Narcissistic personality disorder is characterised by a person believing there are special reasons that make them different, better or more deserving than others.
They may feel upset if people do not recognise their apparent achievements, resent other people’s successes and get upset if other people do not put their needs above their own.
But narcissists - exemplified in fiction and film with the likes of Dorian Gray and vain wizard Gilderoy Lockhart in the Harry Potter movies - may also suffer low self-esteem and can be described as vulnerable.
The new research, led by Queen’s University Belfast, was published in the journals Personality and Individual Differences and European Psychiatry.
Dr Kostas Papageorgiou, from Queen’s school of psychology, said: “Narcissism is part of the ‘dark tetrad’ of personality that also includes Machiavellianism, psychopathy and sadism.
“There are two main dimensions to narcissism – grandiose and vulnerable.
“Vulnerable narcissists are likely to be more defensive and view the behaviour of others as hostile, whereas grandiose narcissists usually have an over-inflated sense of importance and a preoccupation with status and power.
“Individuals high on the spectrum of dark traits, such as narcissism, engage in risky behaviour, hold an unrealistic superior view of themselves, are overconfident, show little empathy for others, and have little shame or guilt.”
The three studies informing his work involved more than 700 people in total.
It found that grandiose narcissism increased mental toughness, which can offset symptoms of depression.
People who scored highly for grandiose narcissism also had lower levels of perceived stress and were less likely to view their life as stressful.
Dr Papageorgiou said: “The results from all the studies that we conducted show that grandiose narcissism correlates with very positive components of mental toughness, such as confidence and goal-orientation, protecting against symptoms of depression and perceived stress.
“This research really helps to explain variation in symptoms of depression in society – if a person is more mentally tough they are likely to embrace challenges head-on, rather than viewing them as a hurdle.
“While of course not all dimensions of narcissism are good, certain aspects can lead to positive outcomes.
“This work promotes diversity and inclusiveness of people and ideas by advocating that dark traits, such as narcissism, should not be seen as either good or bad, but as products of evolution and expressions of human nature that may be beneficial or harmful depending on the context.”