He was a member of a band in Niterói, a city in Brazil surrounded by beaches, and he practiced playing the guitar by watching tutorials online. YouTube had recently implemented a new artificial intelligence system that learned from user behavior and recommended videos based on that. One day, he was directed to a guitar teacher named Nando Moura, who had a large following for his videos on heavy metal, video games, and politics. Moura's far-right views on feminists, teachers, and politicians intrigued him. As he spent more time on YouTube, he was recommended videos from other far-right figures, including Jair Bolsonaro, who was then a lesser-known political figure but popular among YouTube's far-right community in Brazil. Last year, Bolsonaro became the President of Brazil. YouTube became a significant platform for the Brazilian right, according to Matheus Dominguez, a 17-year-old who credits the platform for influencing his shift towards far-right political views. YouTube has thrived by keeping users engaged, but this has inadvertently benefited extremist groups. An investigation in Brazil, the company's second-largest market, has revealed troubling outcomes. The rise of the far-right movement in the country, from local activists to government officials, has been greatly accelerated by YouTube's recommendation system. Recent studies suggest that the platform has consistently directed users towards far-right and conspiracy content. The New York Times' investigation in Brazil uncovered instances where YouTube-promoted videos have disrupted daily life, such as students disrupting classrooms with conspiracy theories or parents receiving harmful health advice from the platform. This has hindered the nation's efforts to combat diseases like Zika. Viral videos have led to death threats against public health advocates, while in politics, right-wing YouTube stars have run for office alongside Bolsonaro, with some winning by large margins. These stars still use the platform to govern the world's fourth-largest democracy through trolling and provocation. YouTube's recommendation system is designed to increase watch time but not to favor any political ideology. However, it often suggests videos that play automatically, aiming to keep viewers engaged. Emotions like fear, doubt, and anger, which are common in conspiracy theories and right-wing extremism, are key features that draw people in. The system can lead users to extreme content they may not have otherwise found and new topics to spark new interests. The system now drives 70% of total time on the platform, generating over $1 billion a month in revenue. Some analysts believe that YouTube is one of the most powerful radicalizing instruments of the 21st century. Company representatives dispute these claims, stating that their systems do not favor any particular viewpoint or direct users towards extremism. However, representatives from the company acknowledged some of the findings and assured that changes would be made. Farshad Shadloo, a spokesperson, stated that YouTube has made significant investments in policies, resources, and products to decrease the spread of harmful misinformation. He also mentioned that authoritative content is performing well in Brazil and is highly recommended on the platform. Danah Boyd, the founder of Data & Society, attributed the disruption in Brazil to YouTube's strong emphasis on viewer engagement and the revenues it generates. Despite the country already facing corruption scandals and a deep recession, Boyd expressed concern about YouTube's increasing impact on democracies globally. Maurício Martins, the local vice president of Bolsonaro's party in Niterói, credited YouTube for recruiting "most" of the party members, including himself. Martins shared that YouTube's auto-playing recommendations served as his political education and had a similar effect on others. The platform's political influence has even reached Brazilian schools, where students are increasingly making extremist claims, often citing YouTube stars like Moura as evidence. Carlos Jordy, a City Council member, exemplifies YouTube's influence as he turned to the platform for political inspiration and posted videos accusing local teachers of conspiring to indoctrinate students into communism. He said that the videos brought him a large national audience and helped him quickly rise to the federal legislature. He believes that without social media, he would not have achieved this success. Researchers at the Federal University of Minas Gerais analyzed YouTube videos and comments to understand how the platform influences users' perceptions. They discovered that right-wing channels in Brazil were growing rapidly and shaping the site's political content. Positive mentions of Bolsonaro increased after YouTube changed its algorithm, even though he was still unpopular in polls. A team from Harvard's Berkman Klein Center investigated whether YouTube's recommendation engine had contributed to the far right's popularity in Brazil. They programmed a server to explore popular channels and search terms, tracking how users were led from one video to the next. They discovered that following the viewing of a video on politics or entertainment, YouTube tended to recommend right-wing, conspiracy-laden channels like Moura's. Importantly, users who watched one extreme right channel were often shown multiple more. The algorithm had brought together channels that were once on the fringes and then helped them build a following, according to the researchers. One of these channels belonged to Bolsonaro, who had been utilizing the platform to spread hoaxes and conspiracy theories for some time. Despite being an early adopter of YouTube, his online following had not significantly expanded his political support, which was minimal at a national level. However, as Brazil's political system began to collapse, YouTube's popularity in the country soared. Bolsonaro's views had not changed, but his presence on YouTube's far-right channels, where he was a prominent figure, attracted a large audience, influencing many Brazilians at a time when the country was ready for a political shift. YouTube disputed the researchers' methodology and claimed that their internal data contradicted the findings, but they refused to provide the data or certain statistics that would confirm the accuracy of the researchers' conclusions. The spread of conspiracies was not limited to politics. Many Brazilians who turned to YouTube for information on health care were exposed to videos that caused fear and misinformation. These videos spread false claims, such as Zika being spread by vaccines or insecticides meant to control the disease. The alarming content of these videos kept viewers engaged, leading to a public health crisis as people refused vaccines and insecticides. This crisis has been particularly severe in poorer communities like Maceió, where the consequences of misinformation have been felt deeply. Health workers have struggled to combat the spread of false information, with patients often citing YouTube and Google as their sources of information. The medical community in Brazil is facing challenges in addressing and countering the impact of misinformation on public health. The Harvard researchers discovered that YouTube's algorithms often steered users searching for information on Zika or watching reputable health videos towards conspiracy channels. A YouTube spokesperson acknowledged the findings and stated that the company would adjust how videos related to Zika were displayed in search results. The rise of the far right led to conspiracy videos being weaponized, targeting individuals such as women's rights activist Debora Diniz. Conspiracy theorists accused her of being involved in Zika plots and suggested that those working to help Zika-affected families were actually behind the disease. As far-right and conspiracy channels began referencing each other, YouTube's recommendation system started linking their videos together, creating the illusion of multiple sources confirming the same falsehoods. Diniz received threats of violence, including details of her daily routines, as a result of these conspiracy theories. Diniz stated that many claims from Küster's videos were echoed, although Küster never explicitly endorsed the threats, which kept him just within YouTube's rules. When the university where Diniz taught received a warning of a potential shooting and the police could no longer guarantee her safety, she left Brazil. Diniz criticized YouTube's system of recommending videos, stating that it has created a culture of hate. She emphasized the importance of companies taking responsibility for their role in this issue. "Morally, they are accountable." As misinformation spreads on YouTube, content creators are targeting aid organizations involved in controversial issues like abortion. Even individuals who have previously supported these organizations began to question the accuracy of the videos and started avoiding them. In Brazil, this online trend is referred to as "linchamento" or lynching. Bolsonaro was an early adopter of this tactic, sharing videos in 2012 that falsely accused left-wing academics of planning to distribute "gay kits" in schools to promote homosexuality. Jordy, a protege of Bolsonaro from Niterói, was not bothered to learn that his own YouTube campaign, accusing teachers of promoting communism, had disrupted their lives. One teacher, Valeria Borges, described being inundated with hateful messages, creating a climate of fear. Jordy admitted that instilling fear was his intention, as he sees it as part of the culture war he is fighting. The political group, Movimento Brasil Livre, based in São Paulo, is at the forefront of using YouTube for political purposes, particularly in advocating for the impeachment of President Dilma Rousseff in 2016. The group's members are typically young, middle-class, right-wing, and heavily engaged in online activities. Renan Santos, the national coordinator, pointed out the importance of the YouTube Division as the core of their operations, where young men were seen editing videos to promote their views. Some members are concerned about the potential negative impact of social media on democracy, referring to it as a powerful tool that can be misused. The co-founder mentioned the concept of the "dictatorship of the like," suggesting that public perception is often shaped by what goes viral. A controversial YouTube video titled "1964" was mentioned, which aimed to justify Brazil's military coup in that year. The video had different effects on individuals, with some feeling convinced while others were reminded of the harsh realities of military rule. YouTube's success was based on its ability to engage users, but this has inadvertently benefited extremist groups. A study in the company's second-largest market revealed significant negative outcomes. (Dado Galdieri/The New York Times)