WHEN physicist Niels Bohr watched westerns, he noticed that the cowboy who drew his gun first and so had an advantage, was often the one shot.

The Nobel laureate's favoured solution to his "gunslinger's paradox" has now been confirmed in part: people move faster when reacting than when they initiate the same actions.

Such reactive responses are about 21 milliseconds quicker than planned actions, according to research. It means that the gunslinger who draws last, draws faster. But it does not mean that you should wait for your opponent to move first. While drawing and shooting might take less time, any advantage is lost by the 200 milliseconds it takes the brain to notice that the enemy has gone for his gun.

"You'd still be the one who gets shot, but you'd at least die satisfied that you were faster," said Andrew Welchman, of the University of Birmingham, who led the study, which was funded by the Biotechnology and Biological Sciences Research Council.

Responsive actions are also less accurate than planned ones, so you are more likely to miss the target.

The results suggest that making swift but rough responses might have evolved as a defence against dangers such as predators or falling trees.

Dr Welchman said: "21 milliseconds may seem like a tiny difference, and it probably wouldn't save you in a Wild West duel, but it could mean the difference between life and death when you are trying to avoid an oncoming bus.

"Apparently Bohr tested his theory in toy pistol fights with a colleague."

In the new study, which is published in the journal Proceedings of the Royal Society, volunteers sat opposite one another and had to press three buttons in a particular sequence more quickly than the other.

"There was no 'go' signal so all they had to go by was either their own intention to move or a reaction to their opponent - just like in the gunslinger's legend," Dr Welchman said. When players moved first they took an average 10 per cent longer to complete the task than when they moved reactively.

It could be that brain circuits that inhibit movement, like mental brakes, are more rapidly switched off when responding to stimuli than when we plan to act. The team is now investigating whether different parts of the brain affect the responsive and deliberate actions. There is evidence that this may happen in Parkinson's disease.

These patients often struggle to initiate movement but can respond. They will struggle to throw a ball, but may be able to catch one. This could indicate that parts of the brain affected by Parkinson's control deliberate actions while less affected areas contribute more to reactive ones.

Why does the first to draw in westerns normally lose ?