Would I lie to you?

On the matter of the correct receptacle for draining spaghetti, my husband demonstrates a bewildering pigheadedness. He insists that the colander is the appropriate choice, despite the manifest ease with which the strands escape through the draining holes.

Clearly the sieve, with its closer-knit design, is a superior utensil for this task. Yet despite his apparent blindness to the soggy tangle of spaghetti in the sink that results from his method, my husband claims to be able to observe starchy molecules clinging to the weave of the sieve for weeks after it's been used for draining pasta. We have had astonishingly lengthy discussions on this issue but after three years of marriage it remains unresolved. By which, of course, I mean that my husband hasn't yet realised that I'm right. The longevity of these sorts of disagreements is well known to us all. I can confidently predict that until somebody invents a colander/sieve hybrid, we will not be able to serve spaghetti to guests.

The writer David Sedaris, describing an argument with his partner over whether someone's artificial hand was made of rubber or plastic, also foresaw no end to their disagreement: "'I hear you guys broke up over a plastic hand,' people would say, and my rage would renew itself. The argument would continue until one of us died, and even then it would manage to rage on. If I went first, my tombstone would read IT WAS RUBBER. He'd likely take the adjacent plot and buy a larger tombstone reading NO, IT WAS PLASTIC."

What is it about brains that makes them so loyal to their beliefs? We don't seek refreshing challenges to our political and social ideologies from the world; we prefer newspapers, magazines and people that share our own enlightened values. Surrounding ourselves with "yes men" limits the chances of our views being contradicted.

Nixon supporters had to take this strategy to drastic levels during the US Senate Watergate hearings. As evidence mounted of political burglary, bribery, extortion and other hobbies unseemly for a US president, a survey showed that the Nixon supporters developed a convenient loss of interest in politics. In this way they were able to preserve their touching faith in Nixon's suitability as a leader of their country.

In other words we like evidence that affirms our pre-set worldview - and discount what doesn't. This was tested by a psychological study in which people already declared either for or against the death penalty were asked to evaluate two research papers. One showed that the death penalty was an effective deterrent against crime; the other showed that it was not. One research design compared crime rates in the same US states before and after the introduction of capital punishment. The other compared crime rates across neighbouring states with and without the death penalty. Surprise, surprise: which research strategy people found the most scientifically valid depended mainly on whether the study supported their views.

So evidence that fits with our beliefs is quickly waved through the mental border control, while counter-evidence must submit to close interrogation and, even then, will probably not be admitted. As a result, people can end up holding their beliefs even more strongly after seeing counter-evidence. It's as if we think, "Well, if that's the best that the other side can come up with then I really must be right." This phenomenon, called "belief polarisation", may help to explain why attempting to disillusion people of their perverse misconceptions is so often futile (just think "colander").

Part of this attachment may be because there is a sense in which our important beliefs are an integral part of who we are. To bid a belief adieu is to lose a cherished portion of our identity. Interestingly, people who have recently indulged in extensive contemplation of their best qualities are more receptive to arguments that challenge their strongly held beliefs about issues such as capital punishment and abortion. By hyping up an important area of self-worth, you are better able to loosen your grip on some of your defining values. Effusive flattery dulls the sword of an intellectual opponent more effectively than mere logical argument.

Flattery plays very effectively on another of our brain's constitutive characteristics: vanity. The vain brain that embellishes, enhances and aggrandises you. The vain brain that excuses your faults and failures, or simply rewrites them out of history. The brain so very vain that it even considers the letters that appear in your name to be more attractive.

Of course, the positive illusions fostered by your brain are essential to survival. Without a little deluded optimism, your immune system begins to wonder whether it's worth the effort of keeping you alive. And most extraordinary, it seems that sometimes the vain brain manages to transform its grandiose beliefs into reality. Freud suggested that the ego "rejects the unbearable idea", and experimental psychologists have since been peeling back the protective layers encasing self-esteem to reveal the multitude of strategies the brain uses to keep the ego plump and happy.

When asked, people will modestly, reluctantly confess that they are, for example, more ethical, more nobly motivated employees, and better drivers than the average person. In the latter case this even includes people interviewed in hospital shortly after extraction from mangled car wrecks.No one considers themselves to fall in the bottom half of the heap which, statistically, is of course not possible.

Likewise we are quick to assume that our successes are due to our own sterling qualities, while responsibility for failures can often be conveniently laid at the door of bad luck or damn-fool others. This "self-serving bias", as it is known, is all too easy to demonstrate in the psychology lab. People arbitrarily told that they did well on a task (for example, puzzle-solving) will take the credit for it, whereas people arbitrarily told that they did badly will assign responsibility elsewhere, such as with their partner on the task. The bigger the potential threat, the more self-protective the vain brain becomes. In a final irony people think that others are more susceptible to the self-serving bias than they are themselves.

Memory is one of the ego's greatest allies. All brains contain an enormous database of personal memories that bear on that perennially fascinating question "Who am I?", or the "self-concept". But the self-concept, psychologists have discovered, is conveniently shifting. If the self-concept you are wearing no longer suits your motives, the brain simply slips into something more comfortable. The willing assistant in this process is memory. It has the knack of pulling out personal memories that better fit the new circumstances.

Two Princeton researchers observed this metamorphosis directly, by tempting volunteers with an attractive change of self-concept. They asked a group of students to read one of two (fabricated) scientific articles. The first article claimed that an extroverted personality helps people to achieve academic success. The second article, handed out to just as many students, claimed instead that introverts tend to be more academically successful.

You can guess what's going to happen. Imagine: you're not any old vain brain; you're a vain brain at Princeton, for goodness' sake. Which-ever personality trait the students thought was the key to success, the more highly the students rated themselves as possessing that attribute.

There are two morals to be drawn. One, never trust a social psychologist. Two, never trust the brain. They both manipulate perceptions of reality, tricking us into embarrassing vanities. But don't feel angry with the brain for shielding us from the truth. There is in fact a category of people who get unusually close to the truth about themselves and the world. Their self-perceptions are more balanced, they assign responsibility for success and failure more even-handedly, and their predictions are more realistic. They are the clinically depressed.

Psychologist Martin Seligman and colleagues have identified a pessimistic "explanatory style" that is common in depressed people. When pessimists fail they blame themselves, and think that the fault is in themselves ("I'm stupid", "I'm useless"), will last for ever, and will affect everything they do. This is a far cry from the sorts of explanations that happy, self-serving people give for failure. And it seems that this pessimism can seriously endanger health. Pessimists are less likely to survive cancer, are more likely to suffer recurrent heart disease and are more likely to meet with untimely death. It may be hard to cultivate a more optimistic perspective in the face of such data, but it's worth trying.

This is an edited extract from A Mind of its Own - How Your Brain Distorts and Deceives, by Cordelia Fine, published by Icon Books