Social proof is a decision-making shortcut: "If other people chose this, it's probably a good choice." Robert Cialdini identified it as one of the six principles of persuasion, and it's arguably the most powerful on landing pages.
It works because decision-making is expensive. Evaluating a product from scratch — reading docs, comparing alternatives, assessing risk — takes real cognitive effort. Social proof shortcuts this: "2,400 teams use this, including Stripe and Notion. Probably fine."
Six types of social proof (ranked by effectiveness)
1. Expert proof — endorsements from recognized authorities. "Recommended by Y Combinator" or a quote from a known industry leader. Highest impact, hardest to get.
2. User proof — testimonials, case studies, and reviews from real customers. The backbone of most landing pages. Quality > quantity.
3. Crowd proof — sheer numbers. "50,000 users" or "4.8 stars from 2,400 reviews." Works when the numbers are large and specific.
4. Peer proof — logos and testimonials from companies similar to the visitor's. "Used by teams at companies like yours."
5. Certification proof — awards, certifications, compliance badges. Important in regulated industries.
6. Earned media proof — press mentions, "As seen in TechCrunch." Effective but fading in impact as earned media becomes easier to get.