Writer/Director Joseph Gordon-Levitt and Scarlett Johansson star in Relativity Media's "Don Jon". The consequences of this are worse than you might think. The thin end of the wedge is less enjoyment during sex. Jon's dissatisfaction with real life sex is something he has in common with a lot of habitual porn users. In his book, The Brain That Changes Itself, the psychiatrist Norman Doidge writes about a phenomenon he began to notice among his male patients in the mid 1990s. They watched porn – “everybody does,” they told Doidge – and were experiencing “increasing difficulty in being turned on by their actual sexual partners, though they still considered them attractive.” They found themselves having to fantastise about porn scenes to get turned on. That's because, along with a great number of porn users, they had rewired the arousal pathways in their brains. “Pornography,” writes Doidge, “satisfies every one of the prerequisites for neuroplastic change,” – that is, the brain's ability to form new neural circuitry. The most important condition is the release of dopamine, the neurotransmitter that gives us a feeling of exciting pleasure, which porn triggers. The more often you watch porn and get the dopamine hit it delivers, the more the activity and the sensation become entwined in your brain. Doidge puts it like this: “since neurons that fire together wire together, these men got massive amounts of practice wiring these images into the pleasure centres of the brain.” And, “because plasticity is competitive, the brain maps for new, exciting images increased at the expense of what had previously attracted them.” A related problem is what addiction experts call “tolerance”, in other words the need for more of a given stimulant (harder and weirder porn) for the same amount of dopamine. In the end, the result is what Doidge politely calls “potency problems”. Compulsive pornography users become unable to maintain erections.

Even among more casual users, porn is wreaking havoc in the bedroom. Last year, American GQ's sex columnist, Siobhan Rosen, complained about the “pornified sex” men seemed to expect – not in a relationship, when trust has been established, but from the very first encounter. She wrote about men she had just started seeing who brandished ball gags, ejaculated on to her body and used really nasty language during sex. “You don't want to do those things with someone you hardly know,” she tells me. Men recreating the money shot is something that “has happened to every single one of my girlfriends,” she says. The advertising executive, Cindy Gallop, became so irritated by this very thing that she made it the central complaint of her TED talk when launching her website, makelovenotporn.com, in 2009. The talk went viral. Gallop is 53 and “only dates younger men, usually men in their twenties,” she tells me over a drink in London. She's encountered many of the same porn-inspired behaviours as Rosen (who is 30), which is why she decided to set up Make Love Not Porn, to promote “real sex”. “Guys watch porn and when they go to bed with a real woman, all they think about is recreating that scenario,” she says. And women, who are watching porn in ever-greater numbers themselves, “start believing that that is what they have to be like in bed as well.” The American porn star and sex educator Nina Hartley calls this “doing it on someone else's template”. Gallop agrees. “The poor guy is going, 'That porn actress loved it when he did that, so why doesn't she?' Meanwhile, the girl is lying there thinking, 'The porn actress really loved it when he did that so why don't I?'” But, she goes on, “in real the world , every single partner you will ever have is different. Different things will turn them on.”

Unfortunately, Rosen says, “a lot of girls don't speak up” about not enjoying a new partner's aggressively porn-y approach. “Instead, they're just like, 'I guess this isn't going to work and, you know, it's over.' And the men I've said something to have felt horrible when I've told them. They have no idea and are so sorry. Most of the time they really don't know they're doing anything wrong.” All they know is that girls keep breaking up with them. This romantic failure is especially apparent among boys in their teens and early twenties. The psychologist, Catherine Steiner-Adair, interviewed a thousand children aged between four and 18 across America for her new book, The Big Disconnect. Among her findings was a marked tendency among boys to approach girls they liked in a sexually aggressive manner. “That is a trend,” she tells me. “There's no question about it.” They send extremely crude messages – Steiner-Adair gave me a dozen examples, none of which I can reproduce here – and, unsurprisingly, “the girls don't like it.” And they rebuff the boys. “The boys are very confused about how to approach girls,” she says. “Their sexual education is porn. And it's very misogynistic and violent porn.” Porn has become more extreme over the last two decades, probably because its users' “tolerance” has rapidly increased with the ubiquity of internet connections. Steiner-Adair had conversations with boys who wanted to know why women liked being choked when they were having sex or why women liked being urinated on. Because young men lack the experience that would allow them to differentiate between an extreme sexual performance and real sex, says Steiner-Adair, some of them “are surprised when the girls don't want to play out the scenarios that they have been watching.” The result is mutual unhappiness, frustration and disappointment. And, according to Doidge, a potentially permanently addled sexuality thanks to the presence of porn during this highly plastic period of brain development. Ultimately, says Gallop, “the issue isn't porn. The issue is the complete absence of a counterpoint to porn.” Nina Hartley agrees.“Young people are going to find information wherever they can get it,” she tells me. “But watching porn to learn to have sex is like watching Vin Diesel movies to learn how to drive. I'm paid to give this performance. It's not the kind of sex that I have at home.”

Nine months ago, Gallop launched MakeLoveNotPorn.tv – a collection of videos of real couples having sex. She was surprised (and delighted) when several porn stars sent their own home videos in. They show just how true Hartley's words are. “Let boys know they don't have to be hyper-masculine in order to get girls. Let girls know you don't have to be doormats in order to be popular,” says Hartley. Joseph Gordon-Levitt hopes that Don Jon will “entertain audiences with laughter and a healthy dose of honesty, and perhaps inspire them to consider what real intimacy means. It's not only a movie about how people objectify each other. It's about how we connect with each other, and how that feeling really is better than anything else. Telegraph, London