Don't be a panda in your bedroom

Couples locked together often find animal passions wane. Olivia Judson mulls advice on shaking the cage.

The other day, a friend confided that she and her husband had had sex just four times since their wedding a couple of years earlier. "I love him," she said, "but we live like brother and sister. He won't talk about it - and I don't know what to do."

One thing she could do is read Esther Perel's Mating in Captivity . Perel is a psychotherapist based in New York who specialises in helping couples whose sex life has died. For my friend is hardly alone. The dirty secret of monogamy is that many - perhaps most - couples find that, as time goes by, their sex life together withers. This is not because libido is dead. On the contrary. It's just dead with the partner or spouse. And if you don't think this matters, consider: in a recent YouGov poll for The Sunday Times , 56 per cent of divorced women and 44 per cent of divorced men said unsatisfactory sex was the reason their marriage had broken down.

Why does this happen? Why is it so often so hard to sustain desire for the same person, year in, year out? It's easy to speculate. Perhaps it has a genetic component - perhaps we find it hard because we haven't evolved to have sex with just one other person for years and years. In our evolutionary past, desire for new mates may have been an advantage. Or perhaps the present social climate is such that we have unrealistic expectations, both of sex and of marriage. Maybe we're overoptimistic about how much better sex might be with someone else. Or perhaps there are lots of factors that conspire - or that affect different people in different ways. We really don't know.

Perel offers a psychological explanation; it's as plausible as any. The essence is this. Sex is primal and dangerous: it unleashes passionate and irrational behaviour. Worse, the more in love you are with someone, the more frightening the thought of losing them. So we try to tame passion, and make it safe. Add in today's ideals of equality and intimacy - we do the chores together, we are best friends as well as lovers - and monogamy becomes monotony. In short, the problem arises from tension between what is erotic and exciting, and what is familiar, comforting, intimate, safe - and dull.

Unlike the authors of most sex-help books, Perel does not prescribe exercises to improve performance. There are no suggestions that men should drape towels over their penises and start the day with 25 cock-ups, or that women should engage in strange exercises involving ping-pong balls. This is not a book of radical new positions or amazing tantric techniques. As far as Perel is concerned, a focus on sexual prowess misses the point. Her aim is to help couples rekindle desire.

The good news is, she thinks it can be done. The bad news is, it won't necessarily be easy or comfortable, and success is by no means assured. In so far as her advice can be summarised, here it is. Preserve a sense of mystery. Celebrate your differences, don't try to fuse your lives completely. And don't take the other person for granted: keep making the effort to dress up, to flirt, to seduce. Slippers and sweatpants may be cosy, but they're probably not sexy. Be surprising. Experiment: play.

Scrutinise your own attitudes and behaviour: maybe you're the one who's not being sexy. And wonder about the other person - you may think you know them well, but do you? If you're fantasising about someone else, or even having an affair, perhaps they are, too.

To illustrate her points and her methods, Perel uses anecdotes about her clients. Here's one. Candace and Jimmy come to see her; they cuddle a lot, but rarely have sex. Perel tells them to stop cuddling. Candace protests; Jimmy chimes in that a friend called Mrs Monahan recently gave Candace a backrub - and that he'd suddenly wondered whether it mattered whether it was him or Mrs Monahan. Perel replies that the goal of therapy is going to be to "differentiate between Jimmy and Mrs Monahan".

Predictably, Perel holds to the usual therapists' belief that your current sex problems stem from how your parents behaved - a kind of post hoc explanation that I find facile and irritating. And her writing sometimes lapses into psychobabble, with talk of "interiority", of how society has become "ontologically more anxious", and of the fact that in our "adult love we seek to recapture the primordial oneness we felt with Mom". Right.

But that doesn't change the fact that, in many ways, this is a brave book.

Perel has taken on a subject that, for many people, is a source of anxiety and shame. She frankly discusses everything from why the arrival of children is so often a death-knell for sex to the possibility that, for some couples, an open marriage may be just the thing; she's not afraid to suggest that some couples may find it thrilling to play out sexual fantasies of bondage, masochism, dominance and other forms of kink. It's also refreshing to read someone who doesn't think it's always better to talk about problems and who recognises that affairs are not always symptomatic of problems within a marriage. Some will find her suggestions uncomfortable or even threatening. She implies, for example, that it might be better if men didn't watch their wives and girlfriends giving birth. But if you are bored in bed, if your lover is more like a brother, if because of that you are thinking of leaving - shake the cage.

Olivia Judson is an evolutionary biologist, Imperial College London.