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For nearly a decade, Reddit, the huge online message board, has been known for its freewheeling stance on letting its users govern themselves. That has resulted in an outpouring of user-generated content — for better or, sometimes, for worse — that attracts nearly 160 million regular users to the site.

Now, Reddit is starting to change its views, if only ever so slightly. Reddit announced a change to its privacy policy on Tuesday that prohibits posting nude photos or videos of people engaged in sex acts without their prior consent to have it posted.

The policy shift will likely upset members of a number of Reddit communities, some of which use the site to trade illicit photos taken of others without their permission. A few years ago, Michael Brutsch created a large sub-community called “Jailbait” dedicated entirely to posting and trading photos of underage girls.

Reddit has undergone a series of changes over the last year, including the departure of its chief executive and a fresh $50 million round of venture capital. The company has re-dedicated itself to proving it can build a long-lasting, sustainable business on top of the huge amount of traffic it regularly attracts, which has caused some degree of self-scrutiny.

“I really want to believe that as we enter the next 10 years of Reddit life, essentially the most trafficked media site on the Internet, the opportunity is here to set a standard for respecting the privacy of our users,” Alexis Ohanian, Reddit’s co-founder and executive chairman, said in an interview.

Mr. Ohanian said he and his co-founder, Steve Huffman, have always taken privacy on the platform seriously, but the changes reflect catching up with the evolution of technology — including the advent of cloud-based online storage and the ubiquity of smartphones — over the ten years since the site was created.

That is particularly pertinent for Reddit, which was embroiled in a scandal last Fall after hackers stole nude photos of celebrities like Jennifer Lawrence and posted them to online message boards, including Reddit.

Since the episode, the company has showed signs of buttoning up. Reddit removed all forums dedicated to trading the photos in response to a flurry of takedown requests, which, according to the Digital Millennium Copyright Act, can be made by those who own the rights to the photos.

Jessica Moreno, Reddit’s head of community, said that someone who wants a photo removed “would need to contact Reddit at contact@reddit.com and alert us to the image with a link.”

Reddit also plans to create a series of moderation tutorials, an effort to help teach its users how to better govern their sub-communities to foster a more positive environment. That includes the appointment of a new head of community, who will work with volunteer Reddit users who moderate their own forums.

Mr. Ohanian emphasized that the site was made up predominantly of positive sub-forums where users help and support one another, and sometimes engage in large-scale altruistic movements to support other members of the community.

But he noted that the new changes are reflective of how he and Ellen Pao, Reddit’s chief executive, have began to think more deeply about the future of how Reddit will look in the years ahead.

“For all of us in social media, we’re all venturing into new territory,” Mr. Ohanian said. “We’ve never seen these platforms — like Reddit, like Twitter — ever before.”