Axios mapped the launch date of 89 news websites over the past quarter century. The data shows there has been an explosion of right-leaning news sites, coinciding with the rise of the Tea Party and alt-right movements beginning in 2010. Many of these sites, in turn, were instrumental in spreading pro-Trump news during the 2016 elections.

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Data: Staff research; Chart: Lazaro Gamio / Axios

The data also shows a similar rise in left-leaning news sites during the Bush Administration and the launch of the Iraq war in 2003. Overall, while there has been a large increase in the number of new news sites over the past 20 years, almost all of them have a partisan angle.

Why it matters: According to experts, digital technology has made it easier to exploit the political divisions that have always existed. Sarah Sobieraj, associate professor of Sociology at Tufts University, told CNN there has been an increase in political polarization in the U.S., but not nearly enough to account for this development. "The technological, regulatory, and media space has shifted into one in which this is profitable, and profit is the driving force."

How they profit: Google and Facebook's algorithmically-driven news distribution platforms have created an environment in which:

a) partisan news sites can easily reach fringe audiences, and

b) news sites are financially incentivized to tilt one way or another.

Facebook, in particular, algorithmically favors content that appeals to user bias and interest. According to comScore Vice President Andrew Lipsman, to elicit high engagement and repeat visitation, "sites must usually speak to a very specific audience." Although this limits the appeal to a broader readership, it creates a sustained and engaged audience that appeals to advertisers.

The Bush burst: The launch of some left-leaning news sites during the Bush Administration captured audiences opposed to the administration's policies, primarily the Iraq War. John Amato, founder and publisher of the liberal, progressive news blog Crooks and Liars, tells Axios he started the site in September of 2004 because he thought that mainstream media wasn't critical enough of the Bush Administration, and he felt motivated to speak out.

The Obama opposition: According to Greg Mueller, president of CRC Public Relations, which has been representing conservative-leaning clients for decades, the rise of right-wing sites around the rise of the Tea Party movement came from the notion that the mainstream media was not fairly covering the scandals in the Obama Administration, like Fast-and-Furious and Benghazi. Vince Coglianese, editor-in-chief of The Daily Caller, which launched in 2010, said the founders, Tucker Carlson and Neil Patel, started the site because they saw a tremendous market opportunity where a conservative news site could report on news with a different form of selection bias. "A different news outlet could come in and report on stories that people weren't seeing covered by mainstream outlets," he said.

What to watch: The same profit motive that created and helped sustain ideological news sites led to the creation of fake news sites. As Google and Facebook figure out their response to being the conduit for all those ad dollars for fake news sites, it might change the business models for ideological sites as well.