New love can look for all the world like mental illness, a blend of mania, dementia and obsession that cuts people off from friends and family and prompts out-of-character behavior - compulsive phone calling, serenades, yelling from rooftops - that could almost be mistaken for psychosis.

Now for the first time, neuroscientists have produced brain scan images of this fevered activity, before it settles into the wine and roses phase of romance or the joint holiday card routines of long-term commitment.

In an analysis of the images appearing today in The Journal of Neurophysiology, researchers in New York and New Jersey argue that romantic love is a biological urge distinct from sexual arousal.

It is closer in its neural profile to drives like hunger, thirst or drug craving, the researchers assert, than to emotional states like excitement or affection. As a relationship deepens, the brain scans suggest, the neural activity associated with romantic love alters slightly, and in some cases primes areas deep in the primitive brain that are involved in long-term attachment.

The research helps explain why love produces such disparate emotions, from euphoria to anger to anxiety, and why it seems to become even more intense when it is withdrawn. In a separate, continuing experiment, the researchers are analyzing brain images from people who have been rejected by their lovers.

Men are from Mars and women from Venus - except when they are in love. During this intense period, men and women become more like each other than at any other time.

We already know that falling in love is a bit like going crazy. Donatella Marazziti of the University of Pisa in Italy showed in 1999 that levels of the neurotransmitter serotonin, which has a calming effect, dip below normal in those who say they are in love as well as in people with obsessive compulsive disorder. Both groups spend inordinate amounts of time obsessing about something or someone (New Scientist print edition, 31 July 1999).

Now Marazziti has looked at the hormonal changes that occur in people who are in love. Her team measured the blood levels of several key hormones in 12 men and 12 women who said they had fallen in love within the past six months. The researchers compared these hormone levels to those in 24 other volunteers who were either single or in stable long-term relationships.

The first finding was that both men and women in love have considerably higher levels of the stress hormone cortisol, indicating that courtship can be somewhat stressful. "But the most intriguing finding is related to testosterone," says Marazziti.

Split the difference

Men who were in love had lower levels of the male sex hormone testosterone - linked to aggression and sex drive - than the other men. Love-struck women, in contrast, had higher levels of testosterone than their counterparts, the team will report in Psychoneuroendocrinology.

"Men, in some way, had become more like women, and women had become like men," says Marazziti. "It's as if nature wants to eliminate what can be different in men and women, because it's more important to survive and mate at this stage."

But is falling in love really responsible for these changes? Andreas Bartels of University College London points out that the hormonal changes could just be a result of increased sexual activity. "There's a high degree of affection, but there's also, without any doubt, extremely high sexual activity," he says.

Marazziti thinks that this explanation is unlikely, however, because in her study those in the control group were having sex just as often as those in the "in love" group.

Love is blind

What is more, other studies suggest that testosterone levels in men rise as sexual activity increases (New Scientist, 27 November 1999). So if the hormonal changes were just the result of sex, testosterone levels would be expected to increase in men, rather than fall.

Converging levels of testosterone may not be the only thing that helps a man and woman overcome their differences. Other research has shown that falling in love really does make us blind to our partner's faults.

Bartels's team has found that when people look at their lovers, the neural circuits that are normally associated with critical social assessment of other people are suppressed (Neuroimage, vol 21, p 1155).

But the blissful state that is romantic love does not last. When Marazziti retested the same people one or two years later, when they said they were no longer madly in love, their hormone levels had returned to normal.

A male baboon named Sherlock sat on a cliff, unable to take his eyes off his favorite female, Cybelle, as she foraged far below. Each time Cybelle approached another adult male, Sherlock froze with tension, only to relax again when she ignored a potential rival. Finally, Cybelle glanced up and met his gaze. Instantly Sherlock flattened his ears and narrowed his eyes in what baboon researchers call the come-hither face. It worked; seconds later Cybelle sat by her guy, grooming him with gusto.

After observing many similar scenarios, I realized that baboons, like humans, develop intense attractions to particular members of the opposite sex. Baboon heterosexual partnerships bear an intriguing resemblance to ours, but they also differ in important ways. For instance, baboons can simultaneously be "in love" with more than one individual, a capacity that, according to anthropologist Helen Fisher, most humans lack.

Fisher is well known for her three previous books (The Sex Contract, Anatomy of Love and The First Sex), which bring an evolutionary perspective to myriad aspects of sex, love, and sex differences. This book is the best, in my view, because it goes beyond observable behaviors to consider their underlying brain mechanisms. Most people think of romantic love as a feeling. Fisher, however, views it as a drive so powerful that it can override other drives, such as hunger and thirst, render the most dignified person a fool, or bring rapture to an unassuming wallflower.

Neuroscience can at last explain why we can't see faults in our partners or children.

Can science help us to understand love? Many argue that a Shakespearean Sonnet, Rachmaninov piano sonata or Jane Austen novel is much better at communicating insights into why we become irresistibly drawn to one person. But now neuroscience promises to offer revealing new insights that could solve some of the mysteries at the heart of love.

A study of whether there are different forms of love has been launched by Dr Andreas Bartels and Prof Semir Zeki from the Wellcome Department of Neuroimaging at University College London. They have attempted to unravel for the first time whether the love between a parent and a child is the same as the emotion shared by lovers and whether all forms of intense attachments are basic variations on the same theme.

Every popular song is about it, half our books and films obsess over it, everybody wants it. But when we come to ask what love is, we are overwhelmed by a myriad different ideas and experiences.

On the one hand, love can lift us up; on the other, it can destroy us. The problem is further compounded because we generally also feel tremendous love for our mothers, our children, our friends - even chocolate. Or maybe especially chocolate.

How can one little word cover so many different nuances of feeling? More importantly, if love means different things to different people, how can we ever effectively communicate it?

Three passions, simple but overwhelmingly strong, have governed my life: the longing for love, the search for knowledge, and unbearable pity for the suffering of mankind.
- Bertrand Russell

"To love and be loved is to feel the sun from both sides."
- David Viscott

A human being is part of the whole, called by us 'Universe,' a part limited in time and space. He experiences himself, his thoughts and feelings, as something separated from the rest, a kind of optical delusion of his consciousness. This delusion is a kind of prison for us, restricting us to our personal desires and affection for a few persons nearest to us. Our task must be to free ourselves from this prison by widening our circles of compassion to embrace all living creatures and the whole of nature in its beauty.
- Albert Einstein

Love is nothing but a pleasurable state, joy, accompanied by the idea of an external cause.
- Spinoza

Math is like love -- a simple idea but it can get complicated.
- R. Drabek

It is easier to forgive an enemy than to forgive a friend.
- William Blake

People need love most when they deserve it least.
- Ernest Eberhard

The Good Life is one inspired by Love and guided by Knowledge
- Bertrand Russell

"The hormone best known for its role in inducing labor may influence our ability to bond with others, according to researchers at the University of California, San Francisco.
In a preliminary study, the hormone oxytocin was shown to be associated with the ability to maintain healthy interpersonal relationships and healthy psychological boundaries with other people. The study appears in the July issue of Psychiatry.

It doesn't interest me what you do for a living.
I want to know what you ache for
and if you dare to dream of meeting your heart's longing.

It doesn't interest me how old you are.
I want to know if you will risk looking like a fool
for love
for your dream
for the adventure of being alive.

It doesn't interest me what planets are squaring your moon...
I want to know if you have touched the centre of your own sorrow
if you have been opened by life's betrayals
or have become shrivelled and closed
from fear of further pain.

I want to know if you can sit with pain
mine or your own