For more than 35 years, psychology professor Robert Cialdini has been studying the science of persuasion to figure out what makes people say “yes.”

He put much of what he’s learned in his 1984 best-seller, “Influence: The Psychology of Persuasion.” Now, newly revised, Cialdini has added some more recent persuasion tricks to the book.

One of those is creating a “we group” mentality.

The key to creating a “we group” is to make the person you want to influence feel the two of you are part of the same group, says Cialdini.

“People are inclined to say yes to someone they consider one of them,” Cialdini writes in his newly revised book “Influence: The Psychology of Persuasion.”

“It’s about tribe-like categories that individuals use to define themselves and their groups, such as race, ethnicity, nationality and family, as well as political and religious affiliations,” he writes.

“We” relationships are ones that allow people to say, “Oh, that person is one of us,” to trigger the feeling of shared identity, he adds.

For example, Cialdini says a couple of years ago, he was on deadline with a grant proposal and realized he needed to include some of his colleague’s data.

“This is a guy who is known in my psychology department as being irascible, sour and difficult,” Cialdini tells CNBC Make It.

Cialdini emailed his colleague that he desperately needed the information right away because the grant was due the next day. The colleague refused to give Cialdini the data because he couldn’t be responsible for Cialdini’s poor time management skills, he said, according to Cialdini.

So Cialdini replied: “You know, we’ve been members of the same psychology department now for 12 years. I really need this.”

Cialdini says he had the information he needed by that afternoon.

“The key is where you have long standing partnerships, relationships, common memberships in ‘we groups,’ you just need to point to them, and all of those features that are associated with that kind of unity come to the surface,” Cialdini adds.

Those who place themselves within the boundaries of “we,” get more agreement, trust, help, liking, cooperation, emotional support and forgiveness and are even judged as being more creative and humane, Cialdini writes.

CNBC