The Fonz (Henry Winkler). But hold on a second. What if this is not true? What if young man as sex-crazed emotional retard is not only an unhealthy stereotype that damages boys and misleads young women, but one that doesn't match how the vast majority of young men really think or feel? Professor Smiler, from the psychology department of Wake Forest University in North Carolina, says we need to challenge Casanova. ''[The Casanova stereotype] lives in the culture, it shows up a lot in the popular media, it even shows up a lot in sex-ed curricula,'' Smiler says.

Russell Brand. ''It's only about 40 years ago that we started to think of that version of male sexuality as the typical version. Even into the 1960s and early 1970s when people talked about male sexuality, they mostly talked about boys or young men being in charge of their hormones, able to control themselves and really wanting to find a ''nice'' girl or the ''right'' girl. ''Then over the last 30-40 years we've really changed our conception, and now we talk about boys being driven by their hormones, or boys and men as being evolutionarily intent on spreading their seed widely. ''In the mass media we now have all of these examples of promiscuous guys that we hold up and make the star of TV or a movie, whereas 40 years ago they were always a bad example.'' In modern comedies like Two and a Half Men, it is the Casanova character who usually gets the best lines, is more often proved right after the plots have played out, and typically has the stronger support of the laugh track. The type traces back to Hawkeye in M*A*S*H, or the Fonz in Happy Days.

It's not just the media reinforcing the message. Before becoming a developmental psychology academic, Smiler was a family therapist in the Philadelphia area working with male teenagers. Often when the boys were ''acting out'', both they and the parents would make a comment like ''well, guys are just like that'' or ''that's just what guys do''. When he moved into academia, he realised there was a disconnect between what the experts know about male sexuality - from years of research and surveys - and what the rest of the population believe about it. This is why he felt the need to write a book (Challenging Casanova, published last month in the US) for the general public. ''The folks that do research on sexual behaviour … have always known that boys and young men really don't quite fit the stereotype,'' he says. ''That knowledge hasn't made it out into the wider culture.'' So what does the science say? Smiler reports in his book that about 15 per cent of young men - or about one guy in seven - has more than two sexual partners in any given year. Over a longer period the numbers fall even lower: barely 5 per cent of guys have more than two partners per year for three years in a row.

These are hardly Casanova-level conquests. In survey after survey, it turns out that most guys, whether they are 15 or 50, have only a small number of sexual partners. Only a tiny minority are the ''players'' who boast Wilt Chamberlain-level bed-hopping. Errol Flynn's swordplay is the exception, not the rule. Another study of university undergraduates found three-quarters of the guys would prefer dating - with its higher levels of commitment and emotional closeness - to the sexual excitement of ''hooking up''. When asked what they wanted from a relationship, sex was not top of the list - instead they looked for qualities like companionship, support, intimacy, and passion. They wanted a partner who was funny, nice, confident and decisive - not the passive victim of the pick-up artist handbooks. Then there's the International Sexuality Description Project, a massive survey run across 62 countries by 118 researchers, which quizzed young men and women about their sex lives, opinions and desires. One of the most hotly debated survey questions asked ''ideally, how many different sex partners would you like to have?'' over a series of different time frames, from one month to the rest of their lives.

The study found that, on average, young men would ideally like nearly two partners in the next month, while women wanted nearly one partner in the next month. This pattern has been found in survey after survey, and was used to back a theory that men have evolved to instinctively want to ''spread their seed'', to give their DNA a better chance in the Darwinian fight for genetic immortality. The man-animal - a compulsive Casanova. Not so fast, says Smiler. Look more closely at the figures. ''Approximately one guy in four (25 per cent) said he wanted two or more partners in the next month,'' he writes in Challenging Casanova. ''That means the majority - 75 per cent - want one or zero partners in the next month. ''Shouldn't we take that majority as telling us about the average or typical guy? If guys are biologically programmed to want multiple partners, why is it that these guys - undergraduate men who are at or near their physical and sexual peak, who are rarely married, who mostly live away from their parents in dormitories, and who are usually surrounded by lots of unmarried, healthy women - overwhelmingly say they're looking for only one partner when completing anonymous surveys?''

There's plenty of other evidence that young men are becoming more, rather than less, emotionally mature - and less, rather than more, sexually promiscuous. Earlier this year, sociologist Amy Schalet wrote in The New York Times that the ''sluts and studs'' labels were losing their power. In the past two decades, the proportion of American adolescents in their mid-teens claiming sexual experience has decreased, and for boys the decline has been especially steep. Today, though more than half of unmarried 18 and 19-year-olds have had sex, fewer than 30 per cent of 15-to-17-year-old boys and girls have, down from 50 per cent of boys and 37 per cent of girls in 1988. Theories abound. One is that sex-ed has succeeded in scaring boys about the chances of becoming a teen dad, or getting an STD. Another is the global economic downturn has made young people more pessimistic about the future, and scared of the financial consequences of having a family. But Schalet said fear was probably not the only reason for gender convergence. She pointed to a series of surveys and interviews by sociologists, reported in 2006, that found teenage boys to be just as emotionally invested in their romantic relationships as girls. ''Such romanticism has largely flown under the radar of American popular culture,'' she wrote. ''Yet the most recent research … indicates that relationships matter to boys more often than we think.'' For example, one survey found that of young males between 15 and 19 who had not yet had sex, more said it was because they ''hadn't met the right person'' than reasons of religion or morality.

''Boys have long been under pressure to shed the 'stigma of virginity','' Schalet said. ''But maybe more American boys are now waiting because they have gained cultural leeway to choose a first time that feels emotionally right.'' Of course, even the modern young male is no saint. The long-standing rule that men are more likely to cheat on their partners than are women seems to still hold. Men have, on average, a higher sex drive. But ''dating'' seems to have fundamentally changed from the rigid, socially defined process it used to be. ''Part of the shift is that most teenage boys and young men today have female friends, and have always had female friends,'' Smiler says. ''We have lots more hanging out in mixed sex groups than we used to before. So instead of going on a first date or a second date with somebody, you just 'hang out'. ''It's 'let's hang out and have coffee', 'let's hang out and watch a movie'. But that hanging out often replicates what might have happened as, say, a first or second date 30 years ago.''

This partly explains the so-called ''hook-up culture'' that has some cultural commentators convinced that universities have turned into unbridled sex romps. The alternative explanation is that men and women spend more time together socially, so the lines between a date and a casual hook-up have blurred. Smiler says that, in the face of all this evidence, we need to re-educate ourselves and our children. There is no one description of male sexuality. There are romantic men, emo men, religious men, and they want different kinds of sexual relationships with different kinds of partners. And, yes, some want lots of sex with multiple partners. ''The Casanovas do exist,'' says Smiler. ''But they're the minority. Most guys are not out there trying to sleep with hundreds, or scores, or even dozens of women. Most guys really are choosing to have sex with people they know and people that they are often in some kind of relationship with.'' One of the big problems with the current situation, he says, is that young men feel pressure to behave in ways that they don't really want to - and the behaviour they are being encouraged into is risky. But it is also about simply acknowledging the truth.

''[The Casanova Complex] gives guys a message that they can't really be their authentic selves,'' says Smiler. ''It also produces a problem for girls, because we tell them that all guys just want sex, they're not interested in relationships. And from there, OK, so if you're a 13-year-old girl and you're trying to get your first boyfriend and you don't have a lot of experience, maybe what you need to do is 'sext' him a half-naked picture of yourself, because what you've always heard is that guys are only interested in girls for sex. ''So we present the guys with an incorrect version of what most guys are like and it distorts how they behave, and we present girls with that same bad information and it distorts how they behave.'' Smiler wants changes to sex education curriculums, and to Hollywood scripts. He wants us to raise boys who know it's OK to want sex in the context of a relationship, rather than just for its own sake. He wants conversations, not just about the importance of using a condom, but about the difference between being a ''friend with benefits'' and a girlfriend or boyfriend. ''We really leave them to sort it out by themselves,'' he says. ''There's no other part of their life where we just give them some information and leave them to figure it out.

''We routinely ask our teenagers, 'What do you want to be when you grow up?' We have that conversation again and again. So why we don't have those kinds of conversations about what it means to be sexual is kind of beyond me.''