A newly published study from Oxford’s Jon Penney provides empirical evidence for a key argument long made by privacy advocates: that the mere existence of a surveillance state breeds fear and conformity and stifles free expression. Reporting on the study, the Washington Post this morning described this phenomenon: “If we think that authorities are watching our online actions, we might stop visiting certain websites or not say certain things just to avoid seeming suspicious.”

The new study documents how, in the wake of the 2013 Snowden revelations (of which 87 percent of Americans were aware), there was “a 20 percent decline in page views on Wikipedia articles related to terrorism, including those that mentioned ‘al Qaeda,’ ‘car bomb’ or ‘Taliban.'” People were afraid to read articles about those topics because of fear that doing so would bring them under a cloud of suspicion. The dangers of that dynamic were expressed well by Penney: “If people are spooked or deterred from learning about important policy matters like terrorism and national security, this is a real threat to proper democratic debate.”

As the Post explains, several other studies have also demonstrated how mass surveillance crushes free expression and free thought. A 2015 study examined Google search data and demonstrated that, post-Snowden, “users were less likely to search using search terms that they believed might get them in trouble with the U.S. government” and that these “results suggest that there is a chilling effect on search behavior from government surveillance on the internet.”

The fear that causes self-censorship is well beyond the realm of theory. Ample evidence demonstrates that it’s real — and rational. A study from PEN America writers found that 1 in 6 writers had curbed their content out of fear of surveillance and showed that writers are “not only overwhelmingly worried about government surveillance, but are engaging in self-censorship as a result.” Scholars in Europe have been accused of being terrorist supporters by virtue of possessing research materials on extremist groups, while the British Library refuse to house any material on the Taliban for fear of being prosecuted for material support for terrorism.

There are also numerous psychological studies demonstrating that people who believe they are being watched engage in behavior far more compliant, conformist and submissive than those who believe they are acting without monitoring. That same realization served centuries ago as the foundation of Jeremy Bentham’s Panopticon: that behaviors of large groups of people can be effectively controlled through architectural structures that make it possible for them to be watched at any given movement even though they can never know if they are, in fact, being monitored, thus forcing them to act as if they always are being watched. This same self-censoring, chilling effect of the potential of being surveilled was also the crux of the tyranny about which Orwell warned in 1984:

There was of course no way of knowing whether you were being watched at any given moment. How often, or on what system, the Thought Police plugged in on any individual wire was guesswork. It was even conceivable that they watched everybody all the time. But at any rate they could plug in your wire whenever they wanted to. You have to live — did live, from habit that became instinct — in the assumption that every sound you made was overheard, and, except in darkness, every movement scrutinized.

This is a critical though elusive point that, as the Post notes, I’ve been arguing for years, including in the 2014 TED talk I gave about the harms of privacy erosions. But one of my first visceral encounters with this harmful dynamic arose years before I worked on NSA disclosures: It occurred in 2010, the first time I ever wrote about WikiLeaks. This was before any of the group’s most famous publications.

What prompted my writing about WikiLeaks back then was a secret 2008 Pentagon report that declared the then-little-known group a threat to national security and plotted how to destroy it: a report that, ironically enough, was leaked to WikiLeaks, which then published it online. (Shortly thereafter, WikiLeaks published a 2008 CIA report describing — presciently, it turns out — how the best hope for maintaining popular European support for the war in Afghanistan would be the election of Barack Obama as president, since he would put a pretty, popular, progressive face on war policies.)

As a result of that 2008 report, I researched WikiLeaks and interviewed its founder, Julian Assange, and found that the group had been engaging in vital transparency projects around the world: from exposing illegal corporate waste dumping in East Africa to political corruption and official lies in Australia. But they had one significant problem: funding and human resource shortfalls were preventing them from processing and publishing numerous leaks. So I wrote an article describing their work, and recommended that my readers support that work either by donating or volunteering. And I included links for how they could do so.

In response, a large number of American readers expressed — in emails, in the comment section, at public events — the fear to me that while they supported WikiLeaks’ work, they were petrified that supporting the group would cause them to end up on a government list somewhere or, worse, charged with crimes if WikiLeaks ended up being formally charged as a national security threat. In other words, these were Americans who were voluntarily relinquishing core civil liberties — the right to support journalism they believe in and to politically organize — because of fear that their online donations and work would be monitored and surveilled. Subsequent revelations showing persecution and surveillance against WikiLeaks and its supporters, including an effort to prosecute them for their journalism, proved that these fears were quite rational.

There is a reason governments, corporations, and multiple other entities of authority crave surveillance. It’s precisely because the possibility of being monitored radically changes individual and collective behavior. Specifically, that possibility breeds fear and fosters collective conformity. That’s always been intuitively clear. Now, there is mounting empirical evidence proving it.

Correction: May 10, 2016

An earlier version of this article incorrectly stated that British libraries refused to house any material on the Taliban. It was the British Library, the national library of the United Kingdom, that refused to house the material.