"You're better looking in real life!" A spontaneous remark from a woman seeing the Liberal Democrat leader Nick Clegg in the flesh for the first time. It was a small incident that briefly brightened up the day's campaign trail, but for psychologists it also throws some light on to how voters form judgements about politicians.

The sensible way, surely, to come to a decision is read the party manifestos and assess the policies? Failing that, there's a plentiful supply of election news as well as websites promising to match your values with what the parties are offering.

But, as psychologists like to remind us, we're not rational beings. Setting aside individuals who earn a living listening to politicians (mainly journalists and economists at the Institute for Fiscal Studies), most people probably don't read manifestos or carefully digest the Today programme's interviews with politicians. Of course, many people keep across the news - but how many of us could recite each party's position on eliminating the deficit?

Trying to understand why we vote the way we do has kept sociologists busy for decades.

There are theories that we vote according to social class, tribal loyalties to a party or because of strong ideological beliefs.

It could simply be a matter of self-interest - that we'll be "better off" with one party's policies over another.

No one theory can explain the complicated business of voting preference. But now, psychologists are now wading into the debate offering their perspective.

Gut reaction

Dr Peter Bull from York University says many theories are losing relevance.

"The prime thing now is the perceived competence and perceived responsiveness of the politician," he says.

"You shouldn't underestimate looks, either. It's not about physical attractiveness, it's more about how they look under pressure. There's a crude rule of thumb operating. Voters have a gut reaction to politicians."

Why do we say we "like" or "dislike" certain politicians we've never even met? Is it relevant if a politician has odd mannerisms or a grating voice? Does it matter how a politician makes us feel? This is all the stuff of political psychology.

Dr Tereza Capelos, an expert in political psychology from Surrey University, says: "British politics is becoming increasingly personalised and presidential."

That means appearance and character play a bigger role in a voter's decision-making.

There's nothing new about image-conscious politicians, but what has changed is the amount of attention it gets.

Things were rather low-key in the 1960s and 1970s when Tom McNally was an adviser to two prime ministers, Harold Wilson and Jim Callaghan.

Now a Liberal Democrat member of the House of Lords, Lord McNally said: "I remember when I started working with Harold Wilson, we had the revolutionary idea of making sure his suit didn't look crumpled.

"Politicians mainly just turned up and made their speeches in those days."

Lord McNally says politicians are responding to new ideas about voting behaviour.

"A lot more thought goes into the psychology of a campaign, partly because we know a lot more about what makes voters respond."

Evoking fear

Many top sportsmen and women employ a sports psychologist. Lord McNally wonders if politicians will follow suit - and have a psychologist on the team.

Dr Martin Rosema, co-editor of the journal Political Psychology, says American academics are already advising political parties.

An experiment in the US monitoring the brain activity sparked by politicians found that Republicans tended to evoke emotion much more than Democrats.

Dr Rosema says: "If you evoke fear in people about something they will start to pay closer attention to it.

"So for example, if people start to get worried about the SNP, they will seek information about them. Good politicians know these things by instinct."

If British politicians did decide to consult Dr Capelos, she would offer this advice: "Ensure that you make the safe choice. Cater for the average voter. Don't invite a lot of surprises. Safety and comfort is always where human beings try to be."

Easy on the eye

She adds, though, that going for the safe choice can create a culture in which, to a casual observer, politicians look and sound the same. Think of our main party leaders - Cameron, Clegg and Miliband. In appearance terms, they're cut from similar cloth - tall, pleasant-looking and in their forties.

As Lord McNally notes: "It makes sense for your lead candidate to be easy on the eye so he can get the message over. If Eric Pickles wanted to run for leader nowadays his shape would be held against him."

How do politicians - beavering away at constituency casework and legislating in Parliament - feel about the suggestion that voters care about their appearance?

Graham Allen, a Labour MP for nearly 30 years, ruefully admits that appearance does count. He reveals that people in his area often comment on how smartly dressed he is.

He says: "Image is very important because people take in an immediate perception and it's important that you ensure that perception isn't negative."

If politicians can project an image people feel comfortable with, he says, they stand a better chance of being listened to.

Mr Allen agrees the UK is becoming increasingly presidential - much to his irritation - and that what actually counts isn't his well-cut suit but the appearance and personality of the party leaders. This is borne out by his experience campaigning.

"People don't necessarily talk to me about policies. They want to talk about what the media are saying about Miliband, Cameron and Clegg."

Competence and warmth

When it comes to personality, Dr Capelos says voters are mainly looking for competence - but also integrity, leadership and warmth.

Don't worry if that doesn't sound rational enough for the tough world of politics, though.

Dr Capelos reassures us that when it comes to processing information, how a politician makes voters feel is just as important.

She says: "Thinking and feeling go hand in hand. In any decision-making - including political decisions - how people feel about the information they're being given is important.

"Emotions help people make decisions."

The former Conservative minister, David Willetts - nicknamed "two Brains" for his intelligence - recognises that policies don't always hit the spot. He says one of the greatest assets to the Conservatives is the character - notably the leadership skills - of David Cameron.

He says: "It's an unpredictable world. What people are trying to do is assess someone's character, how they respond to unexpected events. That's a perfectly reasonable aspect of voting."

Or as Dr Capelos would put it, it's about who makes us feel safe.

Bad news sticks

For a politician struggling to come across well, the good news is that initial impressions can be shifted.

The bad news is that negative information tends to stick in voters' minds more than positive information.

That's because, Dr Capelos says, we're psychologically hardwired to look out for potential threats.

There's an added complication for politicians to contend with. According to Dr Capelos's research, a lot depends on the type of voter we're talking about.

The people who know a lot about politics - a group she calls sophisticates - will take facts and policies into account. The might even read the manifestos. People who are less engaged will form a view based on feelings and general impressions.

So the message seems to be that political parties need to sharpen up their policies for the Today programme and the Institute for Fiscal Studies, but for everyone else it's about projecting a competent - yet warm - personality. That's a tough call for an election strategist.

Elections, with the relentless media focus on politics, do provide more time and space for politicians to convey their personalities - for better or worse.

Mr Allen reckons it has benefited Ed Miliband. "Now he has been on TV and hasn't been mediated as much, people are saying he seems like a decent guy, that he can stand up for himself."

And Lord McNally reckons his man, Nick Clegg, has put himself in the right place. "In terms of psychology Nick has clearly positioned himself as the reasonable guy, in the middle."

Dr Bull has been watching how the leaders project themselves with great interest.

The leader who stands out for him is the SNP's Nicola Sturgeon.

He says: "She deals with awkward questions by rephrasing the language of the question. She has softened her image. She is honest and direct."

Ms Sturgeon, and UKIP's Nigel Farage, have received a lot of attention. Could it be because they seem a bit different - may dangerous, even?

Dr Capelos concedes that while it's generally safety first for voters, being too boring can work against you too - so add a bit of excitement and a dash of hope.

When it comes to the vagaries of human nature, as we all know, it's difficult to get it right.