5 things that happen to couples in long-term relationships, according to science

It also changes you.

More importantly, close relationships may spark an entirely different way of thinking and acting, something Joshua Wolf Shenk, author of " rel="noFollow" target="_blank"Powers of Two," chalks up to having a "shared mind." Shenk, who has written extensively about psychiatry and psychology for outlets including The Atlantic and The New York Times, also directs the Erikson Prize for Mental Health Media.

So, how do you and your significant other stack up? Check out these signs psychologists have observed in long-term couples that they say point to having such a shared mind.

You and your partner develop your own private language.

Ever get a text from your significant other that means absolutely nothing on its own but carries a certain significance that you can't quite explain?

This "insider" language is one of the first signs that the two of you are operating in sync, writes Shenk. According to a study from Robert Hooper, a University of Texas professor of communication, secret communication accomplishes two things: It helps deepen your bond — romantic or platonic — and establishes a unique, shared identity.

Private language can include everything from inside jokes to nicknames, writes Ohio State University psychologist Carol Bruess in a study of romantic couples. Bruess' research suggests a link between how often partners use these private words and how satisfied they are with their relationship. Bruess found that the more often couples used secret words and phrases, the happier they tended to say they were.

You stop self-censoring when you're together.

The way most of us speak with strangers, acquaintances, and even close friends is markedly different from how we talk when we're alone with our partner.

When we're with others, most of us "self-monitor." That is, we try to please the people around us by adapting our behavior to suit theirs.

But when we're with an intimate partner, we often let go of this pattern of behavior and instead "talk fluidly and naturally," Shenk writes. In other words, we stop having to constantly check ourselves before we speak. We're more candid and open.

Many of the pairs Shenk talks to in his book have such a relationship. University of California at Berkeley psychologist Daniel Kahneman, for example, tells Shenk: "Like most people, I am somewhat cautious about exposing tentative thoughts to others." But after he'd spent a few years working with his research partner, cognitive psychologist Amos Tversky, "this caution was completely absent."

You have a bunch of inside jokes that no one else finds funny.

Research suggests that couples are more likely to mirror each other's body language — which in turn makes them look alike — because they're drawing from a wealth of knowledge that only they share. This "insider info" — all of your shared experiences and memories — informs your gestures and posture and the words and phrases you use with each other.

A 2007 study, for example, found that people were more likely to copy each other's eye gaze when they both heard the same background information before their conversation.

You might even start to sound alike.

In addition to having their own private vocabulary, long-term couples eventually "start to match each other in the basic rhythms and syntactical structures of their speech," writes Shenk.

Part of that is a result of a phenomenon that psychologists call "emotional contagion." Basically, when two people spend enough time together, they begin to match each other's speech patterns. We mimic everything from the other person's accent to the amount and length of pauses he or she puts between words and sentences.

There's some evidence to suggest that these changing speech patterns can even serve as one indicator of how long a couple might stay together.

Part of a 2010 study of language use among couples that looked at their text messages, for example, found that when two people "sounded" more alike — in terms of the words and language structure they used in their messages — they were also more likely to still be dating three months later.

... or, in some cases, look alike.

In his influential 1987 study, psychologist Robert Zajonc found that there's a very obvious reason that married couples start to look alike. They use the same muscles so often that, over time, they start to mirror each other.

This coordination of movement isn't accidental, says Shenk. Instead, it "reflects what psychologists call a "shared coordinative structure," which includes how we harmonize our gaze and body sway and the little mannerisms and idiosyncrasies of how we speak.