ALTENA, Germany — When you ask locals why Dirk Denkhaus, a young firefighter trainee who had been considered neither dangerous nor political, broke into the attic of a refugee group house and tried to set it on fire, they will list the familiar issues.
This small riverside town is shrinking and its economy declining, they say, leaving young people bored and disillusioned. Though most here supported the mayor's decision to accept an extra allotment of refugees, some found the influx disorienting. Fringe politics are on the rise.
Everyone here has seen Facebook rumors portraying refugees as a threat. They've encountered racist vitriol on local pages, a jarring contrast with Altena's public spaces, where people wave warmly to refugee families.
Many here suspected — and prosecutors would later argue, based on data seized from his phone — that Mr. Denkhaus had isolated himself in an online world of fear and anger that helped lead him to violence.
This may be more than speculation. Little Altena exemplifies a phenomenon long suspected by researchers who study Facebook: that the platform makes communities more prone to racial violence. And, now, the town is one of 3,000-plus data points in a landmark study that claims to prove it.
Karsten Müller and Carlo Schwarz, researchers at the University of Warwick, scrutinized every anti-refugee attack in Germany, 3,335 in all, over a two-year span. In each, they analyzed the local community by any variable that seemed relevant. Wealth. Demographics. Support for far-right politics. Newspaper sales. Number of refugees. History of hate crime. Number of protests.
One thing stuck out. Towns where Facebook use was higher than average, like Altena, reliably experienced more attacks on refugees. That held true in virtually any sort of community — big city or small town; affluent or struggling; liberal haven or far-right stronghold — suggesting that the link applies universally.
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Their reams of data converged on a breathtaking statistic: Wherever per-person Facebook use rose to one standard deviation above the national average, attacks on refugees increased by about 50 percent.
Nationwide, the researchers estimated in an interview, this effect drove one-tenth of all anti-refugee violence.
The uptick in violence did not correlate with general web use or other related factors; this was not about the internet as an open platform for mobilization or communication. It was particular to Facebook.
Other experts, asked to review the findings, called them credible, rigorous — and disturbing. The study bolstered a growing body of research, they said, finding that social media scrambles users' perceptions of outsiders, of reality, even of right and wrong.
Facebook declined to comment on the study, but a spokeswoman said in an email, "Our approach on what is allowed on Facebook has evolved over time and continues to change as we learn from experts in the field."
The company toughened a number of restrictions on hate speech, including against refugees, during and after the study's sample period. Still, experts believe that much of the link to violence doesn't come through overt hate speech, but rather through subtler and more pervasive ways that the platform distorts users' picture of reality and social norms.
We visited Altena and other German towns to retrace each step from the site's algorithm-driven newsfeed to real-world attacks that its users might not otherwise commit — and that hint at subtle but profound ways that the social network reshapes societies.