Why Couples Look Like Each Other + 5 More Ways Partners Become More Alike Over Time

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couple with similar traits

There are many things that change when you start falling in love with someone, but there is nothing like science to explain why those changes happen.

Though its common to read relationship advice about how important it is for people to maintain their sense of individuality when they become part of a couple, would it really surprise you to learn that being in a loving, long-term relationship can — and does — often change who you are and how you look and behave?

It's true — as we grow close to our partners, we mirror each other's mannerisms and facial expressions, and we share diets and lifestyles (among countless other things).

Ultimately, this infleunces us to grow more and more similar — not just inside, but outside, in physical appearance (or on the face of it, you could say!).

Why do couples look like each other? 

Many factors play into how similarities can be drawn from the ways in which we look like our significant others. Some are there from when the attraction first started building, before the relationship had even happened yet, while other resemblances occur so incrementally — day by day — that we might not notice it ourselves until someone external points it out.

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In fact, Robert Zajonc, a psychologist from the University of Michigan, came to the conclusion that we begin to look alike over time because we end up developing the same laugh or frown lines, after having shared so many years of life together. He believed this to be especially true of older couples.

However, it should be noted that Zajonc's determination is a theory, not fact, because he could only prove a correlation, and not the causation (or, more simply, he could recognize the effects, but not prove exactly what caused them).

Interestingly enough, scientists were able to prove that some people subconsciously seek partners who closely resemble themselves from the start, referred to as "positive assortative mating" and sometimes "homogamy," which is common among the animal kingdom, including our own, as humans.

Biologically, we can be hardwired to look for "kinship" and genetic traits similar to our own, which are markers for fertility and genetic compatibility.

If you think about it simply for reproductive purposes alone, this "narcissism" the researchers discovered makes some sense.

But what about all the other reasons why couples look alike?

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Here are 5 more ways couples become more alike over time.

1. They begin to adopt the same mannerisms.

To add to how couples may aesthetically look more like one another over time, humans participate in something called "the chameleon effect," which is an unconscious way of adopting each other's postures, speech patterns, facial expressions, and other behavioral cues.

Over time, couples can become so synced up in how much they share these otherwise distinctive behavioral signals with one another that they come across to others as having such similar verbal and non-verbal cues for how they communicate that these similarities become apparent to those around them.

Similar findings could be concluded from a study out of the Department of Psychology at the University of Cincinnati, which found that couples start using the same muscles more often and begin to "mirror" each other, which leads to them having the same mannerisms.

Mirroring is considered a more conscious way of mimicking someone else, whereas the chameleon effect is nonconscious.

2. They start to sound like each other — and even create their own language.

If you think you're the only ones who have made up secret ways to communicate, you're wrong. Psychologists found that couples frequently make nicknames, inside jokes, secret words, and phrases as a way of connecting more intimately.

Joshua Wolf Shenk, a lecturer on relationships and mental health, told Business Insider that couples change their basic rhythms and syntactical structures to become more similar after spending so much time together.

Further, a study published in the Journal of Communication explained how couples develop their own idioms. The researchers wrote, "In an intimate relationship between two persons, certain words, phrases, and gestures may come to carry unique meanings only in the context of the relationship." 

3. They start eating the same types and amounts of food.

Many men try to make women feel bad for gaining weight after getting into a relationship. While there are many reasons why men and women gain weight while in a relationship, the result of any of these reasons all look the same — often bearing some resemblance to their partner's waistline.

Dr. Lisa Young, PhD, RDN, CDN, a Nutrition, Food Studies, and Public Health Professor at New York University, says that men eat so much more because they are just physically bigger.

So when they date a woman, that woman often picks up his habits and starts eating as much as the man. Conversely, if they jointly choose to go on a health kick or one is required to go on a significant diet change, this also changes the eating habits of both individuals.

4. They learn socio-economic conventions and adopt cultural traditions from each other.

One study found that people who marry "up" or "down" tend to teach the other certain things to get on the same page.

A person who is working class learns how to have a more stable life from the other, who is middle class. The spouse who is middle class learns more about spending time with family from the working-class spouse.

Similarly, if one is from another country or otherwise different cultural background, it may be important to that person to preserve certain traditions and incorporate them into the relationship.

These could range anywhere from adopting specific rules on decorum or etiquette, to changing what clothing one might wear, to switching religious affiliation or at least observing certain holidays and customs, to learning another language, to following a particular set of values or system of beliefs.

5. They show "ingroup biases" toward people in similar social circles.

While it is becoming increasingly common to blend socio-economic, ethnic, and cultural backgrounds in romantic relationships today, science has proven that people still have an unconscious tendency — or bias — toward others in the same social circles; for example, those from the same racial backgrounds.

In a UCLA study published in the European Journal of Social Psychology, researchers confirmed that the following demographics have a tendency to favor choosing a partner who is of the same race as themselves: Euro-Americans (whites), African Americans, Asian Americans, and Latinx Americans.

The data collected in the study also helped researchers conclude that people may not only have a stronger attraction toward what is familiar or similar, but they are also deeply influenced by their status and social authority among their own "group."

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Nicole Weaver is a senior writer for Showbiz Cheat Sheet whose work has been featured in New York Magazine, Teen Vogue, and more.