The White House recently hosted a Social Media Summit, inviting about 200 conservatives and right-wing activists to discuss their allegations that Facebook and Twitter censor their messaging. The summit capped a year of charges by the right that Silicon Valley tech firms have a liberal bias.

But these charges come in the face of considerable evidence that conservative news outlets outperform others on social media. Last week, the charges turned bipartisan. Rep. Tulsi Gabbard (Hawaii), a Democratic presidential candidate, filed a $50 million suit against Google, alleging that a temporary account suspension infringed on her free speech.

Why do these censorship charges persist?

Our research reveals one likely reason: Facebook and Google don’t make clear either their guidelines or reasoning for accepting or rejecting paid political content or the process by which they make those decisions. This lack of transparency may lead outsiders to believe the worst, especially when tech chief executives make political statements that don’t accord with their prospective customers’ beliefs.

Facebook and Google’s rules are vague but important

Over the past two years, we interviewed former employees of Facebook and Google and political practitioners from campaigns, political organizations and digital political consultancies. We also analyzed email exchanges between Facebook and campaigns to investigate how these firms moderate paid political speech such as campaign advertising. We focused on advertising, or paid content — the domain where these companies are likely to have the most formal policies and transparency around their decisions.  […]