Hindu nationalism and the plummeting cost of mobile phone data services are driving the explosive growth of fake news in India, according to a study that suggests WhatsApp is struggling to limit the spread of online disinformation on its service.

The research by the BBC World Service sheds light on the conundrum surrounding online disinformation and fake news in the world’s biggest democracy, where the explosive popularity of the group-messaging app has been linked to a string of murders and growing anti-Muslim sentiment. The research was conducted as part of the BBC’s Beyond Fake News series.

Dr Santanu Chakrabarti, the head of audience insight at the BBC World Service, said research into fake news tended to focus on western countries, and the motivations of Indian users sharing disinformation were often dismissed as a result of “evil, stupidity or malice”.

He said research by his team had concluded that the rise of the Hindu nationalist prime minister, Narendra Modi, had made many Indians feel as though they had a patriotic duty to forward information. “They are effectively looking for validation of their belief systems,” he said. “On these platforms, then, validation of identity trumps verification of the fact.”

The first wave of panic over online fake news and misinformation, which became a global concern following the 2016 US presidential election, largely concerned links spreading on social media services such as Facebook.

Focus is now shifting to communications on WhatsApp, which are fully encrypted and therefore impossible to track – making it very difficult for researchers and journalists to accurately judge the scale of the problem and understand how information is spreading.

While many people use WhatsApp to communicate one-on-one, there are growing concerns about how large WhatsApp groups of friends and acquaintances are being used to share information, creating the perfect environment for forwarding political images and videos.

The BBC researchers found Indians tend to read and believe information forwarded to them by individuals they know or trust, rather than discerning between news stories based on the original source of the information.

“It is not that people don’t know that there are more credible and less credible sources,” the researchers said. “Nor is it the case that they don’t care about consuming incorrect information. It’s that on the digital platforms, while contending with the flood of onrushing information, they simply cannot be bothered.”

They also found that images, screenshots and stories with minimal text were increasingly overtaking links to traditional websites as the preferred method of sharing news stories. In India, this is being driven by a price war that is driving down the cost of mobile phone data services and making 3G internet accessible to even the poorest members of society.

Rather than rely on large-scale polls, Chakrabarti’s team decided to adopt ethnographic methods and scrutinise WhatsApp and Facebook use by 40 people from different parts of the country for a week, before conducting an in-depth interview with each one to understand their usage of social media.

“You can’t use big data techniques to look into WhatsApp and Facebook private feeds,” he said. “You’ve got to spend serious time with these people, who are neither malicious creators or political activists.”

Facebook Twitter Pinterest WhatsApp took out full-page ads in Indian newspapers in July to warn users about fake news. Photograph: Prakash Singh/AFP/Getty Images

Earlier this year, following government pressure, WhatsApp introduced a “forwarded” tag on some messages and put limitations on Indian users in an attempt to make them question the source of information, though the researchers found this had made little difference.

“That forwarded thing hasn’t quite worked,” said Chakrabarti. “It’s trying to tell you that it hasn’t come from the person who forwarded it on. How people are ending up interpreting this is that the sender is all important and it doesn’t really matter if it originated elsewhere.”

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One respondent told the BBC that he thought the tag was actually an encouragement to forward messages on rather than a warning.

Chakrabarti said Indians may be more susceptible to sharing fake news because the country has a history of a trusted independent media – meaning users assume that material forwarded to them has been checked at some stage.

A parallel BBC study in Kenya and Nigeria found that a natural distrust of the media meant people were more likely to question the original source of online information.

A WhatsApp spokesperson said: “WhatsApp has made private communication easier and more reliable for millions of people including doctors, educators, community organisations and the local police.

“Though sadly some people also use WhatsApp to spread harmful misinformation. We believe this is a challenge that requires action from technology companies, civil society and governments. That’s why we’ve stepped up broad education with ads on the radio and online to encourage people not to share rumours, and have created limits on how forwarded messages can be sent.

“We will continue to take action within WhatsApp and in partnership with others to address the underlying challenge at hand.”