Commit the phrase "but you are free" to memory now — these are the four magic words the can double your persuasive powers. The "but you are free" (BYAF) compliance-gaining technique works like this: You have a request for someone, so you tack on the phrase "but you are free" or "but you are free to refuse" after stating it. Why does this work?

In 2013, communication researcher Christopher Carpenter published a meta-analysis of the research on the BYAF technique in the journal Communication Studies. In it, he describes the very first experiment with the technique in 2000 by French researchers Nicholas Guéguen and Alexandre Pascual: "One of the experimenters approached individuals walking alone in a shopping mall in France. In the control condition, the experimenter made a simple, direct request: 'Sorry, Madam/Sir, would you have some coins to take the bus, please?' In the experimental condition, the experimenter added: 'But you are free to accept or to refuse.' Those in the experimental condition were substantially more likely to comply with the request. Moreover, those who gave in the experimental condition gave twice as much as those in the control condition." Carpenter went on to say that the BYAF technique may work so well because it eases the target's perception that his or her ability to say "no" is being taken away.

As you might expect from research that was originally performed in French, the wording of the phrase doesn't really matter. Guéguen and Pascual also tried "but obviously do not feel obliged" and found that to be just as effective. According to the meta-analysis, "The factor most consistently emerging has been the importance of verbally recognizing the target's freedom to say 'no.'"

Why not take it a little further? How does this sound: You can get what you want and make someone like you at the same time. Try coupling the BYAF technique with a request for a favor from someone you want to make a good impression on. According to the Benjamin Franklin effect, you can gain someone's admiration by asking him or her to do a favor for you. We dissect all the science behind it right here.

Get stories like this one in your inbox or your headphones: sign up for our daily email and subscribe to the Curiosity Daily podcast.

For even more persuasion tricks, learn to "Communicate Like A Mind Reader" on the Curiosity Podcast. Stream or download the episode using the player below, or find it everywhere podcasts are found, including iTunes, Stitcher, and Gretta.