The modern equivalent of "what does Google rank first" is "what does ChatGPT cite first." The question has an answer, and the answer is neither open, neither neutral, nor stable.

For a generation, Google PageRank was the most consequential algorithm in information distribution. It decided which pages surfaced first, and in doing so decided which institutions, brands, and individuals were visible. Every communications strategy, every SEO budget, every earned-media placement, every Wikipedia entry was built around a single foundational question: what does PageRank reward?

That question is now a legacy question. Its successor is more consequential, more measurable, and far less well understood: what do ChatGPT, Claude, Perplexity, Google AI Overviews, and Gemini actually cite when they assemble an answer? Between August 2024 and April 2026, six of the largest citation-tracking studies ever conducted produced overlapping datasets covering more than 680 million individual citations. Everything-PR Research has synthesized that data into the first consolidated ranking of the fifty sources that compose the modern AI answer.

The findings contradict most prevailing assumptions about AI behavior. Platforms do not treat the open web as a level playing field. They concentrate citation share in a remarkably small set of domains. They privilege user-generated community content over most institutional journalism. They treat a single encyclopedia as near-foundational training material. And they exhibit volatility so severe that a domain's citation share can fall by eighty percent inside a month — as ChatGPT's Reddit share did in September 2025.

For communicators, publishers, educators, institutions, and brand owners, this is the working map of the new discovery terrain. The organizations that understand it will compound visibility over the next 24 months. The organizations that do not will discover, too late, that they have been running campaigns against a set of rules whose mechanics were already documented.

Methodology

This index synthesizes verified citation data from six of the largest published studies of generative AI citation behavior conducted between August 2024 and April 2026, covering more than 680 million individual citations across the five dominant AI answer engines.

Source studies and scope

• Profound's longitudinal citation study (August 2024 – October 2025), covering 680 million+ citations across ChatGPT, Google AI Overviews, and Perplexity, with updates through Reddit's own Q2 2025 shareholder disclosure.
• Semrush analyses of 150,000 citations (June 2025) and 230,000 prompts over 13 weeks (August – November 2025), documenting citation volatility after the September 2025 Google num=100 parameter change.
• Ahrefs Brand Radar analysis of the 100 most-cited domains in ChatGPT (September 2025), broken out by US and global coverage.
• Search Engine Land consolidated cross-platform analysis (October 2025), covering Reddit, YouTube, LinkedIn, Wikipedia, and Forbes as the leading multi-platform citation sources.
• Muck Rack's journalism citation study (July 2025), analyzing more than one million citations to measure how often AI platforms rely on journalistic content and which publishers are cited most.
• Visual Capitalist's synthesis of LLM citations across more than 150,000 queries (August 2025), documenting aggregate ranked frequency data.

Everything-PR Research consolidated these studies into a cross-referenced citation-share signal — defined as the average rank position of each domain across the source studies, weighted by each study's citation volume — and constructed a ranked index of the 50 domains that collectively define the modern AI answer. We grouped the index into six functional categories — Community & Conversation, Encyclopedic & Reference, Professional & Identity, Video & Audio, Editorial & News, and Commerce & Review — to reflect the distinct strategic approaches each category requires. The work was conducted in coordination with 5W, the AI communications firm whose AI Visibility practice operationalizes this index for brands, institutions, and category leaders.

Platform-by-platform: each answer engine cites the web differently.

What communications, PR, and institutional leaders should do with this map.

The Citation Source Index is not an academic exercise. It is a working map of where modern earned reputation compounds. The strategic implications fall into seven concrete priorities for any organization operating a 2026 communications program.

1. Audit presence across the top fifteen sources before the long tail. Most communications strategies still treat the web as a uniform landscape. It is not. An organization, brand, or leadership team without verified presence across Reddit, Wikipedia, YouTube, LinkedIn, Forbes, Reuters, and four to five category-specific domains is invisible to 68% of the AI citation pipeline.
2. Build Wikipedia as infrastructure, and resource it accordingly. ChatGPT cites Wikipedia at 26–48% of its top-10 share. Brands and individuals without accurate, sourced, sufficiently comprehensive Wikipedia presence are missing the single largest input to the single largest consumer AI platform. Wikipedia editing is a discipline: notability standards, neutral tone, citation hygiene, and sustained maintenance over years.
3. Treat Reddit as a strategic channel, not a tactical one. Reddit is the single most-cited domain across AI platforms. The average Reddit post cited by AI in 2025 was roughly a year old and originated in late 2023 or early 2024. This is an evergreen visibility asset, not a social-media quick-hit. Identify the ten to fifteen subreddits most relevant to the category and build sustained, transparently-affiliated presence inside them.
4. Map journalism targets to platform-specific citation patterns. ChatGPT favors Forbes, Business Insider, Reuters, New York Post, and TechRadar. Perplexity favors NIH, Microsoft, Britannica, and specialized B2B trade press. Claude favors The New York Times, The Atlantic, The New Yorker, and The Economist. Pitching a single story to a generic "trade press" list is no longer optimized; tier outlets by which answer engine serves the target audience. See also: The PR Measurement Gap Study 2026.
5. Treat LinkedIn as a citation channel, not a social-media channel. LinkedIn is a top-five multi-platform citation source, and dominant in B2B and professional queries. Substantive executive thought leadership — named authorship, consistent posting, category depth — now produces measurable AI citation share. The shift from "social media presence" to "citation asset" is new, and most organizations have not made it.
6. YouTube is the only serious video-citation asset. YouTube holds a dominant citation advantage over every other video source, and is the dominant video reference for Google AI Overviews. Any video-content strategy that does not prioritize YouTube is not a serious AI visibility strategy. Short-form vertical video on TikTok, Instagram Reels, and YouTube Shorts is performing for human discovery — not for AI citation, at least not yet.
7. Plan for volatility, not stability. The September 2025 ChatGPT citation swing was the clearest available signal that AI citation patterns are subject to sudden algorithmic repricing. Strategies that concentrate visibility in one or two sources are exposed to model-level and algorithm-level changes that can displace them within weeks. Diversification across the top fifteen sources is no longer optional; it is risk management.

The work of mapping a single brand, executive, or institution against this index — auditing presence, identifying gaps across the top fifteen sources, and building the multi-quarter program required to compound visibility — is now its own discipline. 5W has built its AI communications practice around it, and the research that follows will track how leading firms operationalize the index across categories.