IT'S one of the most horrific tragedies the world has witnessed, but the terrorist attack to pit America in a battle against "evil" has also revealed human nature's brightest side.

When America came face-to-face with its human tragedy its citizens pulled together in a spirit of camaraderie and generosity.

New York's restaurants sent food to rescue workers, while its residents welcomed the shell-shocked into their homes and answered urgent calls to donate blood until the city's blood banks were full.

Like the famous Blitz Spirit, which came to life during the Second World War, the Big Apple's infamous hard underbelly seems to have softened with the terrorist attacks on the twin towers, the Pentagon and in Pennsylvania.

And the change was not just in the US. In Britain too, the murder and mayhem ignited a mood of mourning and a palpable desire to reach out and help those both here and across the Atlantic whose lives have been blighted by those awful events.

London's fire commissioner, Brian Robinson, announced a fund for the American families of dead fire crew, heart-rending messages of condolence from "cockney friends" were posted outside the capital's American Embassy and radio stations played sombre music to mark the tragedy - the significance of which is being compared with Pearl Harbour.

The Queen, too, ordered an unprecedented tribute, a special Changing of the Guard outside Buckingham Palace, to all the victims of the suicide pilots. And a three-minute silence took place across Europe yesterday, organised by the European Union.

But these acts of kindness - some small others large - arenot only borne out of altruism but a far more complex set of influences that alter human behaviour during harrowing times, according to experts.

Peter Hodgkinson, a crisis psychologist, says: "It's easier to help others then face your own problems. When we help other people we are also helping ourselves.

"By offering assistance you are making yourself feel useful and are overcoming an awful sense of powerlessness. To sit by, in the face of disaster, is to be overwhelmed by it."

We are often motivated by previous experience, he says. "If you've recently lost a family member from cancer you might act differently because you know how people need to be treated and how you yourself would have liked to be treated."

During war, the mental health of a population is known to improve, but as soon as the disaster subsides problems shoot-up again. And on Wednesday, there was reported to be a drop in the numbers of patients at British surgeries. The devastation put minor ills into perspective.

Christine Kalus, a consultant clinical psychologist, says disasters push human behaviour to extremes, which is why we often see looting and pillaging as well as charitable acts.

She says kind acts are sometimes borne out of a desire to give meaning to all our lives, which suddenly seem meaningless and fragile when tens of thousands of people could have been wiped out in minutes.

"Extreme circumstances bring into consciousness just how vulnerable our lives are. They show what a tenuous link there is between life and death.

"You live your day-to-day life expecting your family to come back. When something like that happens you realise that the world is not such a safe place. Ordinary acts of kindness become extraordinary acts when you realise the frailty of human life.

"We feel we have no control over the situation but we want to do something. People often say 'I feel so helpless, there is so little I can do' so they will do good things to regain some sense of control and overcome feelings of helplessness.

"They contribute blood, make donations, and give coffee and food to feel they are part of what's going on and to get some meaning and make some sense from what has happened.

"You couldn't stop the plane flying into the towers but by giving some blood or setting up a coffee stall you feel you can make a difference to half a dozen lives," she says.

Christine says goodness also breeds goodness, so that if one person does something charitable others will follow.

And camaraderie helps individuals get over their own personal loss. "If you've suffered some personal individual bereavement, these acts give you a sense of being connected to other people again, and you can start to move on," she says.