And according to Joshua Wolf Shenk, the author of "Powers of Two," these tiny shifts are also the catalyst for a different kind of thought process - a shared mind, so to speak - that allows couples (romantic or not) to come up with more creative solutions to problems than they'd ever think up on their own.
Here are some of the signs psychologists have observed that they say characterize 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 inmate 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 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 couples' 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.
4. 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.
5. You have a bunch of inside jokes that no one else thinks are 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, 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'd both heard the same background information before their conversation.