There have been times when it has appeared as though the best professional to help you to understand Conservative European policy would be a psychiatrist. But right now? I’d consult a behavioural economist.

Barry Legg, the head of the Eurosceptic Bruges Group, wrote a furious article after discovering that David Cameron was no longer intent on putting the Lisbon treaty to a referendum. He said that Cameron had promised that “there would be a vote on Lisbon. And now there won’t, so what’s changed? Well, nothing about Lisbon, for starters.”

But, of course, something very fundamental had indeed changed about Lisbon. Not its contents (which is what Mr Legg presumably meant), but that it had been ratified. And behavioural economists can explain why this is a very big change indeed.

A raft of experiments have tried giving people say, a free coffee mug, and then negotiating a price to buy it back. The price that owners regard the mug as being worth shoots up the moment they possess it. This is known as the endowment effect.

Linked to this effect is what economists call “loss aversion”, the identification of which helped Daniel Kahneman to win the Nobel Prize for Economics. Someone who loses £100 will feel more pain than the happiness they would feel gaining £100. Indeed, the difference between the pain of loss and the pleasure of gain is a very big one.

Now consider the Lisbon treaty.

Before ratification, people might feel anxious about its impact. They might fear loss of their liberty. Loss aversion, with all its power, would assist those opposing the treaty. But once it has been ratified? Loss aversion would work on the other side. Voters might worry that Britain’s relations with the world might be destabilised.

So behavioural economists would advise that a referendum on Lisbon held now might have a very different outcome to one held even last week. That seems like as good an explanation of Cameron’s decision not to have one, as anything else.

If you don’t want to consult a behavioural economist about Europe, why not try a social psychologist?

The founder of the European project — the banker and wartime planner Jean Monnet — had a theory. It’s been given the horrible name of neo-functionalism. But what it really meant was that the creation of one integrated European institution made, by its very existence, the case for further integration.

Monnet started with merging the coal and steel industries of Germany and France and hoped that from this he could progress to defence and then to fiscal policy and so on. It didn’t quite work out like that, but Monnet’s method is still the driver of European integration.

Neo-functionalism is just a fancy Euro-word for a familiar concept in social psychology. In his (incidentally, essential) book Influence: The Psychology of Persuasion, Professor Robert Cialdini calls it “commitment and consistency”.

He starts his account of this idea by citing a Canadian study of betting on horses. Once a bet has been placed, gamblers instantly become much more convinced that they are backing a winner.

Other studies — for instance getting residents to put up small “road safety” signs and then gradually increasing the size of the signs — show that once a commitment has been made, it is quite easy to get people to keep on increasing their involvement. And quite hard to stop it.

No wonder the Conservatives have found what they call the “European ratchet” so difficult to resist.

The BBC began a report this week on a new piece of Australian research with the words “In a bad mood? Don’t worry — according to research, it’s good for you”.

Now that can’t be right. Because according to research it, er, isn’t good for you.

One of the leading books on the relationship between your mental state and your health is Professor Martin Seligman’s Learned Optimism: How to Change Your Mind and Your Life. The author produces a raft of evidence that optimism and an upbeat way of explaining the world to yourself makes you healthier and wealthier.

He provides evidence that optimism extends your life. He has even helped insurance companies to select better salesmen by screening them for optimism, and has demonstrated that the more optimistic candidate almost always wins US presidential elections.

So what could the research referred to by the BBC possibly mean? Well, Professor Joe Forgas believes that being downbeat can lead to better decisions and clearer thinking. And this is quite consistent with Seligman. Optimists may do better and feel better, but pessimists, sadly, are more realistic. Seligman argues that this explains why evolution has allowed pessimists and optimists to co-exist.

daniel.finkelstein@thetimes.co.uk

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