Copycat couples: you see it all over Hollywood: twosomes who look like twins. Cosmo investigates why some mates (and it's not just celebs) appear to merge.

* In movieland lately, it seems that when celebs hook up, they do this funny thing where they start morphing into mirror images of each other. Even pairs who start out as polar opposites will begin dressing the same way, talking alike, and developing similar mannerisms and interests. For instance, Gwyneth Paltrow and Chris Martin may appear to be two peas in a macrobiotic pod, but if you recall, before they met, she was a glamazon hailing from Hollywood royalty, and he was a grungy rocker from a working-class English family.

Is this just freakish famous-person behavior? Experts say no and that all of us outside of La-La Land are just as guilty of partner identity theft. We walk out of the house wearing the same kind of outfit as our man and pick up our boyfriend's hobbies too. Fascinated by the phenom, Cosmo went to experts to explore such mate mimicking. Here are the biggest reasons they gave us for why couples are doing it.

* We Want What We Don't Have

It's common to fall for someone we wish we were more like, according to Judith Kuriansky, PhD, author of The Complete Idiot's Guide to a Healthy Relationship. "We are naturally attracted to people who have qualities that we admire. When we start dating them, we try to emulate those strengths, and after a while, they may rub off on us." The process makes distinctions between twosomes start to blur.

This scenario is very normal and positive, especially if both parties are learning from each other. Think of it this way: If you're type A and superstressed, you might go for a laid-back guy because being around him relaxes you or encourages you to let your hair down. You adopt his chill-out techniques (maybe sleeping in or wearing comfortable clothes), so you start to seem similar. And chances are, your ambition will encourage him to become a tad more motivated. In other words, everyone wins. Just look at Demi and Ashton. It appears being with Ashton has made Demi feel younger and more carefree, and in return, Ashton's picked up some style tips from his fashionable wife and seems more mature and responsible (notice how he's become a devoted step-dad to her three daughters).

* Partners Bring Out Our Best

Sometimes it's not about taking on qualities your partner has that you lack but rather further developing traits that he possesses too. Let's say you can get into the joy of acting like a goofball at times. Being with a funny guy who relates to that side of you will encourage your jokey side to become more dominant. Your sense of humor was always present but may have been lying dormant because, perhaps, it wasn't appreciated by a previous boyfriend, explains psychologist and relationship coach Janice D. Bennett, PhD.