Excessive use of social media - in particular the posting of images and selfies - has been linked to a subsequent increase in narcissism, research has found.
Researchers from Swansea University and Milan University studied the personality changes of 74 individuals aged 18 to 34 over a four-month period.
They also assessed the participants' usage of social media - including Twitter, Facebook, Instagram and Snapchat.
Those who used social media excessively, through visual postings, displayed an average 25 per cent increase in such narcissistic traits over the four months of the study.
This increase took many of these participants above the clinical cut-off for Narcissistic Personality Disorder, according to the measurement scale used.
Narcissism is a personality characteristic that can involve grandiose exhibitionism, beliefs relating to entitlement, and exploiting others.
The study found that those who primarily used social media for verbal postings, such as Twitter, did not show these effects.
However, in this group of participants, their initial levels of narcissism predicted a growth in this form of social media usage over time.
The more narcissistic they were to begin with, the more verbal postings they made later.
All but one of the people in the study used social media, and their average use was around three hours a day, excluding usage for work.
Some reported using social media for as much as eight hours a day for non-work related purposes.
Facebook was used by 60 per cent of participants, while 25 per cent used Instagram, and 13 per cent used Twitter and Snapchat each.
More than two-thirds of the participants primarily used social media for posting images.
Professor Phil Reed, from Swansea University's Department of Psychology, led the research.
He said: "There have been suggestions of links between narcissism and the use of visual postings on social media, such as Facebook, but, until this study, it was not known if narcissists use this form of social media more, or whether using such platforms is associated with the subsequent growth in narcissism.
"The results of this study suggest that both occur, but show that posting selfies can increase narcissism.
"Taking our sample as representative of the population, which there is no reason to doubt, this means that about 20 per cent of people may be at risk of developing such narcissistic traits associated with their excessive visual social media use.
"That the predominant usage of social media for the participants was visual, mainly through Facebook, suggests the growth of this personality problem could be seen increasingly more often, unless we recognise the dangers in this form of communication."
The study is published in The Open Psychology Journal.
