You can’t always get what you want. But if you try sometimes, you can get free pizza on Reddit. And there, in the Random Acts of Pizza subreddit (tagline: “Restoring Faith in Humanity, One Slice at a Time”), Stanford computer scientists say they’ve found the perfect conditions for a natural experiment in human altruism.

“We were kind of interested in what happens if you ask for a favor online. What factors influence the success?” says Tim Althoff, one of the co-authors of the Stanford study. “This Random Acts of Pizza community is really unique because it naturally controls for confounds. It controls for these incentives, because you don’t receive anything back.”

Random Acts of Pizza is indeed unique. It’s where broke, hungry students can go while cramming for finals and ask a random Samaritan for a double-cheese, please. It’s where out-of-work dads request a pizza to feed the kids until the next unemployment check. But not all pizzas are distributed equally. Althoff and his colleagues wanted to see how language had an effect on these requests–what strategies were people using that won them pizza, in the end?





After analyzing 5,700 free pizza requests, the researchers discovered that free pizza corresponded with a hierarchy of needs. The unemployed were most likely to receive free pizza, followed up by poor families. The least likely to eat pizza for free were the people who posted because they simply had a drunk hankering for it. Makes sense.

But researchers discovered a few other interesting factors, too. While they thought that shorter requests for pizza might be more effective, longer word counts actually proved more successful. So was expressing gratitude. Pledging to pay it forward was also another highly rewarding technique. Users who ended their requests by promising to do the same for someone else received more free pizza than those who didn’t. But at the same time, researchers discovered, only 10% of the people who said they would return the favor actually did.

Unlike crowdfunding platforms like Kickstarter, Random Acts of Pizza doesn’t give out rewards for delivering someone else a pizza. Only warm fuzzies. And researchers say that because of these unique conditions, they’ll be able to apply their techniques to even more communities like Reddit. They’ve set their sights on online philanthropy communities like DonorsChoose, in which people can donate to individual public school teachers’ classroom projects.

“We hope that practical implications are that if you understand what factors are important, maybe you set up a community in a way in which people help each other out effectively. What are the factors that are important that you should incorporate into your writing to be successful?” Althoff says. “You can also do that with automatic techniques, automatic tools that could that help you check your writing.”