Personal space: how close is too close?

It's basic etiquette - stay out of other people's personal space.

Now scientists have determined exactly how close is too close by looking at what's happening when people begin to feel threatened by someone else's proximity.

They have defined the invisible boundary that marks the outer limit of our personal space as being between 20cm and 40cm from the face.

They also found that the more anxious a person was, the larger the personal space.

Dr Gian Domenico Iannetti, one of the researchers in the study by the department of neuroscience at University College, London, said the distance appeared to be a defensive mechanism employed by the brain.

He said the findings could help the police and other emergency service personnel in responding to stressful situations and in assessing threats.

"These findings indicate that the defensive peri-personal space has a clear boundary.

"This space is very much related to defence - the point at which we start to feel threatened. It is linked to social personal space but it is not the same.

"We think the defensive space is something that we constantly update [as circumstances change]. The measures we obtained were in a friendly lab environment.

"If you are chatting to someone in your living room, you won't need a large safety margin, but if you are walking through a dangerous area the safety margin will be much bigger.

"Places like the Tube are not threatening so we are probably more accepting of being close to people there. It can be annoying but not threatening."

Invasion of personal space has become a growing problem on overcrowded railway and other public transport networks.

Most people have a limit beyond which they will feel uncomfortable but the latest research measures the point at which people start to feel threatened.

It is thought to be a cultural issue because in countries such as China and India personal space is less important.

Iannetti said he hoped to use people from a number of countries as research subjects to gain a better understand of the cultural factor in setting personal space.

In the current research, which is published in the Journal of Neuroscience, the peri-personal space of 15 volunteers aged between 20 and 37 was measured.

The researchers got the volunteers to hold a hand at various distances from their face before giving them a strong electric shock on the hand.

They assessed their reaction to the shock, which did not change in intensity, by the number of times the volunteers blinked. The more they blinked, the greater the perceived threat to their face.

The subjects were also asked to complete anxiety tests.

Iannetti and his colleagues found that, instead of gradually increasing, the personal space of the volunteers appeared to extend abruptly to its boundary.

• Debrett's, which publishes guides on etiquette, says that if you are close enough to feel the warmth of someone's breath on your face. you are too close and should discretely create space.