A leading psychologist has warned that young men are facing a crisis of masculinity due to excessive use of video games and pornography.

Psychologist and professor emeritus at Stanford University Phillip Zimbardo has made the warnings, which form a major part of his latest book, Man (Dis)Connected.

In an interview on the BBC World Service's Weekend programme, Zimbardo spoke about the results of his study, an in-depth look into the lives of 20,000 young men and their relationships with video games and pornography.

He said: "Our focus is on young men who play video games to excess, and do it in social isolation - they are alone in their room."

"Now, with freely available pornography, which is unique in history, they are combining playing video games, and as a break, watching on average, two hours of pornography a week."

Zimbardo says there is a "crisis" amongst young men, a high number of whom are experiencing a "new form of addiction" to excessive use of pornography and video games.

Zimbardo gave a TED talk in 2011 outlining the problems facing young men's social development and academic achievement, which he puts down to excessive use of porn, video games and the internet.

He cited the example of a mother he met while conducting the study whose son does not see the problem in playing video games for up to 15 hours a day.

Zimbardo said: "For me, 'excess' is not the number of hours, it's a psychological change in mindset."

Giving an example of the mindset of a gaming and pornography-addicted young man, he says: "When I'm in class, I'll wish I was playing World of Warcraft. When I'm with a girl, I'll wish I was watching pornography, because I'll never get rejected."

Zimbardo claims that this relatively new phenomenon is affecting the minds of young men.

Citing the research he and his team conducted for the book, he says: "It begins to change brain function. It begins to change the reward centre of the brain, and produces a kind of excitement and addiction."

"What I'm saying is - boys' brains are becoming digitally rewired."

He also mentioned the growing problem of a disputed phenomenon called 'porn-induced erectile dysfunction', or PIED: "Young boys who should be virile are now having a problem getting an erection."

"You have this paradox - they're watching exciting videos that should be turning them on, and they can't get turned on."

An article from Psychology Today, however, argues that there are no demonstrable scientific links between porn consumption and erectile dysfunction.

In his opinion, the solution is to accept that the problem is serious - parents must become aware of the number of hours a child is spending alone in their room playing games and watching porn at the expense of other activities.

He also blamed negative images of men in the American media, which show men as being "slobs, undesirable, only wanting to get laid and being inadequate in doing that."

He also called for better sex education in schools - which should focus not only on biology and safety, but also on emotions, physical contact and romantic relationships.

The pressing issue of male mental health is now a much more prominent concern than it once was. Last year saw the first Male Psychology Conference at University College London, intended to encourage the British Psychological Society to introduce a male specialist section, to sit alongside its female equivalent.

The charity Campaign Against Living Miserably, or CALM, was started in 2006 and has gained a high profile in recent years, for its efforts to encourage men to discuss mental health problems and bring down the male suicide rate.

Phillip Zimbardo is famous for the 1971 Stanford prison experiment, in which 24 students were asked to play the roles of 'guards' and 'prisoners' in a mock prison at Standford University. Intended to last for two weeks, the experiment was abandoned after six days, after the previously normal 'guards' became extremely sadistic and the 'prisoners' became submissive and depressed.