Carey, Winslet, Firth

What's the common factor shared by Colin Firth, Kate Winslet and Jim Carrey? Of course, there are quite a few: they're all Hollywood actors, parents and native English speakers, for example. They're also all white, but a psychological study shows that people find that particularly hard to remember.

When people took part in a guessing game as part of a psychological experiment, they were given the names of three famous actors. The participants were told that the experimenter had one particular factor they had in common in mind – the experimenter's rule – and they had to guess what it was as quickly as possible.

One group of names given was Halle Berry, Morgan Freeman and Eddie Murphy. Study participants quickly alighted on the fact that all three were black: 90% of study participants guessed that race was the common factor, taking an average time of 7 minutes to come to that answer.

The other group of actors was Firth, Winslet and Carrey. Only 25% of people got to the answer that all three were white before the game was cut off after 20 minutes.

In a subsequent study, participants given the white group of actors were given a clue. Several guesses in, the experimenter told the participant that another actor did not fit the experimenter's rule for the group.

"The clue that did work was, we gave people a bit of information – we would give them Eddie Murphy's name and say he doesn't fit the experimenter's rule," study author Peter Hegarty of the University of Surrey told IBTimes UK. "That vastly improved performance – it was an effective clue."

Such clues weren't necessary for those guessing the common factor shared between the black actors, as the participants guessed the answer so quickly.

Freeman, Berry. Murphy

So why is it difficult to notice that white people are white?

"One reason is, these assumptions fix in the back of your mind and you don't know that you have them, and that they have an effect on your thinking," Hegarty said. "It's not that people don't know that they're white, it's just that it's such a background assumption that they don't notice it."

Jules Holroyd, a philosopher at the University of Sheffield, said: "What needs explaining, crucially, is why the assumption that someone is white, but not the assumption that someone is black, is difficult to retrieve.

Many race scholars have argued that in societies where there is a racial hierarchy, 'whiteness' is the norm against which 'other' racial identities are defined, Holroyd said. It is this status of whiteness as the norm that makes it difficult to realise that white people are white.

"One of the many problems of taking whiteness as the norm is that other racial identities experiences are marginalised or overlooked."

The finding also has implications for medical and scientific research, Hegarty said. "These results suggest why scientists might be much quicker to label something common to black people as race-related, than something common to white people."

The results of the study were published a paper published in The Journal of Experimental Psychology.