It’s no secret that Americans are more divided along partisan lines than at any point in the last two decades. But how did we get here? That’s the question at the heart of historians Julian Zelizer and Kevin Kruse’s new book, “Fault Lines: A History of the United States Since 1974.”

The book, which is modeled on a college course taught by the two at Princeton University, tries to trace the modern origins of our political divide through what they refer to as “fault lines,” various strands of cultural and policy debates that have merged together to form the political earthquakes of the 2016 and 2018 election cycles.

Dozens of books have recently been written about the American political divide, but “Fault Lines” deviates from the norm by not framing its causes as purely the result of political decisions, but the clash between culture and politics in an era of rapidly changing technology. One chapter focuses exclusively on the advent of cable — and not just CNN and Fox News, but how MTV and ESPN allowed television viewers to opt out of news if they wanted to. The fragmentation of media in the 1980s, Zelizer says, allowed Americans to begin to curate their own media experiences, while also amplifying viewpoints not traditionally shown on the Big Three networks (ABC, CBS, and NBC).

“Before the 70s there were only a handful of institutions that people turned to for news, handful of networks, handful of city papers that really defined what the debate was about, and we traced that since that time it has fragmented, fractured and become this wild west of information, that fuels a lot of our polarization,” Zelizer said.

Zelizer also says that partisan divide didn’t begin in the 1970s. Rather, he says a combination of policy decisions helped erode institutions that connected society together, leaving many Americans economically insecure and less able to enter or remain in the middle class.

“It wasn’t as if everyone agreed in the 40s to the 60s, there were institutions that held us together. Unions are a great example, they were a thread that held together different parts of our society,” he said. “The federal government [was also] an important presence in that period, and that would weaken after the 1960s.”

“Fault Lines” isn’t meant to be a cautionary tale calling for a return to an America that existed before 1974. While there have been strong regulatory rollbacks that have led to an economically more unequal society, Zelizer also says that the last few decades have seen an explosion of social movements which have been aided in part to a more decentralized and easier to enter media market.

“One story [we looked at] was the gay rights movement and how they really forced the government to deal with AIDS,” Zelizer said. “It starts with Ronald Reagan not even uttering the word and his press secretary mocking people in a press conference ... and it ends with George W. Bush launching the biggest AIDS initiative overseas that we’ve ever had. That was a result of grassroots protest and energy, and that can happen again.”