Mainstream advertising is still showing up on polarizing and misleading sites — despite efforts to stop it

Online advertising systems regularly put mainstream ads alongside content from the political fringes — and dollars in the pockets of those producing polarizing and politically charged headlines. This mismatch of online content and ads, which digital advertising companies have been working to fix, goes to the heart of how the Internet economy works. Tens of billions of dollars are at stake in the promises of online ad systems to match advertising pitches with receptive targets. But the automated systems that place many of these ads are driven by sophisticated, proprietary algorithms that are hard for advertisers to understand or control, many complain. These systems, which have powered the rise of Google, Facebook and dozens of lesser-known technology companies, also can have the unintended effect of fueling the creation and spread of extreme content online — on both the political left and the right — independent researchers and industry experts say. Sensationalized headlines bring clicks. Clicks bring ads. Ads bring revenue. And the advertisers often have limited options for avoiding potentially objectionable websites.