"In general, for "good" traits, such as generosity, friendliness, and sense of humor, most people rate themselves as above average; for "bad" traits like snobbishness and dishonesty people typically describe themselves as below average (less snobbish, less dishonest). Most of us thus believe we are less biased than other people, less racist, less prone to conform, and less prone to be influenced by advertising. Yet, while good at spotting bias and prejudice in others, we are routinely blind to it in ourselves.
These happy illusions extend to those we identify with.

People expect that members of their own ethnic groups are more likely to smile (even in situations where a smile is inane, such as being alone in a room waiting for a computer to start up). Asked to pick out photographs of people likely to support the same political party as themselves, they pick more beautiful people than they do for supporters of an opposing party. In general, people tend to hold more favorable views of their "in-group," to exaggerate differences with a perceived out-group, and to treat members of their in-group more generously.

When it comes to studying ourselves — to trying to understand how we compare to other animals on the planet — we run into similar problems. We consistently overestimate human uniqueness and underestimate the abililities of other animals.

On the overestimation side, we only need to look at history to see that humans tend to have any number of self-aggrandizing beliefs — we have a long tradition of believing ourselves to be the center of the universe, for example, or to think the planet was created especially for us. We often forget that for the first two billion years of its existence, the planet was home only to bacteria, and that bacteria make all other lifeforms possible: we are as dependent on the bacteria in our guts as a termite or cow. And when the chimpanzee genome was published, there was a big disappointment. The genes that have been evolving fastest between our lineage and theirs turned out not to be those involved in head size or intelligence, but those involved in reproduction and the immune system—the same pattern you see between any other pair of closely related species of mammal.

Moreover, in our assessments of other animals, we are consistently surprised. My favorite example of this comes from a headline in Nature a few years ago that announced that "sheep are not so stupid after all." The reason for the re-evaluation of ovine intelligence was a series of elegant experiments that showed that sheep can recognize and remember other sheep. But sheep are social animals: they live in flocks. It would be astonishing if they could not do this. (A sheep newspaper would no doubt have run the headline, "Humans Amazed Again!")

The idea that we need outside help in assessing ourselves isn't new. The great 19th century scientist Thomas Huxley, in his classic text about the evolution of humans and their similarities to chimpanzees and gorillas, wrote:

Let us endeavour for a moment to disconnect our thinking selves from the mask of humanity; let us imagine ourselves scientific Saturnians, if you will, fairly acquainted with such animals as now inhabit the Earth, and employed in discussing the relations they bear to a new and singular "erect and featherless biped," which some enterprising traveler, overcoming the difficulties of space and gravitation, has brought from that distant planet for our inspection, well preserved, may be, in a cask of rum.

Huxley goes on to argue that only a human could deny the extraordinary resemblances between humans and their primate cousins.
Since then, the genuine difficulty in disconnecting the "mask of humanity" has grown more apparent. As we continue to learn about the inherent human tendencies towards bias, and the flattering illusions we like to maintain, it may get easier to guard against the problem, and to assess ourselves more clearly. Yet perhaps — probably — there are some biases that our brains have that we simply can't see at all, blind spots that we, as a species, can never discover we have.
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Olivia Judson, writing in the Times.