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I’ve known my two best friends since 9th grade. In that time, a lot has happened, and I’ve forgotten a lot of it. It’s not unusual now for one of them to say “Remember in high school, when this happened?” and for me to reply “Well now I do.” They’re always reminding me of things I’ve forgotten. They’re an extra hard drive for my limited memory capacity.

In science, this is known as a transactive memory system. Transactive memory systems (TMS) are repositories of knowledge that are shared between two or more people. A shared memory of events, like with me and my friends, above, can be part of it, but it’s also a way of calling up facts that other people know. If you say “Oh, what’s the movie that starts with that whistling cartoon rooster?” and I say “Robin Hood,” that’s transactive memory. You have access to my knowledge, and vice versa. But, it only works if we trust each other that we both know what we’re talking about, and that we know we can call on each other for the knowledge if we need it.

These systems have so far been studied in romantic relationships and work groups (coworkers, classmates, groups slapped together in the lab). A new paper published in the Journal of Social and Personal Relationships looks at transactive memory in the context of best friendships. The researchers asked people to answer questions about their relationship with their best friend—how satisfied and committed they were to the friendship, how long they’d been friends, things like that, as well as questions designed to measure the strength of their TMS, like “I trust that my best friend has credible knowledge.”

“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.”

One question this research raises is whether you really need your own personal trivia team at your disposal in the age of Google. “Relying on our computers and the information stored on the Internet for memory depends on several of the same transactive memory processes that underlie social information-sharing in general,” one study from 2011 concludes. Does this mean we are outsourcing our memories more to the internet than to each other?

Ledbetter doesn’t think so. For one thing, the internet makes it easier to use our friends’ knowledge. “We can turn to our social network and ask for advice about repairing the car, finding a doctor, or what fun books to read during the summer,” he writes. And while you could just Google those questions too, Ledbetter writes: “We have a wealth of information at our fingertips with Google—but with our friends, we have a wealth of trust.”

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