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Between 2008 and 2010, polarization in society increased dramatically alongside a significant shift in social behavior: the number of close social contacts rose from an average of two to four or five people. The connection between these two developments could provide a fundamental explanation for why societies around the world are increasingly fragmenting into ideological bubbles.

"The big question that not only we, but many countries are currently grappling with, is why polarization has increased so dramatically in recent years," says Stefan Thurner from the Complexity Science Hub (CSH), explaining the study's motivation. The research was in Proceedings of the National Academy of Sciences.

The researchers' findings confirm that increasing polarization is not merely perceived—it is measurable and objectively occurring. "And this increase happened suddenly, between 2008 and 2010," says Thurner. The question remained: what caused it?

The friendship shift: From two to five close contacts

To investigate, Thurner and his team examined whether social networks had changed—specifically, whether people's close friendships had shifted. "For decades, sociological studies showed that people maintained an average of about two close friends—people who could influence their opinions on important issues," explains Thurner.

Here too, the researchers identified a striking change: "Around 2008, there was a sharp increase from an average of two close friends to four or five," explains CSH scientist Jan Korbel.

The paradox: More connection, more division

Are these two developments related? Do more close friends—and thus denser social networks—lead to network fragmentation and ultimately societal polarization?

Using a model based on real data, the researchers discovered this could indeed be the case: "When network density increases with more connections, polarization within the collective inevitably rises sharply," says Markus Hofer from CSH.

"This finding impressed us greatly because it could provide a fundamental explanation for the peculiar form of polarization we're currently observing simultaneously across many parts of the world—one that definitely threatens democracy," Thurner continues.

"When people are more connected with each other, they encounter different opinions more frequently. This inevitably leads to more conflict and thus greater societal polarization," adds Korbel.

Polarization has always existed, but what is happening now goes far beyond historical patterns. Greater connectivity has led to the formation of fewer but more tightly-knit groups with strongly differing opinions, between which there is hardly any exchange.

"There are few bridges between these 'bubbles,' and when they exist, they are often negative or even hostile," says Korbel. "This is called fragmentation, and it represents a new social phenomenon," adds Thurner.

Behind the numbers: Tracking polarization through decades of data

For their study, the researchers analyzed extensive existing survey data on both polarization and social networks.

"To measure political polarization, we used over 27,000 surveys from the Pew Research Center, which regularly records political attitudes of people in the US," explains Hofer.

"The key advantage of this data is that the questions have remained virtually unchanged over time, enabling reliable long-term comparisons."

The researchers found that political attitudes became significantly more one-sided between 1999 and 2017. For example, only 14% of respondents consistently expressed liberal views in 1999, but by 2017, this had risen to 31%. Conversely, only 6% of respondents consistently held conservative views in 1999, compared to 16% in 2017.

"More and more people are clearly aligning themselves with one political camp rather than holding a mixture of liberal and conservative views," explains Hofer.

To analyze friendship networks, the researchers combined 30 different surveys totaling over 57,000 respondents from Europe and the US, including the General Social Survey (US) and the European Social Survey.

"Despite minor differences between individual surveys, the data consistently show that the average number of close friendships rose from 2.2 in 2000 to 4.1 in 2024," says Hofer.

"The decisive contribution of this study is that it reconciled both phenomena using a mathematical social model," explains Thurner.

"This enabled us to show that increasing connectivity must lead to sudden polarization once a critical connectivity density is exceeded—just like a phase transition in physics, such as water turning to ice," adds Hofer.

"It is fascinating that these phase transitions also exist in societies. The exact location of these critical thresholds still needs clarification. According to our results, for close relationships, it lies somewhere between three and four people," the researchers note.

The smartphone era: When connection may have become fragmentation

The sharp rise in both polarization and the number of close friends occurred between 2008 and 2010—precisely when social media platforms and smartphones first achieved widespread adoption. This technological shift may have fundamentally changed how people connect with each other, indirectly promoting polarization.

"Democracy depends on all parts of society being involved in decision-making, which requires that everyone be able to communicate with each other. But when groups can no longer talk to each other, this democratic process breaks down," emphasizes Stefan Thurner.

Tolerance plays a central role. "If I have two friends, I do everything I can to keep them—I am very tolerant towards them. But if I have five and things become difficult with one of them, it's easier to end that friendship because I still have 'backups.' I no longer need to be as tolerant," explains Thurner.

What disappears as a result is a societal baseline of tolerance—a development that could contribute to the long-term erosion of democratic structures. To prevent societies from increasingly fragmenting, Thurner emphasizes the importance of learning early how to engage with different opinions and actively cultivating tolerance.