Female nipples will be allowed in posts only in the context of breast-feeding, birth or health, including post-mastectomy or gender reassignment surgery.

“There are no shortage of sites on the internet that feature adult content,” chief executive Jeff D’Onofrio wrote in a blog post Monday. “We will leave it to them and focus our efforts on creating the most welcoming environment possible for our community.”

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The company did not say what prompted the change. For years, Tumblr has been known for the sex-focused subcultures active on the image-forward service: Those sharing and making explicit fan fiction and art, amateurs and hobbyists, and LGBT erotica and pornography.

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The decision comes just weeks after Apple dropped the company’s software from the App Store after child pornography was found on the site, raising questions about whether Tumblr’s decision was related.

“With its massive distribution and strict rules, Apple’s App Store has had a broad homogenizing and sanitizing effect on the internet,” Motherboard wrote on Monday.

The site noted the declining number of websites that support pornography and nudity, writing that “much of that attitude has grown out of Apple’s strict controls over the App Store and the iOS ecosystem.”

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Apple’s aversion to pornography on its platforms has been well publicized.

Many took to social media to complain about Tumblr’s change, sharing innocuous posts that had been flagged by the company.

Others joked about the idea that the platform, with significantly less reach than Facebook and Twitter, banned one of the main things people visited it for.

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“What a lousy decision,” the writer Roxane Gay posted on Twitter. “Adult content is the only reason I use Tumblr.”

Statistics from the web analytics service SimilarWeb, cited by TechCrunch in 2017, found that “adult” content was the top driver of traffic to Tumblr’s desktop site, responsible for some 20 percent of clicks.

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Some were quick to connect Tumblr’s decision to Congress’s recently passed Stop Enabling Sex Traffickers Act, which changed federal laws to make websites responsible for any prostitution and sex trafficking that is facilitated on them.

Websites like Backpage.com, certain message groups on Reddit and the personals section of Craigslist were shut down in response. Congress said the law was meant to better police sex trafficking, but some sex workers have complained that it eliminated a safe venue to advertise their services.

Tumblr’s decision was a reminder of the way that social media companies have, at times, more effectively policed nudity than more destructive content like misinformation and racial hatred.

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On Monday, searches for sexually explicit terms on Tumblr did not turn up any results, while racist and white supremacist content, including Nazi propaganda, was easily surfaced, despite the company’s prohibition on “hate speech.” The company has been changing its search policies over the years to filter out more adult content, but it was not immediately clear if it had made any more changes in recent weeks.

Tumblr was founded in 2007 by David Karp. In 2013, the company was bought by Yahoo and later became part of a unit called Oath, which is owned by Verizon. Karp left last year.

Its previous policies allowed users to filter out adult content from their feeds, describing their philosophy as “live-and-let-live.”

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