The English language is full of metaphors linking moral purity to both physical cleanliness and brightness. We speak of "clean consciences", "pure thoughts" and "dirty thieves". We're suspicious of "shady behaviour" and we use light and darkness to symbolise good and evil. But there is more to these metaphors than we might imagine. The mere scent of a clean-smelling room can take people down a virtuous road, compelling them to choose generosity over greed and charity over apathy. Meanwhile, the darkness of a dimmed room or a pair of sunglasses can compel people towards selfishness and cheating.

These new results are the latest from psychologist Chen-Bo Zhong. Back in 2006, he showed that people who brought back memories of past wrong-doings were more likely to think of words related to cleaning, or to physically crave cleaning products. He called this the "Lady Macbeth effect". Subsequently, another group found that it works the other way too. People judge moral transgressions more leniently if they had previously washed their hands or if they had been primed with words related to cleanliness, like 'pure' or 'immaculate'.

Now, Zhong, together with Katie Liljenquist and Adam Galinsky, have expanded on these studies by showing that clean smells can make people behave more virtuously. They ushered 28 volunteers into a room that was either unscented or that had been lightly sprayed with a citrus air freshener. In either case, they had to play a trust game, where a "sender" has a pot of money and chooses how much they want to invest with a "receiver". The investment is tripled and the receiver decides how much to give back.

The volunteers were all told that they had been randomly chosen as receivers. Their anonymous partner had invested their entire $4 pot with them, which had been tripled to $12. Their job was to decide how much to give back. On average, they returned a measly $2.81in the unscented rooms but a more equitable $5.33 in the scented ones. The single spray of citrus nearly doubled their tendency to reciprocate.

In a second experiment, the trio again ushered 99 students into either a scented or unscented room. They were given a pack of miscellaneous tasks, including a flyer requesting volunteers for a charity called Habitat for Humanity. Those in the citrus-scented rooms were more likely to be interested in volunteering, and almost four times more willing to donate money to the cause.

You could argue that nice smells make people feel better, and these positive moods underlie their sudden burst of charitable behaviour. But questionnaires handed out after the tasks showed that neither room affected the volunteers' mood. Nor did the volunteers realise what was going on. In both experiments, they didn't believe that smells were influencing their behaviour and they didn't think that their room was any cleaner or dirtier than usual.

The idea that a simple scent can influence behaviour to this degree may be surprising to many people, but I've blogged about many such studies before. Social exclusion can make you feel cold, a warm cup of tea can make people behave more warmly or charitably to others, and holding heavy objects can make us see things as more important. All of these are examples of a fascinating concept called "embodied cognition", where many of the abstract concepts we use daily, like virtue, are related to concrete parts of our environment, like smells.

Zhong's new study also provides some indirect support for the broken windows theory, which suggests that signs of petty crime, like the eponymous broken windows, can trigger yet more criminal behaviour. Disorder breeds disorder. So far, Zhong has only shown that clean smells promote charity and generosity, not that dirty smells promote self-interested behaviour. However, he has found that darkness will do the job.

Darkness can obviously shroud one's identity, giving people a licence to misbehave. As Ralph Waldo Emerson once wrote, "Gaslight is the best nocturnal police, so the universe protects itself by pitiless publicity." But Zhong thinks that darkness doesn't need to provide anonymity to affect our behaviour - just creating the illusion of anonymity is enough.

Someone sitting in a dim room or wearing sunglasses can't think that they're less likely to be seen (unless they come from a Douglas Adams novel), but they may well feel a sense of secrecy all the same. It's a throwback to the childish behaviour of closing your eyes and believing others can't see you.

Together with Vanessa Bohns and Francesca Gino, Zhong set out to test this idea. They brought 84 students into either a well-lit or a dimly lit room, where two-thirds of the fluorescent lights were out. The students were faced with 20 mathematical problems and $10 in small change. They had to solve as many as possible in five minutes, which is typically enough to do around seven. For every correctly answered question, they could keep 50 cents. They had to put the remaining money in a white envelope and drop it into a collection box.

To the recruits, the process was entirely anonymous. They could have kept all the money without anyone knowing. But Zhong incorporated a few special numbers into the problems that allowed him to match tests with takers. He found that people in the darker room were more likely to lie about how many problems they solved, claiming that they answered an average of 11.5, while those in the well-lit room said that they'd done just 7.8. In reality, both groups had actually answered around 7.3 questions. The proportion of recruits who lied about their performance was also higher in the dimmer room (61% compared to 24%).

Obviously, neither group of recruits would actually have thought that their identities were more secret. The same applied to a second experiment, where 50 students were asked to test either a pair of clear glasses or sunglasses. They played a dictator game, where they were given $6 and had to allocate part of it to a partner. The games were played anonymously, with the alleged partner sitting in another room.

As expected, the sunglasses-adorned recruits were stingier, offering up an average of just $1.81, compared to the $2.71 tendered by those who wore clear glasses. Again, wearing sunglasses doesn't really do very much to conceal one's identity, and all the games were played across an internet connection anyway.

However, a short questionnaire filled in after the study revealed that the students who wore sunglasses did indeed feel more anonymous than those who wore clear ones. As Zhong says, "Darkness appears to induce a false sense of concealment, leading people to feel that their identities are hidden." And this perception of anonymity was enough to breed dishonesty and self-interest.

The results of studies like these are very compelling. Every day, reams of column inches and hours of airtime are devoted to discussing ways of tackling crime and encouraging altruism. And for all that talk, it is fascinating to learn that something as simple as a fresh smell or a darkened room could be used as a way of enforcing righteousness or promoting selfishness.

References: Zhong et al. 2010. Good Lamps Are the Best Police: Darkness Increases Dishonesty and Self-Interested Behavior. Psychological Science http://dx.doi.org/10.1177/0956797609360754

Liljenquist et al. 2010. The Smell of Virtue: Clean Scents Promote Reciprocity and Charity. Psychological Science http://dx.doi.org/10.1177/0956797610361426

Images: by Beautiful Tima and Andre Karwath

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