Health: Revealed: the scientific basis for falling in love: A study of identical twins and their partners offers a disturbing picture of how we choose a mate, says Jerome Burne

AS USUAL, Shakespeare said it best. Here is Oberon bewitching Titania in A Midsummer Night's Dream: 'What thou seest when thou dost wake, do it for thy true-love take; Love and languish for his sake'. What makes Titania's subsequent infatuation with Bottom so delightful is that we can all recognise the blind desire for someone that everyone else can see is totally unsuitable.

Most of us are comforted by the belief that, however powerful it feels at the time, infatuation is a condition we can recover from - and that when we make a serious choice of mate we are influenced more what we have in common than by blind passion. But David Lykken, professor of psychology at the University of Minnesota, has just completed a research project that seems to show that the Titania syndrome is actually how we all choose our marriage partners. We may think we have some control over our choice, but Professor Lykken believes that while we may not be at the mercy of vengeful faeries, we are all equally in thrall to our genes when it comes to mating.

He argues that, just as newly hatched ducklings are genetically programmed to treat as a mother anything large that walks in front of them in their first few days, so we have a genetic programme that switches on at key biological times in our life - making us fall for the first remotely suitable person we meet.

The theory is based on his new and remarkable findings about the husbands and wives of heterosexual identical twins. It might seem reasonable to suppose that twins, who share the same genes and very much the same upbringing, would fancy similar people. After all, identical twins have been found to have almost everything else in common: although there are minor variations, they look almost the same, have very similar IQs and personalities, and make the same choices in clothes, interior decor, leisure pursuits and even holidays.

But for his new study, due to be published later this year in the Journal of Personality and Social Psychology, Professor Lykken questioned more than 900 pairs of twins and their spouses about their love life, with surprising results. It turns out that the partners of pairs of twins are on average barely more alike than any other two adults passing in the street. Not only that, but 40 per cent of twins actually dislike their sibling's mate.

Just as one might expect twins to fancy similar people, so one might expect their partners to feel a parallel attraction: that anyone who fancied one twin might also be expected to fancy the other. Again, Professor Lykken has found this not to be true. In most cases, one twin did not particularly like or dislike their twin's mate; and the partners of twins did not like or dislike the twin any more than someone might like the partner of a friend. 'As far as we can see,' says Professor Lykken, 'whom people fancy and then marry is not governed by any logical rules. You could say that this was good evidence for Cupid and his arrows.'

While psychologists still admit defeat when it comes to explaining why one person seethes with lust for A and remains unmoved by B - who is better looking and altogether more suitable - there still exists a strong belief that the main reason why people get married is because they are similar in outlook and have a lot in common.

But Professor Lykken's study found that the similarities between each twin and his or her partner were smaller than expected. Having things in common 'still leaves an awful lot unexplained,' he argues. 'At best, similarity in things like age, education, attractiveness and political leanings narrows the field of suitable candidates down by about 50 per cent. But that still leaves almost half the eligible population as possibilities.

'Given that people usually surround themselves with people who are broadly similar anyway, how do they choose Jane instead of Gill, or John rather than Henry? If similarity really was so important you'd expect the spouses of twins to fancy their sister- or brother-in-law, and they don't'

This disturbing picture of people having no conscious control over whom they love or loathe comes as no surprise to Julian Boon, lecturer in personality at Leicester University; he believes that when it comes to relationships, basic evolutionary principles are at work. 'All men want the same things when selecting a mate. They want a girl who is young and in good health and that's it; all the rest of the stuff about similar interests in fishing or sharing political views is just ribbons and bows. Women, on the other hand, want a man who is powerful but also sensitive.'

He also believes that there is a powerful sexual pecking order. 'If there are four blokes at a disco and there are four girls dancing round their handbags, the strongest and warmest male will get off with the most attractive female, the next with the next, and so on.'

But if evolutionists can greet Professor Lykken's results with equanimity, they pose rather more of a problem to the social psychologists, who are wedded to the idea that our upbringing has some effect on whom we lead to the altar. Maryon Tysoe, author of Love Isn't Quite Enough: the psychology of male-female relationships, points out that even identical twins have differences in outlook and personality and suggests that those could account for Professor Lykken's findings that they choose different types of partner.

Duncan Cramer, senior lecturer in social psychology at Loughborough University, admits that the findings were surprising but makes the point that when people choose between two equal things, they will tend afterwards to come up with a reason why the one they have chosen was the better. 'I would expect the spouses of twins to say that their twin was the one they wanted as opposed to the twin'.

Of course, if we are all in the grip of the Titania syndrome it will be no surprise to the poets, who have always described love as blind, mad or a fever. The moral implications of the imprinting theory were considered in 1579 by John Lyly: 'I am of this mind that both might and malice, deceit and treachery, all perjury, any impiety may lawfully be committed in love which is lawless.'

The only modern twist is that billionaire status is assured to the first person who discovers how to turn the love gene on and off.

(Photograph omitted)