THE virtuous are often said to be as “pure as the driven snow” while villains are frequently described as having hearts of coal or blackened souls. And the metaphor is made flesh (or, at least cloth) in many plays and films where the baddy wears black and the goody white. But how deep does the metaphor actually run, psychologically speaking?

That is the question which was asked by Gary Sherman and Gerald Clore of the University of Virginia. They were pondering a well-known tendency, called the “Macbeth effect”, for people to want to clean themselves physically if they have acted unethically or even had thoughts of corrupt behaviour. (The name comes from the scene in Shakespeare's play in which Lady Macbeth desperately tries to wash phantom bloodstains from her hands after encouraging her husband to murder the king.) This association of cleanliness with moral probity is further bound up with the now well-established link between moral disgust (eg, at unusual sexual practices) and physical disgust (eg, at handling dirty objects or eating polluted food). The researchers' ponderings led them to wonder if the moral roles of black, which is roughly the colour of dirt, and white, which shows up the dirt so well, were connected with the Macbeth effect.

To explore this, they devised an experiment involving 27 Caucasian, six Asian, three African-American and two Hispanic students at the university. The students were all asked to state the colour of words that they were presented with as quickly as they could.

Such a technique is not new. Psychologists have long known that if people are presented with, say, the word “blue” printed in a blue font, they will be able to state the colour of the font much faster than if the word “red” is printed in the same blue font.

The study conducted by Mr Sherman and Dr Clore presented words of moral goodness, like “virtuous” and “honesty”, and of badness, like “cheat” and “sin”, in either black or white fonts on a computer screen. As they report in Psychological Science, the two researchers found that when “good” words were presented in black it took the participants about 510 milliseconds to state the colour of the word. When these same words were presented in white it took roughly 480 milliseconds—a significant difference. A similar effect was seen with “bad” words. Responding to white ones took around 525 milliseconds, whereas black ones needed only about 500. These results are remarkably similar to those found when words are printed in colours that clash with their meaning.

Besides illuminating an interesting corner of the Macbeth effect, Mr Sherman and Dr Clore suggest that their findings may have implications for understanding racial prejudice. Given that their work hints that blackness and immorality are psychologically connected, and that the labels “black” and “white” are often applied to race, dark skin might thus easily be associated with immorality and impurity.

Because most of the study group were white, and only three black, there is no way of detecting from the original data whether black people react in the same way as others, and thus whether the link between colour and disgust is independent of race. As a result, the two researchers are now conducting a follow-up study to look for racial differences.

That study is not yet complete but, according to Mr Sherman, “initial results are suggesting that this effect is not confined to Caucasians”. Perhaps, then, one reason that so many black Americans get a bad rap is not just that their skin colour differs from that of the majority, but that their psychology, being the same as everybody else's, does not.