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You have probably spotted one of these yourself: a neatly printed placard in your hotel room imploring you to reuse your towel. 鈥淓very day millions of gallons of water are used to wash towels that have only been used once,鈥� it says, before going on to instruct you where to place your towel if you intend to reuse it (like a decent human being). Helpfully, your own brain adds the last bit.

We now know that such signs are an effective way to get people to reuse their towels. More effective still is telling people that most other guests are doing it too. That was the insight from a classic by researchers at Stanford University in California.

The behavioural scientists applied the lessons of nudge theory 鈥� which has been used to coax people to do everything from aim straighter at urinals to take the stairs instead of the escalator 鈥� to get people to cut back on water use.

That鈥檚 great for hotels looking to save on bills and burnish their green credentials, but can this type of messaging really make a difference on how people use water in their own homes? The question is far from academic. Several states in the western US are facing water shortages due to a megadrought that has now stretched past two decades.

More drastic actions will be needed to contend with water shortages and the permanent impact of severe drought on the landscape. But researchers and government officials alike increasingly believe that changing individual water consumption habits is crucial too. So what actually works?

The United Nations estimates that people need for both personal and domestic needs such as showering or washing clothes. The average person in the US, in contrast, uses 鈥� more than three times the necessary amount.

Sometimes getting people to cut back is as simple as getting them to pay attention to how much water they are using.听 A 2014 study of 630 households in Australia found that installing in-home water use displays led to a . Even after the displays were removed, the effects lingered: On average, water use was still down 6.4 per cent the third year.

鈥淐onnecting behaviour to water use is a really important first step so that people recognise where they鈥檙e using water the most,鈥� says at California State University San Marcos.

But education alone has a limited impact. A of households in Atlanta, Georgia, found that there was no difference in water consumption between more than 70,000 homes not sent any messaging compared with over 11,500 homes that received a tip sheet about reducing water use. Interestingly, though, when a second group of about 11,500 homes got the same tip sheet along with a letter from a city official explaining the drought conditions, their water consumption decreased by an average of 2.7 per cent over the next six months.

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Is your neighbour watering their lawn?

As with the hotel towels, what really seems to make a difference is getting people to compare themselves to others. When Cape Town, South Africa, risked running out of water during , the city made a publicly available map depicting how much water individual households used. That, along with other initiatives like educational campaigns on how to reduce water during daily activities such as showering or flushing the toilet, helped the city avoid 鈥淒ay Zero鈥�, when water supply would have turned off.

鈥淲e want to do what everyone else is doing,鈥� says at the University of Maryland, Baltimore County. 鈥淚f a big percentage of the group is reducing water, it might be because it鈥檚 a good idea to do so.鈥�

In 2015, during California鈥檚 last serious water shortage, Schultz and his colleagues sent in Sacramento, informing residents about how much water they used the month prior, and, critically, how that compared to similarly sized homes in their area. The researchers then tracked the group鈥檚 water usage over the next six months and compared it to over 10,000 households that did not receive a report.

They found that people sent the comparative report reduced their water use by more than 8 per cent on average.

鈥淪ocial comparison really calls [people] out. It says you鈥檙e consuming more than the average, more than maybe your fair share,鈥� says Schultz. That can have a powerful effect, particularly for those who are consuming the most. Schultz and his colleagues found that high-consuming homes reduced their water use by an average of 13.5 per cent over six months, or about 1.7 times the average reduction.

鈥淭here鈥檚 pressure on those who are above the norm to reduce their consumption in order to conform to the standards of their community,鈥� he says.

Finding out what your neighbours are up to doesn鈥檛 always encourage better behaviour, however. In households with the lowest water use, this type of messaging can actually increase water consumption. 鈥淧eople who are below the norm feel like they鈥檙e being taken advantage of, and so there鈥檚 pressure then to use more water,鈥� says Schultz.

To mitigate against this, he and his team added another layer of social messaging praising people for reducing their water use. Properties with lower-than-average water consumption received a report shaded green with a smiley face, and those with higher-than-average consumption received a red report with a frowny face.

Adding the colours and faces didn鈥檛 just prevent potential increases in water usage by low-consuming households, it actually reduced their water use further 鈥� by an average of 5 per cent over six months.

鈥淚f you communicate that there鈥檚 a sense of approval, then you can induce feelings of pride, and people will continue to reduce their consumption,鈥� says Schultz. 鈥淩ecognising that you are engaging in a behaviour that鈥檚 approved by others gives you the motivation to stay at that low level.鈥�

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How long do these good habits last?

Most studies find that the effects of these kinds of nudges peak after a few months before tapering off. For instance, in Schultz鈥檚 study, households that received a water report returned to their pre-experiment water levels after six months. But there are ways to strengthen the effects.

In the 2007 study in Atlanta, a third group of over 11,000 households received a tip sheet, a letter from a city official explaining the drought and a letter comparing their water usage to that of their neighbours. After four months, homes given all three messages had cut their water use by about 5 per cent on average.

Bernedo Del Carpio and her colleagues, who were not involved in the original experiment, wanted to see how long this would last. In a follow-up study, they found that households given all three messages were still using slightly less water four years later than households that received no messages.

When the researchers only looked at homes where the residents hadn鈥檛 moved since the study was conducted, they found that these properties had lower water use even after seven years.

This suggests that people are conserving water by changing their habits or purchasing easily movable water-saving technologies, such as low-flow showerheads or efficient washing machines. If residents had invested in more permanent water-friendly upgrades, like an in-ground sprinkler system, their homes would probably continue to use less water regardless of who lived there next, says Bernedo Del Carpio.

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Does saving water save you money?

Getting people to invest in permanent infrastructure changes is the ideal, says Schultz. Keeping up water-saving habits requires people to keep the notion at the top of their mind. 鈥淚 think that鈥檚 why you get the fading of the benefit over time,鈥� he says.

In some places, water-saving upgrades are provided for free by utility companies or local governments. In the UK, for instance, London area water provider 鈥� though they recently halted this scheme. In California, the is mailing free water-efficient showerheads and faucets to residents who request them. In theory, homeowners who purchase these themselves should earn back the cost via lower water bills, but in practice, Bernedo Del Carpio has found that isn鈥檛 the case.

She and her colleagues have found that once installed, such technologies reduce water consumption by only 9 per cent, according to research still underway. This is despite original estimates of 20 to 30 per cent reductions for some products. Actual savings are lower because reduced water flow doesn鈥檛 change the fact that some activities, like cooking a pot of pasta, require fixed amounts of water.

鈥淭he net value for the individual is not positive. It鈥檚 zero. Because of the cost of installing these technologies, this might not be something people are willing to do,鈥� says Bernedo Del Carpio. She suggests that subsidies for household infrastructure upgrades may be the most effective way to get people to make these changes in drought-stricken regions.

Some states are already doing this. Residents of southern Nevada can get up to $200 back on their installation of a , for instance. Governments in both Nevada and California will also pay households .

While social comparisons are a cheap, easy and promising intervention for curbing water use, they simply won鈥檛 be enough.

鈥淚f you expect each individual household to cut back water by 25 per cent, [behavioural nudges] only get you a fraction of the way there,鈥� says at the Georgia Institute of Technology in Atlanta.

Mandatory cutbacks have been used with some success in the past. In 2014, when California was facing water shortages, the state mandated that residents slash water use by 25 per cent or risk paying fines. Two years later the policy paid off 鈥� average household water consumption decreased almost 24 per cent.

These types of mandatory restrictions are some of the most effective measures for getting people to cut back on water, says Wichman. His research has shown that bans on activities such as watering lawns or washing cars can reduce water consumption by about .

With no end to the drought in sight, lawmakers in states most severely affected are once again enacting laws to curtail residents鈥� water use. In June 2021, Nevada passed a law set to go into effect in 2026 that outlaws non-functional grass 鈥� that is, lawns around office parks or at the entrances to housing developments. Cities in other states, such as Colorado, New Mexico, Montana and Utah during which residents can water lawns.

Yet mandatory cutbacks are largely unpopular among residents who sometimes view them as examples of government overreach, which is why most officials encourage people to reduce water use voluntarily.

For instance, in April, officials in Salt Lake City, Utah, asked residents to reduce their water . Similarly, in July 2021, California governor Gavin Newsom asked residents to voluntarily cut their water consumption by 15 per cent. Newsom has so far stopped short of enacting mandatory restrictions 鈥� except for prohibiting the watering of non-functional grass. But since he asked Californians to cut back, water use state-wide has only decreased .

Such results not only underscore the limitations of voluntary reductions, but of states relying solely on households to stave off water crises.

Californians today still use than they did before the 2014 drought. 鈥淚鈥檓 not sure how we will continue to rely on these big, short-run reductions,鈥� says Wichman. 鈥淚f you鈥檝e already exhausted that margin of which people can adjust [their water use], you might not be able to get as much of a reduction in water use the next drought.鈥�

Residential water mandates may well become commonplace in the future, as will the need for publicly provided resources such as tools to identify and repair leaks, but Wichman says the private sector will also have to step up water conservation efforts. In California, .

With climate change set to make droughts both more common and more severe, preserving this most precious resource will undoubtedly require a combination of efforts 鈥� and when it comes to changing our own habits, we shouldn鈥檛 throw in the towel.

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