Whatever your opinion of President Barack Obama, it isn't hard to find someone who disagrees. A recent poll in the US found that Obama is the most divisive president since the 1950s: 81 per cent of fellow Democrats think he's doing a good job but only 13 per cent of opposing Republicans agree.
How can so many people make a judgement about the same person and come to such different conclusions? The obvious explanation is that they are biased - by their political affiliations, by the media, by their friends and family and much else.
This obvious explanation is correct. But who, precisely, is biased? It depends who you ask. Those who approve of Obama think the conservatives, and their media, are the biased ones. Those who don't, think it's the liberals. In fact, they are both right.
As any psychologist will tell you, pretty much everything you think and do is coloured by biases that you are typically totally unaware of. Rather than seeing the world as it is, you see it through a veil of prejudice and self-serving hypocrisies.
To get a handle on this, think about your own opinion of Obama. You probably believe your view to be an honest and objective assessment based on a range of evidence from both sides. Perhaps you'll grudgingly acknowledge that you feel the way you do because you are liberal/conservative, but then reassure yourself that being liberal/conservative is the only rational choice, so that's OK.
You have just experienced the illusion of naive realism - the conviction that you, and perhaps you alone, perceive the world as it really is, and that anybody who sees it differently is biased. According to Emily Pronin, a psychologist at Princeton University, this conviction is "inescapable and deep".
If, at this point, you are thinking: "Yeah, right, that might be true of other people, but not me," then you have fallen foul of yet another aspect of the illusion: the bias blind spot. Most people will happily acknowledge that such biases exist, but only in other people. "It's not that we're blind to the concept of bias, or to the fact that it exists," says Pronin. "We're just blind to it in our own case."
Why are we so blinkered? The problem is that our biases - which form and solidify in childhood and early adulthood - operate below the radar, in our subconscious. It is not that people do not look inwards to question their own judgements and beliefs. Many do. But their biases are not consciously available for inspection, so they leap to the conclusion that their beliefs are correct and based on rational reasoning.
Many of the biases are a harmless variant of the positive illusions we routinely entertain in order to shelter our fragile egos from reality, such as a tendency to take credit for success but deny responsibility for failure.
Others are more serious. Few people believe that they are racist or sexist, and their beliefs are honestly held, and yet time and again they are betrayed by their actions. In one experiment, people were shown a picture of a man and a woman and asked to say which they would prefer as police chief. They were also told that the male candidate was "streetwise" and the female candidate "formally educated", or vice versa. Most people chose the male candidate and then, when asked why, justified their decision by saying that whichever quality had been attributed to him was more important for the job.
While opinions are obviously ripe for bias, facts are also at its mercy, with people adept at interpreting the world to fit with their existing beliefs. For example, environmentalists interpret the fact that most scientists and governments are convinced that humans are changing the climate as open-and-shut evidence that we are. But sceptics just see a conspiracy. No amount of new information will change their minds, and yet on the whole, both camps sincerely believe their views are unbiased and rational.
Similarly, we seek out information that fits with our beliefs and ignore or dismiss information that doesn't. This "confirmation bias" has been shown time and again, for example in experiments in which people are asked to read a range of evidence about a contentious topic such as capital punishment. Even when exposed to arguments on both sides, most people interpret the evidence in a self-serving way, accepting the data that supports their views and dismissing or ignoring the rest. The scary thing is that they have no awareness of doing it. Similarly, confronting people with new information that contradicts their beliefs more often than not ends up hardening their position.
Sadly, even knowing that you are biased doesn't necessarily help. "I know that I am susceptible to all sorts of biases because I'm a human being," says Pronin. "But in a given instance, I'm still not likely to be aware of it."
