Many men in the 'friend zone' may seem kind, caring and affectionate - but one psychologist has warned they have a hidden agenda.

Dr Scott Kaufman says men with the 'Nice Guy Syndrome' who are often relegated to the friend zone often have a sense of entitlement, and so are likely to be narcissists.

'A lot of nice guys who complain about being in the friend zone are not really that nice,' Dr Kaufman, a professor at the Positive Psychology Center at the University of Pennsylvania told MailOnline.

In the film '500 days of Summer', actress Zoey Deschanel (pictured left) plays Summer Finn who 'friend zones' nice guy Tom Hansen, played by actor Joseph Gordon-Levitt (pictured right)

'They feel entitled to women - they are narcissists in nice guy's clothing.

'They think "well I'm friends with her, why won't she sleep with me" and that's not a very nice guy way of thinking.'

There are some exceptions, he says. 'You also have a lot of really shy guys who aren't trying to be manipulative but they're scared of expressing their desire.

'A lot of them don't say anything and wait for it to happen.'

For men who find themselves relegated to the friend zone, Dr Kaufman recommends taking the plunge and being honest with their feelings.

'If you have a girl that you're attracted to and you have a romantic attraction to them, signal your attention right away and say "hey can I take you out for a coffee".'

There is also research that suggests that couples aren't likely to be friends first - so if people are attracted to someone, it may be best for them to avoid the 'friendship first' approach if they want to be liked back.

In a study on 626 high school students, researchers found that romantic partners weren't likely to be friends in the previous year or share the same friendship group, and opposite-gender friends are unlikely to transition to dating (stock image)

In a study on 626 high school students led by Dr Derek A Kreager at Pennsylvania State University, researchers found that romantic partners weren't likely to be friends in the previous year or share the same friendship group, and opposite-gender friends are unlikely to transition to dating.

Dr Robert Glover, a psychologist who wrote 'No More Mr Nice Guy', says that a nice guy's primary goal is to make other people happy and that they're guided by 'covert contracts' such as believing: 'If I meet other people’s needs without them having to ask, then they will meet my needs without me having to ask.'

Because they believe they have kept their side of the 'contract', they often feel helpless and resentful when other people, and the world, don't keep their side of the 'contract'.