Mourner: Social networks spread the grief

The academics, led by anthropologist H Russell Bernard from the University of Florida, have developed their formula to estimate the "ripple effect" - the impact on the friends and associates of victims - of disasters and epidemics.

Based on figures of 6,333 reported dead or missing after the New York attack; 20,000 actually present at the scene and another 50,000 who regularly visited or did business at the centre, the total number of people "involved" turns out to be at least 20.3 million.

If the academics are right, it means that around one in person in every 17 in the US knew somebody who was "directly involved" in the tragedy. And as many as one US citizen in every 140 is likely to have known one or more people who were actually killed during the attack.

The calculation is based on research that shows the average US citizen "knows" 290 others - mostly, but by no means exclusively, who are US citizens.

Degrees of separation

By "knows" the academics mean two people can recognise each other by sight or by name; that they feel able to contact each other by telephone or mail; and that they have been in contact at some time during the past two years.

UK too: Friendship networks span the Atlantic

The work is an application of the social science principle of the "six degrees of separation" which holds that no person is ever more than six personal friendship steps away from any other person.

In recent years the theory has been developed to estimate the number of people in a community likely to be affected by knowing someone in that community who is HIV-positive.

The crucial statistic is the estimated size of the average person's social network. Dr Bernard's estimate of 290 is based on extensive research and, he says, is likely to be an underestimate.

The second "degree of separation" - people around the world who "know somebody who knows somebody" directly affected by the attack - accounts for hundreds of millions of people.

Ripples fade

The multiplier effect declines with each "degree of separation" because networks of relationships start to overlap. Just like the ripples on a pond, the effect eventually dies out.

The large networks of business, communications and social contacts maintained by many of those directly involved in the WTC attacks magnifies its horrific scale even further than many people thought.

And as the ensuing anti-terror crackdown continues hundreds of millions more people are being drawn into the complex security, economic, political ramifications of the attacks.

The fact is, just five weeks after the 11 September attacks, there is scarcely a person on earth whose life has not been touched in some important way by the world's worst act of terror.