Wearing face masks may cause people to flout social distancing advice because they feel more comfortable sitting or standing close to others, a study suggests.
Mask-wearers are also more likely to keep a smaller distance from others wearing face coverings, according to a team of behavioural scientists at Warwick Business School.
The research further suggests that those who believe face masks are effective at preventing the spread of coronavirus are more likely to relax the distance if they or the other person is wearing a mask.
The scientists say their findings could make it difficult to re-establish social distancing measures if they are needed to help control the virus in the event of a second wave of cases.
Ashley Luckman, a research fellow at Warwick Business School and lead author of the study, said: "Our findings appear to be a classic case of risk compensation.
"Wearing masks brings down the overall risk of spreading COVID-19, so people feel safer and are more willing to take other risks, such as decreasing the physical distance between them and others.
"If the government's aim is to minimise transmission of the virus, its guidelines must be clear enough to prevent this trade-off, emphasising that masks are not an alternative to social distancing."
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Researchers showed 800 participants in the UK pictures of people sitting, standing, or walking and asked them to indicate how close they would be willing to stand in different scenarios.
These included indoor and outdoor settings and whether both people, just one, or neither wore a mask.
In each scenario, people were more likely to tolerate a reduced distance if they or the other person wore a mask, the pre-print study, which is yet to be peer-reviewed, found.
On average, participants wearing a mask felt comfortable standing 1.8 metres from another person.
Without a mask, they preferred to remain more than two metres apart.
Those with the strongest belief that masks prevented them catching COVID-19 were prepared to stand closer to others if they wore masks.