A few hundred miles away from Niteri, a team of researchers led by Virgilio Almeida at the Federal University of Minas Gerais hunched over computers, trying understand how YouTube shapes its users reality.
The team analyzed 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 sites overall political content.
In the months after YouTube changed its algorithm, positive mentions of Mr. 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 Harvards Berkman Klein Center set out to test whether the Brazilian far rights meteoric rise on the platform had been boosted by YouTubes recommendation engine.
Jonas Kaiser and Yasodara Crdova, with Adrian Rauchfleisch of National Taiwan University, programmed a Brazil-based server to enter a popular channel or search term, then open YouTubes 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, YouTubes recommendations often favored right-wing, conspiracy-filled channels like Mr. Mouras.
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 Mr. 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 Brazils political system collapsed just as YouTubes popularity there soared. Mr. Bolsonaros views had not changed. But YouTubes 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. ... As the far right rose, many of its leading voices had learned to weaponize the conspiracy videos, offering their vast audiences a target: people to blame. Eventually, the YouTube conspiracists turned their spotlight on Debora Diniz, a womens rights activist whose abortion advocacy had long made her a target of the far right.
Bernardo Kster, a YouTube star whose homemade rants had won him 750,000 subscribers and an endorsement from Mr. 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 Brazils abortion ban or even make abortions mandatory.
As far-right and conspiracy channels began citing one another, YouTubes recommendation system learned to string their videos together. However implausible any individual rumor 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, Ms. Diniz said.