“Social proof” is a critical psychological principle that savvy business owners can use to dramatically increase sales and grow their businesses. The principle simply states that people are more likely to do something when they see others doing it. For example, after entering a new restaurant, customers are more prone to sit down and eat if they see others in the restaurant versus if it was completely empty.
Interestingly, there’s one famous example when the power of social proof caused unintended and negative results. The example was Nancy Reagan’s ‘Say No to Drugs’ campaign in the 1980s. While the campaign hoped to decrease drug use, the opposite actually happened. Yes, teen drug use actually increased in the 1980s as the campaign implied that many teens were using drugs. This social proof made other teens think it was ok if they tried drugs too.
On the other hand, there are countless examples of using social proof for benefit, such as the following:
1. Social Proof from Other Users/Customers
Showing other users and customers is the most common form of social proof. Here are some examples:
Even more powerful is when you get your customers to invite their friends to become customers. Hotmail did this extremely effectively by putting “join Hotmail” advertisements in the footer of all email messages. This prompted Hotmail to grow from 500,000 users at the start of 2007 to over 12 million users by year’s end. Likewise, allowing friends to invite friends to play through Facebook helped Zynga grow over 10 times, from 3 million to 41 million average daily users, in just one year.
2. Social Proof from Experts
This form of social proof is when you show approval of your product or service from credible experts.
I used this form of social proof when marketing my book, Start at the End. Specifically, I received, and subsequently promoted, reviews from several experts such as: Marshall Goldsmith, Kevin Harrington, John Jantsch and Brad Feld among others.
A similar example is Sensodyne toothpaste promoting that “9 out of 10 Dentists Recommend Sensodyne” for sensitive teeth.
3. Social Proof from Celebrities
An estimated 25% of television commercials in the US now use celebrities. For example, you’ve probably seen Catherine Zeta-Jones promote T-Mobile over the years. You may have also seen Beyonce promoting milk and William Shatner promoting PriceLine.com.
Even if you don’t have the funds to afford to big celebrity, you can use this form of social proof to your advantage. For example, when luxury pillow manufacturer Pillo1 received positive publicity from Oprah on Oprah.com, Pillo1 effectively showed this on their website. 
4. Social Proof from Research and Past Results
Showing research and past results gives positive social proof to spur new customers to buy your offerings. Here are some examples:
5. Social Proof from “Borrowed Trust”
A final form of social proof is when you “borrow” trust from other brands. Examples of this include:
As you can see, there are numerous ways to use social proof to influence others to take the actions you want. Use these examples as a starting point in brainstorming ideas to leverage social proof in your business. And then use the other proven marketing tactics  to take your business to the next level.