He had it all: champion surfer, skilful guitarist and insouciant owner of a snazzy Japanese convertible with electric windows - cousin Matthew represented the paragon of manhood. Yet I was a little nonplussed when, for my 12th birthday, he handed over what was quite clearly a book.

Once unwrapped, however, its title intrigued: Everything Men Understand About Women. I opened it. Every single page in the book was blank. Matt thumped me on the shoulder and hooted but, as a naive late developer, I didn't really get the joke. Then, quite some time afterwards, I did. Ha. Women, eh? What can you do?

A preparatory text that equips adolescent males for their romantic negotiations with the opposite gender would be very handy indeed. I'm thinking a straightforward primer stressing that, for the next few years, they will be chasing girls who are themselves only barely up-to-speed with their sudden, devastating ability to transfix, and who can - especially en masse - be a bit cruel to boys on occasion. It would urge them to acknowledge the stark fact that young girls often prefer older boys - but add, tantalisingly, that in about twenty years' time this will start working in their favour. Then there would be a soothing epilogue that urges them not to take it too personally, never to generalise about an entire gender because of the behaviour of one individual, and always to resist any temptation to pen poetry, post her a barrage of mix-tapes or become unhealthily keen on heavy metal as a consequence of rejection.

What most young men want to read, though, is a to-the-point manual that guarantees sure-fire seduction. Preferably spectacular, but seduction of any kind will do.

It is cover-lines that promise just this, a former men's magazine editor cheerily attests, that have proved one of the most effective editorial techniques for shifting issues over the years. One day the magazine guys might even crack it, and rustle up a fumble formula that works - but for what reason would any self-respecting 15-year-old buy Zoo, Nuts or FHM then? By the time we've grown up, been around the block a bit and find ourselves in proper, upstanding 21st-century relationships complete with mortgage anxiety, mutual fatigue and early-onset middle-aged spread, it seems self-evident that, whatever FHM touted, there is no Da Vinci Code for the dames. If there really were some hieroglyph that, once discovered and decrypted, could magically transform her from het-up harridan to hot-to-trot hula-girl, we would have unearthed it by now. The pages of Everything Men Understand About Women will, surely, remain blank for ever.

Well, hold on just a minute there, gents: switch off Top Gear, stop scratching and focus. I have discovered what I believe to be the greatest storehouse of knowledge into the Inner Life of Woman known to Man. And it is but a few keystrokes away. What is the name of this holy grail? Thanks to my beloved fiancée - who has yet again failed to shut down my laptop - I can tell you: mumsnet.com.

What you have here are thousands of highly intelligent women let loose online to enjoy one of their favourite pastimes; caustic, no-holds-barred conversation. Because they are all anonymous, they really do let loose - and not just about mastitis, mood swings and other mum-specific topics. They talk about us, their BFs (boyfriends), DPs (dear partners), and DHs (dear husbands), too - quite a lot. It's like watching an episode of Sex and the City, but one that hasn't been co-written by a man. You have to get used to those acronyms. Sex, for instance, is sometimes unsettlingly referred to as BD (baby dancing).

Persist, however, and you soon build up an extremely valuable sketch of their concerns. Most dramatic are the threads in which posters discover their DP has an OW (other woman). Fellow mumsnetters never fail to rally round, but the devastation of the wounded is laid painfully bare. A useful aside is that, although these marriage-wrecking affairs are sometimes discovered via the DP's mobile phone or internet history, the discovery is often accidental, not through snooping. Mumsnetters regard men who stray with weary scorn.

Men who stay, however, are loved. Many mumsnetters have highly developed libidos and want to exercise them. They fondly put up with many of their partners' various idiosyncrasies; even serial forgetfulness (such as that displayed by the man who went out to buy Wellington boots and returned with an accordion), men who repeat the same lame jokes for years on end, and men who love Bob Dylan too much.

Other male behaviours are less tolerable; leisurely reading on the lavatory is simply mystifying, while leaving wet towels on the bed is borderline offensive. Failing to talk to your partner and leaving her to deal with most of the drudgery - especially without giving much affection in return - will slowly but inevitably suck all joy from the relationship. Many mumsnetters strain to understand their DPs and support them through their various midlife woes. It's the DPs who don't return the favour who seem to fall short most often and, presum-ably, end up angry, anti-feminist and alone.

Although it is a bit of a laugh, the notion that men are biologically doomed never to understand women is a very dangerous canard indeed, because it hands us an easy excuse to stop trying. And reading mumsnet reveals that many women want from their relationships exactly what many men do: love, sex and attention.

So, Matthew's present fell a bit flat in the end. Although he did give me some truly useful information at around the same time. Driving through Sydney one day in that fine car of his, he gently suggested something that I'd never really heard of before: wearing deodorant. No wonder he'd kept the roof - and those electric windows - down.

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