or 'It's just sex, I still love my girlfriend, it doesn't mean anything'," Stewart recalled. "These are not, from a psychological point of view, actual reasons. They're not the facilitating driver as to why they actually did it."

To identify that 'driver', Stewart looked to evolutionary theory. The idea is that a behavior that occurs now has its roots in earlier times. According to Stewart, the question that needs to be asked is: Why did men cheat in the past? Back in the beginnings of humankind, behaviors were only replicated if they assured survival of the race. It made sense for men to impregnate several women, so they could ensure that their genes were passed on.

Taking every opportunity

Stewart suggests that evolutionary history has created male brain types which are more or less likely to cheat, and for different reasons. In fact, he has identified the personality types which, he says, explain the reasons why men stray.

"There are 27 different types of men, and women need to be aware of which type of man they're getting into a relationship situation with," Stewart said. "Some of them are more prone to cheating, others less."

One of Stewart's types is the opportunist. He doesn't like to miss out on anything - and never considers the consequences of his actions. DW spoke to some men about Stewart's personality types. Our interview partners didn't reveal their last names, and all said they weren't cheaters themselves.

Forty year old Paul reckoned he knew a few Opportunists and described their behavior.

"The circumstances are right, it's a Christmas party," Paul said. "You have a quick bunk up wherever it's convenient, because both parties have their guard down, and it happens! That's all there is to it, it happens."

No respect for others

Another type is the manipulator. He likes to turn situations to his advantage and will manipulate family members, friends and his partner in order to satisfy his need for immediate gratification, according to Stewart.

Ben, in his twenties and living in London, thinks he knows a manipulator with very asocial behavior: "I've had several arguments with my best mate, because he thinks if he doesn't know the boyfriend of the person he's trying to hook up with, it almost doesn't matter."

The good guys

But not all hope is lost - there are also good men out there, Stewart concluded from his study.

"One example is Mr. Secure," the researcher said. "His aim is to deal effectively with his own strengths and limitations in order to grow and live a more appropriate life. There are also the visionary, the thinker, the family man …"

And which type is Stewart himself? "Ah, no! Everyone wants to know and nobody's getting it out of me."