It is brave for a woman in her 60s to claim to be an expert on the men two generations down. Being nearly such a man myself, I would certainly hesitate – perhaps permanently – before publishing my impressions of Woodstock or how to cope when the kids leave home.

Kay S Hymowitz, however, is a senior fellow at a conservative American thinktank called the Manhattan Institute, so it is her job to have opinions. And in Manning Up, quite rightly, she has decided to say what she believes: that the developed world is now witnessing "a male culture in profound decline". It is the book's ideas, after all, and not the author's identity, that should be decisive. And they are.

Manning Up sets out the view that, thanks to women's increasing success in the knowledge economy, a young man leaving university today does so "with the distinct sense that he is dispensable, that being a guy is a little embarrassing and that given his social ambiguity, he might as well just play with the many toys (and babes – he hopes) his culture has generously provided for him". But as time moves on, says Hymowitz, these men don't. We get stuck, it seems, in the cosy and well-catered furnishings of our inadequacy.

Here is my main problem with this book: it's balls. On the first page, as Manning Up invites us to recognise that men have become more puerile, I found myself thinking: have we? This was followed by: no, we haven't. And shortly after that: how dare you! Yet all that Hymowitz can offer to get this founding premise off the ground are the collected opinions of several other women and a list of films and television shows she doesn't like.

But then this is her recurring problem: she is good on theories, lamentable on proof. Donor-insemination clinics, for instance, are said to contribute to men's sense of domestic redundancy. Yet how often do single women actually use donated sperm? Hymowitz doesn't say. It is reported that modern women let men know that they "want chocolates on Valentine's Day or, better yet, a weekend in Paris". But do they? That is the cliche; we are just expected to accept it unexamined.

Not that we need evidence, on other occasions, to see that she is wrong. The films of Judd Apatow and Maxim magazine certainly do illustrate men's love of childish entertainment (as celebrity magazines, not mentioned, do with women). But in extending the accusation to sophisticated comedies such as South Park, The Colbert Report, Futurama and The Sarah Silverman Program, Hymowitz is way off. In the same way, it sheds an unflattering light on her pop-cultural credentials when she cites the Twitter feed @shitmydadsays but calls it "Shit My Father Says". The error seems only too consistent with somebody who lists similar website names in the apparent belief that this proves the existence of a social movement, when all it actually proves is how easy it is to start a website.

To some extent, all this would be forgivable if Hymowitz were on to something. It is true, after all, that men and women are marrying later; it is certainly true that this has lengthened adolescence; and it sounds reasonable that men would be enjoying these added years of irresponsibility while women are becoming more successful at work. But are men really getting stoned, playing video games and laughing at fart gags in a defeated funk? Might they not merely be doing it because they can?

Frustratingly, there is some good stuff around Manning Up's neglected fringes. Surely the real story here is not that young men are silly (whom does that surprise?), but rather that, in the US, female graduates under 35 now outnumber and out-earn male ones. Or that parents in America have begun to favour daughters. I had no idea that things had gone so far and would love to read a different book explaining why.

Though I'm not sure I want Hymowitz to write it. Her tireless pursuit of perkiness is something one can have enough of, and it leads her regularly into inelegance and inanity. We hear that "the child-man's home sweet home media is the internet"; female ageing is said to seem "like an invention dreamed up by Dr Misogyny". Like many other books of popular scholarship, Manning Up also tends to feel like an extended sales pitch for its own neologisms. "Preadulthood", "the New Girl Order" and "the child-man", in this case, are the phrases that Hymowitz would like to see catch on. They won't, I don't think. But here is a neologism of my own: "slacademic". As is the rule with these terms, it should be self-explanatory.

Leo Benedictus's novel The Afterparty is published by Cape