Why do couples, like Harry Styles and Olivia Wilde, often dress alike? Experts explain the phenomenon.

When Olivia Wilde and Harry Styles stepped out together recently, the couple was pictured wearing similar outfits featuring white T-shirts, ripped jeans and sneakers. Shortly after the paparazzi pictures emerged, internet headlines called out their "twinning" moment.

They're far from the only pair who've been called out for matching. For years, fans have lovingly trolled celebrities like Brad Pitt, who has an impressive record of mirroring the women he's dating, right down to their ensemble and hairstyle — including Gwyneth Paltrow, Jennifer Aniston and Angelina Jolie.

Neil Patrick Harris and David Burtka, Abby Wambach and Glennon Doyle, Gabrielle Union and Dwyane Wade, Benedict Cumberbatch and Sophie Hunter, Jim Parsons and Todd Spiewak, Pauletta and Denzel Washington, and many others, have faced similar allegations.

But coordinating couples is far from just a Hollywood occurrence. There are plenty of pairs who look and dress eerily similar to each other, particularly as their relationship goes on, whether it’s expressed in their mood, fashion and grooming choices — or even the way they smile, laugh, frown, enter a room or speak, matching each other's vocal tone and cadence.

But were their choices conscious or unconscious? What lies behind this kind of phenomenon? And how common is it? These are the bigger questions scientists (and fans) want to know.

Dawnn Karen, a fashion psychologist and author of Dress Your Best Life, explains that mirroring your partner's fashion choices is related to having a keen sense of empathy, and signifies a high level of happiness and content within a relationship.

"I've been seeing a lot of couples who dress alike and even dress their children like them, too," Karen tells Yahoo Life. "Research has [shown] that similar personalities are why people could be attracted to each other, so if I'm just relating that to a form of dress, I will correlate them to a high degree of happiness."

Does that mean guys like Pitt and Styles are just oh-so-sensitive souls? Perhaps.

"There definitely is a correlation there," says Karen. "Mirroring in a conversation shows that you're both on the same page. You're vibrating on the same frequency. You know, they cross their legs and you cross your legs; they move to the left and you move to the left. In terms of fashion, mirroring each other as far as dress, there definitely is a correlation [with highly sensitive people]."

It is important to note that when a person routinely changes themselves, their tastes and their opinions around fashion to please their partner, it can be considered unhealthy. It's something she refers to as "fashion situational code-switching."

"It's when you alternate between different styles, depending on your cultural or social situation," Karen elaborates. "You may dress a certain way with one group or in one context, and a different way in another. So basically, in the fashion and congruence aspect, it becomes unhealthy when you are not being your ideal self. When everything in your closet is centered around your partner or when you put their emotions or their needs before your own, you neglect yourself."

Jaime Bronstein, a relationship therapist and coach, agrees, adding that these types of scenarios aren't purely for young lovers but can "happen at any age."

"I've seen clients in their 20s to their 60s or 70s, where people lose themselves in relationships," Bronstein explains to Yahoo Life. "There's compromise in a relationship, but if you find yourself sacrificing, that's not good."

"Sometimes couples randomly do it and it’s not on purpose. It just shows how united they are," she expands, adding that her own parents recently celebrated their 50th anniversary and still find themselves dressing alike "all the time."

"In general, I think it's a very cute and playful thing," she says. "I think it sends a message of, 'We're a strong and bounded couple and we like to have fun and we're not boring.' I think if you do it in a classy way, not a cheesy way, then it can be cool. It also sends off a message saying, ‘We are bound. We are one. We are a united team.' But also, like, ‘Hands off [to onlookers].'"