NSA whistleblower Edward Snowden's revelations on government mass surveillance have apparently scared a significant number of people away from searching for terrorism-related information on Wikipedia, as a new study shows readers are afraid of being investigated.

The conclusions come as the online encyclopedia giant showed a 30-percent drop in searches on terrorism-related topics – the most direct evidence yet of something called the "chilling effect," described by University of Oxford and Toronto’s Jonathon Penney as the measure of the negative impact on legal conduct that arises out of the leaks by the former NSA contractor who exposed the PRISM program.

Many in academic circles have been denying that such an effect exists, as well as its potential negative fallout. This prompted Penney to delve deeper into the issue, which has steadily been rising in popularity across academic disciplines.

His findings are published in an upcoming paper in the Berkeley Technology Law Journal. There he analyzes the fall in traffic after taking a look at 48 Wikipedia topics monitored closely by the US government on national security concerns. "Al-Qaeda," "jihad" and related searches top the list compiled by the Department of Homeland Security.

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To measure the effect Penney took as his starting point a date 16 months prior to the first revelation in summer 2013. In that period the interest in terrorism had been rising mildly from 2.2 million searches at the low point to 3.0 million just before the NSA leaks. As people woke up to Snowden's revelations, the figure plummeted rapidly back to the average 2.2 million, then further below 2.0, until stabilizing at 2.5 million only in summer 2014, according to Penney’s calculations.

“The Article finds not only a statistically significant immediate decline in traffic for these Wikipedia articles after June 2013, but also a change in the overall secular trend in the view count traffic, suggesting not only immediate but also long-term chilling effects resulting from the NSA/PRISM online surveillance revelations,” Penney writes.

He also found that the more sensitive a topic was, the more likely it was to cause privacy concerns among the public. Penney established a list of more, and less, ‘safe’ topics. The safer topics, which presumably reflected only a general interest in US government security forces, retained steady levels of interest.

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Penney’s study joins a growing body of related research, such as that dealing with the increase in the number of online searches for privacy protection tools, like encryption and special web browsers.

More broadly, the research seeks to “offer important insights about how we should understand such chilling effects and their scope, including how they interact with other dramatic or significant events (like war and conflict) and their broader implications for privacy, US constitutional litigation, and the health of democratic society.”