Behavioral economists say financial bubbles can create an emotional high that turns into a irrationally deep low when the bubble pops -- people begin to ignore fundamentals and have only negative associations with certain investments.

"Something like that is now happening with the euro (zone) where it has become contaminated on a psychological level and hated," said David Tuckett, psychoanalyst and author of "Minding the Markets."

"It actually is quite a good analogy to think of if your girlfriend has an affair or something. It's actually very difficult to get back (together), although it can be done."

Tuckett and other proponents of "emotional finance" -- the intersection of managerial finance and investor psychology -- say in periods of crisis following sustained growth, fear and panic can spread like a virus.

"Sometimes just a little trigger, quite often trivial, can lead to a tipping point unraveling the whole thing usually very quickly. It starts moving in one direction, then panic takes over ... basically driven by herding behavior," said Paul de Grauwe, professor of international economics at the University of Leuven and an expert on monetary union and European debt.

In the case of the euro, the prospect of Greek default was enough to take Greece's debt problems off years of slow boil and eventually prompt Europe to think the unthinkable - breaking up the euro zone.

To emotional finance experts this makes sense; like the example of the troubled relationship, at a certain point everything associated with a former love begins to seem tainted.

Emotions are a way people can tip the scales in a dilemma between two unknowns, and changes in these emotional states drive events like the euro zone crisis, Tuckett said.

"In order to make decisions in a world of uncertainty, if it's falling in love with something or making a big investment or buying a house, your emotions have to be engaged in that decision because it cannot be done on rational grounds alone."

THE PLEASURE PRINCIPLE

According to psychologists the brain's emotional workings can be divided up into three systems: wanting, attachment and anxiety.

Wanting is what psychologist Sigmund Freud called the pleasure principle, and is reinforced by the production of the neurotransmitter dopamine, a highly addictive chemical.

Attachment produces something different -- oxytocin, which is associated with the pleasure received from being part of a relationship, even fleeting ones.

Tuckett says that this same type of attachment pleasure is derived by holding onto investments and decisions.

"There was euphoria about the euro zone like there was during the bubble in the 1990s -- people didn't see the weaknesses," said de Grauwe, author of the authoritative "Economics of Monetary Union" textbook.

"When you are euphoric you are blind. It's like being in love -- you are blind."

The anxiety that unpredictability and uncertainty produces is not appreciated fully enough, said Richard Taffler, a professor of finance and accounting at Warwick Business School.

When investors lose confidence in their ability to know the direction of the market, they turn negative and stop believing in their data, he said.

"It's a cauldron of emotions we're dealing with -- we're saying the markets want this or that or borrowing rates are this or that, but fundamentally it's all about anxiety and lack of trust."

Peter Ayton, professor of psychology at City University London, says that this has been deemed the ostrich effect -- investors bury their heads ignoring fundamentals and just go on emotion.

"You don't want to look when you know that the news is probably bad ... It's not that you don't even sell or trade, you just don't look at what's happening."

The idea of waves and fears dictating market behavior is nothing new.

Economist John Maynard Keynes wrote about animal spirits of optimism and pessimism and Friedrich Hayek wrote in 1945 of the marvel of a market that enabled disparate groups of investors to make correct actions based on limited information.

"It's a combination of animal spirits that you will find everywhere, and a combination of structural weakness in the euro zone that makes governments very much like banks," said de Grauwe.

One senior economist at an international organization said the only sure way to stop the virus spreading was for governments -- and specifically the European Central Bank -- to take decisive action.

"The most serious source of contagion is (the lack of) policy responses," the economist, who asked not to be named, told Reuters.

"Unless you have a very immediate and clear line, it can fester for quite some time."

(Editing by Sonya Hepinstall)

• A Permanent Economic Emergency