“We found that the longer they were friends, the stronger these transactive memory systems were in the friendship,” says Nicole Iannone, a professor of psychology at Penn State University and lead author of the study. “And then trust was really important—the more trust you have in your friendship, the stronger your transactive memory system was.”

And the stronger the TMS, it seems, the stronger the friendship, though it’s not clear which causes which. People who had powerful TMSs with their best friends reported higher friendship quality, even when the researchers controlled for things like trust and how long they’d known each other.

That makes sense to Andrew Ledbetter, a professor of communication studies at Texas Christian University who has studied friendship. “When we develop transactive memory systems, we're able to communicate better,” he told me in an email. “We're psychologically closer. Our lives are integrated with one another. And that forms a friendship bond that's tough to break.”

There are two different structures of a TMS—differentiated and integrated. In an integrated TMS, friends share similar knowledge and are able to reinforce or remind each other of what they know. In a differentiated TMS, they have knowledge of different things, and can consult each other like encyclopedias. The researchers found that in mixed-gender best friendships, TMSs were more likely to be differentiated, while in same-gender best friendships, they were more likely to be integrated. But regardless of the gender makeup, the systems were equally strong.

“If you’re in a mixed-gender best friendship it seems you might have this advantage where you have more information yet the strength is very similar,” she says.

Another thing that surprised Iannone was that there was no relationship between the amount of time friends spent together and the strength of their TMS, even though research on other relationships has found that face-to-face time is important for the development of these systems.

“What I think is happening is that in best friends, oftentimes you don’t even live in the same place,” Iannone says. The sample in this study consisted of college students and adults, and other research has shown that as people grow older, they tend to move farther from their friends. So it may be that friends’ TMSs were built in an earlier time, when they did live in the same place, and now they just maintain them through the myriad long-distance communication tools at their disposal.

Having as I do two best friends with whom I’ve traveled this life in an unholy trinity, I wondered how might TMSs work when more than two people are involved? That’s a question Iannone says she wants to research in the future, but she speculates that group TMSs can be “even more beneficial,” and as more people enter the group, they would be more likely to be differentiated than integrated. “One of the clear applications of this is if you go play trivia at a bar, and you have the friend who’s great at science, and the friend who knows so much about pop culture, and then you have this other friend who knows a lot about say, food and cooking. You can combine these different things together.”