When Matheus Dominguez was 16, YouTube recommended a video that changed his life.
He was in a band in Niterói, a beach-ringed city in Brazil, and practised guitar by watching tutorials online.
YouTube had recently installed a powerful new artificial intelligence system that learned from user behaviour and paired videos with recommendations for others. One day, it directed him to an amateur guitar teacher named Nando Moura, who had gained a wide following by posting videos about heavy metal, video games and, most of all, politics.
In colourful and paranoid far-right rants, Moura accused feminists, teachers and mainstream politicians of waging vast conspiracies. Dominguez was hooked.
As his time on the site grew, YouTube recommended videos from other far-right figures. One was a lawmaker named Jair Bolsonaro, then a marginal figure in national politics — but a star in YouTube's far-right community in Brazil, where the platform has become more widely watched than all but one TV channel.
Last year, he became President Bolsonaro.
"YouTube became the social media platform of the Brazilian right," said Dominguez, now a lanky 17-year-old who says he, too, plans to seek political office.
Members of the nation's newly empowered far right — from grassroots organisers to federal lawmakers — say their movement would not have risen so far, so fast, without YouTube's recommendation engine.
New research has found they may be correct. YouTube's search and recommendation system appears to have systematically diverted users to far-right and conspiracy channels in Brazil.
A New York Times investigation in Brazil found that, time and again, videos promoted by the site have upended central elements of daily life.
Teachers describe classrooms made unruly by students who quote from YouTube conspiracy videos or who, encouraged by right-wing YouTube stars, secretly record their instructors.
Some parents look to "Dr. YouTube" for health advice but get dangerous misinformation instead, hampering the nation's efforts to fight diseases like Zika. Viral videos have incited death threats against public health advocates.
And in politics, a wave of right-wing YouTube stars ran for office alongside Bolsonaro, some winning by historic margins. Most still use the platform, governing the world's fourth-largest democracy through internet-honed trolling and provocation.
YouTube's recommendation system is engineered to maximise watchtime, among other factors, the company says, but not to favor any political ideology. The system suggests what to watch next, often playing the videos automatically, in a never-ending quest to keep us glued to our screens.
But the emotions that draw people in — like fear, doubt and anger — are often central features of conspiracy theories, and in particular, experts say, of right-wing extremism.
As the system suggests more provocative videos to keep users watching, it can direct them toward extreme content they might otherwise never find. And it is designed to lead users to new topics to pique new interest — a boon for channels like Moura's that use pop culture as a gateway to far-right ideas.
The system now drives 70 per cent of total time on the platform, the company says. As viewership skyrockets globally, YouTube is bringing in more than US$1 billion a month, some analysts believe.
Zeynep Tufekci, a social media scholar, has called it "one of the most powerful radicalising instruments of the 21st century."
Company representatives disputed the studies' methodology and said that the platform's systems do not privilege any one viewpoint or direct users toward extremism. However, company representatives conceded some of the findings and promised to make changes.
Farshad Shadloo, a spokesman, said YouTube has "invested heavily in the policies, resources and products" to reduce the spread of harmful misinformation, adding, "we've seen that authoritative content is thriving in Brazil and is some of the most recommended content on the site."
Danah Boyd, founder of the think tank Data & Society, attributed the disruption in Brazil to YouTube's unrelenting push for viewer engagement, and the revenues it generates.
Though corruption scandals and a deep recession had already devastated Brazil's political establishment and left many Brazilians ready for a break with the status quo, Boyd called YouTube's impact a worrying indication of the platform's growing impact on democracies worldwide.
"This is happening everywhere," she said.
The Party of YouTube
Maurício Martins, the local vice president of Bolsonaro's party in Niterói, credited "most" of the party's recruitment to YouTube — including his own.
He was killing time on the site one day, he recalled, when the platform showed him a video by a right-wing blogger. He watched out of curiosity. It showed him another, and then another.
"Before that, I didn't have an ideological political background," Martins said. YouTube's auto-playing recommendations, he declared, were "my political education."
"It was like that with everyone," he said.
The platform's political influence is increasingly felt in Brazilian schools.
"Sometimes I'm watching videos about a game, and all of a sudden it's a Bolsonaro video," said Inzaghi D., a 17-year-old high schooler in Niterói.
More and more, his fellow students are making extremist claims, often citing as evidence YouTube stars like Moura, the guitarist-turned-conspiracist.
"It's the main source that kids have to get information," he said.
Few illustrate YouTube's influence better than Carlos Jordy.
Musclebound and heavily tattooed — his left hand bears a flaming skull with diamond eyes — he joined the City Council in 2017 with few prospects of rising through traditional politics. So Jordy took inspiration from bloggers like Moura and his political mentor, Bolsonaro, turning his focus to YouTube.
He posted videos accusing local teachers of conspiring to indoctrinate students into communism. The videos won him a "national audience," he said, and propelled his stunning rise, only two years later, to the federal legislature.
"If social media didn't exist, I wouldn't be here," he said. "Jair Bolsonaro wouldn't be president."
Down the rabbit hole
A few hundred miles from Niterói, a team of researchers led by Virgilio Almeida at the Federal University of Minas Gerais hunched over computers, trying to understand how YouTube shapes its users' reality.
The team analysed transcripts from thousands of videos, as well as the comments beneath them. Right-wing channels in Brazil, they found, had seen their audiences expand far faster than others did, and seemed to be tilting the site's overall political content.
In the months after YouTube changed its algorithm, positive mentions of Bolsonaro ballooned. So did mentions of conspiracy theories that he had floated. This began as polls still showed him to be deeply unpopular, suggesting that the platform was doing more than merely reflecting political trends.
A team at Harvard's Berkman Klein Center set out to test whether the Brazilian far right's meteoric rise on the platform had been boosted by YouTube's recommendation engine.
Jonas Kaiser and Yasodara Córdova, with Adrian Rauchfleisch of National Taiwan University, programmed a Brazil-based server to enter a popular channel or search term, then open YouTube's top recommendations, then follow the recommendations on each of those, and so on.
By repeating this thousands of times, the researchers tracked how the platform moved users from one video to the next. They found that after users watched a video about politics or even entertainment, YouTube's recommendations often favoured right-wing, conspiracy-filled channels like Moura's.
Crucially, users who watched one far-right channel would often be shown many more.
The algorithm had united once-marginal channels — and then built an audience for them, the researchers concluded.
One of those channels belonged to Bolsonaro, who had long used the platform to post hoaxes and conspiracies. Though a YouTube early adopter, his online following had done little to expand his political base, which barely existed on a national level.
Then Brazil's political system collapsed just as YouTube's popularity there soared. Bolsonaro's views had not changed. But YouTube's far-right, where he was a major figure, saw its audience explode, helping to prime large numbers of Brazilians for his message at a time when the country was ripe for a political shift.
YouTube challenged the researchers' methodology and said its internal data contradicted their findings. But the company declined the Times' requests for that data, as well as requests for certain statistics that would reveal whether or not the researchers' findings were accurate.
"Dr. YouTube"
The conspiracies were not limited to politics. Many Brazilians searching YouTube for health care information found videos that terrified them: some said Zika was being spread by vaccines, or by the insecticides meant to curb the spread of the mosquito-borne disease that has ravaged northeastern Brazil.
The videos appeared to rise on the platform in much the same way as extremist political content: by making alarming claims and promising forbidden truths that kept users glued to their screens.
Doctors, social workers and former government officials said the videos had created the foundation of a public health crisis as frightened patients refused vaccines and even anti-Zika insecticides.
The consequences have been pronounced in poorer communities like Maceió, a city in Brazil's northeast that was among the hardest hit by Zika.
"Fake news is a virtual war," said Flávio Santana, a paediatric neurologist based in Maceió. "We have it coming from every direction."
When Zika first spread in 2015, health workers distributed larvicides that killed the mosquitoes that spread the disease.
Not long after YouTube installed its new recommendation engine, Santana's patients began telling him that they'd seen videos blaming Zika on vaccines — and, later, on larvicides. Many refused both.
Dr. Auriene Oliviera, an infectious disease specialist at the same hospital, said patients increasingly defied her advice, including on procedures crucial to their child's survival.
"They say, 'No, I've researched it on Google, I've seen it on YouTube,' " she said.
Medical providers, she said, were competing "every single day" against "Dr. Google and Dr. YouTube" — and they were losing.
Mardjane Nunes, a Zika expert who recently left a senior role in the Health Ministry, said health workers across Brazil have been reporting similar experiences. As more communities refuse the anti-Zika larvicide, she added, the disease is seeing a small resurgence.
"Social media is winning," she said.
Brazil's medical community had reason to feel outmatched. The Harvard researchers found that YouTube's systems frequently directed users who searched for information on Zika, or even those who watched a reputable video on health issues, toward conspiracy channels.
A spokesman for YouTube confirmed the Times' findings, calling them unintended, and said the company would change how its search tool surfaced videos related to Zika.
An "ecosystem of hate"
As the far right rose, many of its leading voices had learned to weaponise the conspiracy videos, offering their vast audiences a target: people to blame. Eventually, the YouTube conspiracists turned their spotlight on Debora Diniz, a women's rights activist whose abortion advocacy had long made her a target of the far right.
Bernardo Küster, a YouTube star whose homemade rants had won him 750,000 subscribers and an endorsement from Bolsonaro, accused her of involvement in the supposed Zika plots.
The very people working to help families affected by Zika, their videos implied, were behind the disease. Backed by shadowy foreigners, their goal was to abolish Brazil's abortion ban — or even make abortions mandatory.
As far-right and conspiracy channels began citing one another, YouTube's recommendation system learned to string their videos together. However implausible any individual rumour might be on its own, joined together, they created the impression that dozens of disparate sources were revealing the same terrifying truth.
"It feels like the connection is made by the viewer, but the connection is made by the system," Diniz said.
Threats of rape and torture filled Diniz's phone and email. Some cited her daily routines. Many echoed claims from Küster's videos, she said.
Küster gleefully mentioned, though never explicitly endorsed, the threats. That kept him just within YouTube's rules.
When the university where Diniz taught received a warning that a gunman would shoot her and her students, and the police said they could no longer guarantee her safety, she left Brazil.
"The YouTube system of recommending the next video and the next video," she said, had created "an ecosystem of hate."
"'I heard here that she's an enemy of Brazil. I hear in the next one that feminists are changing family values. And the next one I hear that they receive money from abroad" she said. "That loop is what leads someone to say 'I will do what has to be done.' "
"We need the companies to face their role," Diniz said. "Ethically, they are responsible."
As conspiracies spread on YouTube, video makers targeted aid groups whose work touches on controversial issues like abortion. Even some families that had long relied on such groups came to wonder if the videos might be true, and began to avoid them.
In Brazil, this is a growing online practice known as "linchamento" — lynching. Bolsonaro was an early pioneer, spreading videos in 2012 that falsely accused left-wing academics of plotting to force schools to distribute "gay kits" to convert children to homosexuality.
Jordy, Bolsonaro's tattooed Niterói protégé, was untroubled to learn that his own YouTube campaign, accusing teachers of spreading communism, had turned their lives upside down.
One of those teachers, Valeria Borges, said she and her colleagues had been overwhelmed with messages of hate, creating a climate of fear.
Jordy, far from disputing this, said it had been his goal. "I wanted her to feel fear," he said.
"It's a culture war we're fighting," he explained. "This is what I came into office to do."
"The dictatorship of the like"
Ground zero for politics by YouTube may be the São Paulo headquarters of Movimento Brasil Livre, which formed to agitate for the 2016 impeachment of left-wing President Dilma Rousseff. Its members trend young, middle-class, right-wing and extremely online.
Renan Santos, the group's national coordinator, gestured to a door marked "the YouTube Division" and said, "This is the heart of things."
Inside, eight young men poked at editing software. One was stylising an image of Benito Mussolini for a video arguing that fascism had been wrongly blamed on the right.
But even some people here fear the platform's impact on democracy. Santos, for example, called social media a "weapon," adding that some people around Bolsonaro "want to use this weapon to pressure institutions in a way that I don't see as responsible."
The group's co-founder, a man-bunned former rock guitarist name Pedro D'Eyrot, said "we have something here that we call the dictatorship of the like."
Reality, he said, is shaped by whatever message goes most viral.
Even as he spoke, a two-hour YouTube video was captivating the nation. Titled "1964" for the year of Brazil's military coup, it argued that the takeover had been necessary to save Brazil from communism.
Dominguez, the teenager learning to play guitar, said the video persuaded him that his teachers had fabricated the horrors of military rule.
Borges, the history teacher vilified on YouTube, said it brought back memories of military curfews, disappeared activists and police beatings.
"I don't think I've had my last beating," she said.
Written by: Max Fisher and Amanda Taub
Photographs by: Dado Galdiei
© 2019 THE NEW YORK TIMES