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Years ago, social scientists introduced the broken-windows theory of crime control, which posited that if a neighborhood looked orderly and cared for — with no graffiti or broken windows — potential wrongdoers would be dissuaded from committing crimes there. Now psychologists have proposed a similar theory, which suggests that people can be induced to behave virtuously when their environment smells as clean as it looks.

It's the Macbeth principle of morality, says Katie Liljenquist, professor of organizational leadership at Brigham Young University's Marriott School of Management and lead author of the new study, to be published in Psychological Science. "There is a strong link between moral and physical purity that people associate at a core level. People feel contaminated by immoral choices and try to wash away their sins," says Liljenquist. "To some degree, washing actually is effective in alleviating guilt. What we wondered was whether you could regulate ethical behavior through cleanliness. We found that we could."

In two separate experiments, researchers were able to influence participants' behavior by exposing them to "cleanliness" in the form of a common cleaning agent's odor — in this case, citrus-scented Windex. It turned out that people who sat in a room spritzed with Windex were more likely to act fairly and charitably than those sniffing unscented air.

The first experiment involved an anonymous game of trust. The 28 study participants were told they would be "receivers," with whom a group of anonymous "senders" had been instructed to invest money. Participants were told that each sender had been given $4 and told that any part of it invested with receivers would be tripled. The job of the receiver, then, was to decide what portion of the dividends to return to the sender.

In reality, there was no sender, and each study participant received $12, making it seem as though the senders had entrusted them with the full $4 they had been given. But would the receivers reciprocate that trust or exploit their unidentified investors? On average, those in the plain-smelling room returned $2.81 to the sender, pocketing the lion's share of the money. But those bathed in the scent of Windex sent back an average of $5.83, returning the senders' blind faith.

The scientists insist they didn't overdo it with the Windex, just a few spritzes — so we can rule out brain-cell death or intoxication-induced generosity as reasons why those receivers gave back so much of the booty. Rather, Liljenquist says, "a moral awareness was awakened in a clean-smelling environment."

In the second experiment, researchers aimed to manipulate people's propensity toward charity. Ninety-nine participants were assigned to either a Windex-scented room or a neutral-smelling room and given a packet of tasks to complete. Included in the packet was a flyer soliciting volunteers and donations to the charity Habitat for Humanity. As expected, people in the Windex-sprayed room were more inclined to volunteer and give money than those in the unscented room — 22% of those in the clean group said they wanted to donate money, compared with 6% of the controls.

According to co-author Adam Galinsky, a social psychologist at Northwestern University's Kellogg School of Management, society relies on incentives, in the form of rewards and punishments, to encourage people to conform to certain standards of behavior. "Economists and even psychologists haven't been paying much attention to the fact that small changes in our environment can have dramatic effects on behavior. We underemphasize these subtle environmental cues," he says.

Liljenquist says the real-life implications of the study could be as simple as an office investing more in janitorial supplies than in expensive and intrusive surveillance equipment to keep workers in line. Other olfactory researchers suggest, however, that perhaps it wasn't the clean smell that made people more virtuous in the new study, but rather the smell of citrus; that is, people may have behaved better because they smelled something they liked, rather than something "clean." "It could be simply that a positive smell creates a positive mood, which encourages positive behavior. You cannot conclude it is cleanliness per se," says Brown University psychologist Rachel Herz, author of The Scent of Desire. To rule out the confounding factor of good smells, she says, the study's authors could have added a third room to the experiment scented with recently baked chocolate chip cookies, for example.

Nevertheless, both morality researchers and olfactory scientists agree that people do strongly associate physical cleanliness with purity of conscience. It is the notion at the heart of adages like "cleanliness is next to godliness" and evidenced by the widespread use of cleansing ceremonies to wash away sins in various religions around the world. (Truth be told, that practice is merely an extrapolation of an evolutionary strategy to avoid disease.)

For their part, Liljenquist and Galinsky say they controlled for the good-mood effect by giving participants in the second experiment a mood-screening questionnaire. They also say their results are consistent with existing literature on cleanliness and morality. For instance, in one of Liljenquist's earlier studies, she found, among other things, that cleaning hands after writing about a moral transgression made people feel less guilty about it. Other researchers have also tackled the issue of morality and smell, but from the opposite end of the spectrum. A paper published last year in the Personality and Social Psychology Bulletin revealed that people are more critical and judgmental about certain moral issues when exposed to the vapors of a — ahem — fart-scented spray.

Yes, fart spray is a commercially available product. Incidentally, according to a psychologist who has worked with it in experiments, it is nearly impossible to rid upholstery of it. Citrus-scented Windex certainly makes for a nicer lab environment, which perhaps has something to do with Liljenquist's continued interest in this line of study. "Research on how to stay on the moral high ground and promote virtue," she says, "is something I find refreshing."

• Catherine Elton