Science says these 5 things happen to couples who have been together a long time

Getty Images/Pascal Le Segretain

Being with someone for a long time changes the way you see the world.

It also changes you. 

More importantly, close relationships may spark an entirely different way of thinking and acting, something Joshua Wolf Shenk, author of "Powers of Two," chalks up to having a "shared mind." 

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:

1. 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 University of Texas professor of communication Robert Hopper, secret communication accomplishes two things: First, it helps deepen your bond — romantic or platonic. Second, it 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. 

2. You stop self-censoring.

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 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 more open. 

Many of the pairs Shenk talks to in his book have such a relationship. University of California 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 Tverksy, "this caution was completely absent."

3. You start to 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, body sway, and the little mannerisms and idiosyncrasies of how we speak.