A forty-year old psychology experiment reveals why we should be careful about what we think we know.

Name of concept: Confirmation Bias.

Where did it come from: Psychology, philosophy of science.

Who came up with it: Peter Wason.

What it is about: Human beings are easy to fool. Almost all of us select information that confirms what we think we know, and downplay or ignore information that conflicts with our beliefs. People often remark how extraordinary it is that someone telephones them just as they were thinking about that person, but forget all the times when the telephone did not ring. Scientists call this confirmation bias.

Again, unwise political leaders tend to make decisions by selecting information that supports their beliefs and ignoring data that does not. It seems clear that many of those that decided to go to war in Iraq were victims of this syndrome. The US administration placed great significance on information that suggested they should go to war, such as apparent weapons of mass destruction programmes and links between Iraq and international terrorist organisations, but sidelined information that suggested otherwise.

Philosophers and scientists have long been aware of the danger of this process. George Bernard Shaw once remarked: "The moment we want to believe something, we suddenly see all the arguments for it, and become blind to the arguments against it."

Karl Popper, the mid-twentieth century philosopher, said: "it is easy to obtain confirmations or verifications for nearly every theory – if we look for confirmation". Popper's main contribution to scientific philosophy – that no hypothesis can ever be proven, only disproven – helps scientists to guard against confirmation bias. Instead of an experimenter having a theory and seeking to prove that it is true, their task is to test a hypothesis to see whether you can disprove it.

Peter Wason, who coined the phrase 'confirmation bias', conducted an experiment, published in 1960, that illustrates how susceptible people are to this approach. In the test, he asked the participants to discover a rule that makes sequences of three numbers. To discover the rule, he gave the participants an example of a sequence that fitted the pattern, 2-4-6, and asked them to produce their own sequences. He would tell them whether their guesses tallied with the rule or not. Most of the participants guessed that the pattern was 'goes up by two', and produced sequences like 8-10-12 or 233-235-237.

However, though the students with the hypothesis 'goes up by two' thought they had the right answer because Wason approved all their suggestions, they had got it wrong. The correct hypothesis was 'any ascending sequence'. Only a small minority of participants in this experiment ever find the correct rule because few participants seek to disprove their initial hypothesis.

What is it good for: Knowing about confirmation bias is a good way to look smart if you ever find yourself in a psychologist's experiment, but it has many other uses. Being aware of one's own confirmation bias helps us all guard against poor decision-making. In a sense, the human mind is just a hypothesis-testing machine, so having an insight into how it works is useful.

Confirmation bias is rife in the world of politics, all the way from the top to the bottom. While Prime Ministers and Presidents often make poor decisions based upon flawed hypotheses, people that write about politics – and their readers – are often just as guilty of capitulating to this bias themselves.

Reactions to the war in Iraq provide a good example of this process. People that had strong opinions about the war quickly highlighted the information that supported their own hypothesis and ignored any alternative facts.

Commentators and bloggers that supported the war said the chemical plants, dodgy-looking munitions and apparently incriminating papers were strong evidence that backed up their opinions. On the other side of the debate, the Americans' takeover of the Iraqi oil ministry, the civilian casualty figures and the lack of substantial finds of unconventional weapons supported the beliefs of those that opposed the conflict.

Similarly, news outlets, as ever, tended to prioritise stories that justified their existing positions. The audiences of Fox News and Al Jazeera ended up seeing entirely different wars.

Again, the tribalism of party politics reflects the workings of confirmation bias in the national political arena. Speak to a committed party follower and you will find that any sins of their own party are entirely excusable or relatively insignificant, while the actions of their opponents are incredibly important and impossible to ignore.

Such processes work amongst single issue campaigners too. Speak to an environmentalist, for example, and you will find that every environmental threat is likely to end the world as know it, whereas other dangers, such as terrorism or economic collapse, are relatively insignificant.

Right from childhood, people theorise about the world; these theories shape the way that we understand new information, particularly events, during our lifetimes. Depending on your hypothesis, banging your thumb whilst building a flat-pack piece of furniture might just be a minor, if painful, occurrence, or it could be confirmation that you are no good at building things, never have been and there was no point in trying in the first place.

How information slots into place depends on the hypotheses that you use to understand the world, and confirmation bias is one of the main mechanisms of this process.

In politics, good policy derives from a well-tested hypothesis. While any hypothesis will not be entirely correct, an open process of examination, either by competing politicians, organisations, the media or individuals, will ensure that governments make fewer glaring errors.

Bit dodgy because: It is very likely that it is only through confirmation bias that I believe confirmation bias is worth writing about in ak13. I could be completely wrong and only looking for evidence that supports my theory.

Why not also take a look at . . . Why Paul Wolfowitz, Deputy Defence Secretary of the US, is a tragic victim of confirmation bias. Or Wason's confirmation bias experiments.