WHAT turns smouldering discontent into direct action? Why does one protest over fuel prices fizzle out like a soggy firework while another engulfs the country in days?

And why do some consumers rush to stock up for a lengthy siege at the first sign of shortages, ''just in case'', while others are as prone to panic as musicians on the Titanic?

As Scotland ground gracelessly to a halt yesterday, academics who managed to reach their desks were making a psychological assessment of the action so far.

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Dr Geoff Scobie, a senior psychologist at Glasgow University, says the turning point in the current dispute was last week's capitulation of the French government.

''The initial British petrol pump boycott didn't take off because people simply didn't believe it would work. But after the French action, farmers and truck drivers here started to think: 'If they can do it, so can we','' he said.

Dr Sally Hibbert, lecturer in marketing at Strathclyde University, believes media speculation that British workers ''didn't have the stomach for such action here'' may have actually helped the protest to leapfrog the Channel. ''It was like waving a red flag,'' she said.

It gave the FFAUK (Farmers For Action UK) just the opportunity its leaders had been looking for, despite denials from leader David Handley that his blockade of Shell's Stanlow refinery in Cheshire last Thursday was ''copy-cat action''. By yesterday, he was admitting the action had spiralled far beyond his control.

''The action has spread quickly because it has strong public support. A lot of people are aggrieved about the level of tax on petrol and feel the protest is justified,'' said Dr Scobie.

This in turn has given those whose livelihoods depend on fuel - farmers, truckers, taxi drivers - the impetus that catalyses resentment into direct action. Then the psychology of crowd behaviour takes over, enabling people to in-dulge in actions they would never consider taking as individuals.

''Individuals rely on the anonymity of the crowd. It's not them doing something, it's the group. That's what turns a group of people into a lynch mob,'' Dr Scobie said.

The danger of the authorities reacting to such a situation is that any heavy-handedness is interpreted as retaliation and this is likely to produce an escalation in the action.

On the other hand, the Government may be relying on support melting away when services such as school buses, home care visits, and routine operations are threatened.

''As soon as they start to suffer personally from the action or see others suffering, self-interest tends to intervene,'' said Dr Scobie.

Meanwhile, the impact of their action has been massively magnified by another psychological phenomenon - panic buying.

Yesterday there were already indications that some shoppers are starting to hoard essentials such as bread and milk, in case fuel shortages start to bite into food distribution.

The pumps would still be flowing in garages across Scotland if car owners had not rushed to them to fill their tanks at the first sign of trouble.

Dr Hibbert divides the queuing motorists into two groups: those who need fuel for their work or because of caring commitments or where they live, and panic buyers.

''The first group have made a risk assessment and have worked out the consequences of running out. The second group are acting at least partly irrationally. These are the ones who see a crowd running and run after them, without taking time to make a considered decision.

''Some people are more risk-averse than others. They make a selective perception process. While others pick up all the information and make a balanced judgment, the panic buyers just draw off the information that fits with their perception of how bad the situation is.

''If you're a worrier, you pick up on all the worrying bits of information.

''Older people tend to be more risk-averse, and so are people who have experienced this kind of suffering before.''

One group particularly vulnerable to panic buying are likely to be those who remember shortages and rationing during and after the Second World War.

At the other end of the scale from the hoarders and panic buyers are those who, for moral reasons, refuse to change their usual shopping habits.

''Their motivation is governed by the welfare of the group or wider society as opposed to the individual,'' said Dr Hibbert.

''So the gap between picking up the information and taking action varies enormously.

''But once you see everyone else doing something, it becomes progressively harder to maintain a selfless attitude,'' added Dr Hibbert.