SHARON POSET: I had a nice childhood. But I just always remember being lonely. I always wanted someone my age. I wanted a playmate.
DEBBIE MEHLMAN: It was very similar. I grew up always wanting to have a friend and always wanting somebody my age. We wanted another one of us,
SHARON POSET: Yeah, for real.
DEBBIE MEHLMAN: Yeah,
NARRATOR: Psychology is the study of behavior and thinking. Identical twins Sharon Poset and Debbie Mehlman offer psychologists a particularly rich story to examine. Sharon and Debbie were separated a week after birth and adopted by different families. Neither woman had any idea that her twin sister lived just 20 miles away and was leading an oddly similar life.
SHARON POSET: We were both like Daddy's little girl. We could not do enough to please our dads.
DEBBIE MEHLMAN: Both families were religious. Like my father was practically Orthadox Jewish. And Sharon's parents were very Catholic. We both got married right after college.
SHARON POSET: Like as soon.
NARRATOR: The twins went into similar fields of work, and each gave birth to one child. Then in 1997, at 45 years old, Sharon and Debbie learned of each other's existence. They went into their first conversation with very low expectations.
SHARON POSET: We really thought we had nothing in common. We talked on the phone, and we thought, how long is this phone conversation going to last? We just talked about a few things. We like tea, and we both had a lot of dogs in our life.
DEBBIE MEHLMAN: Which is part of the trying to fill the void.
SHARON POSET: --void of looking for each other. And I thought that was going to be it. We decided that I would fly up to meet her. Yes, I wasn't looking forward to a pleasant experience.
DEBBIE MEHLMAN: She's dressed kind of very casually, and I'm like all dressed up. And then her hair was really curly, and mine's straight. I'm like, oh, we don't even look like each other.
SHARON POSET: And I'm like, what a stick in the mud.
DEBBIE MEHLMAN: We were so underimpressed with each other. I don't think it could be any worse.
SHARON POSET: And then it's like our faces locked, and I'm like, oh, my gosh!
DEBBIE MEHLMAN: And she was saying, don't look at me.
SHARON POSET: And I kept saying, don't look at me. I can't take it.
[INTERPOSING VOICES]
DEBBIE MEHLMAN: I kept saying, it's my face, but it's on somebody else. I just burst out laughing. And all of a sudden it's like, and it starts--
SHARON POSET: Because our laugh is the same. And then we're doing this.
DEBBIE MEHLMAN: And we're doing this with our hands.
SHARON POSET: And started laughing like that, where we couldn't talk.
DEBBIE MEHLMAN: You know she's out here?
SHARON POSET: I did.
DEBBIE MEHLMAN: Oh.
NARRATOR: Three months after meeting, Sharon and Debbie became participants in the Minnesota Study of Twins Reared Apart, a research project that examined the lives of 137 sets of identical and fraternal twins that had been raised in different families.
SHARON POSET: Chronologically that's when we first met, and that was November 17th, I think.
NARRATOR: The study was a unique opportunity to explore nature and nurture, the ongoing scientific conversation about how much of our personality is influenced by genetics, or nature, and how much is influenced by our environment, or nurture. Sharon and Debbie had already reached their conclusion.
SHARON POSET: They're genetics.
DEBBIE MEHLMAN: They're genetic. These sets of twins that never met each other, and they both like wear exactly the same outfit and would have a rubber band on-- how can that be environmental? These people never met.
We had to run up a flight of stairs three times. I'm like, oh, well, I exercise more. I'll probably be like a second faster. It was exactly the same.
NARRATOR: The twin study was primarily descriptive-- research that describes behaviors but can't establish cause-and-effect relationships. Experimental research, meanwhile, seeks to uncover cause-and-effect relationships.
DEBBIE MEHLMAN: I remember when you sent this to me.
NARRATOR: Even the best research cannot always draw a clear line between nature and nurture. While identical twins shared nearly 100% of their genes, countless environmental factors make every twin unique.
DEBBIE MEHLMAN: We're not clones of each other. There's still environmental influences.
SHARON POSET: She exercises a lot more and cares for her health better than I do.
DEBBIE MEHLMAN: And the dogs.
SHARON POSET: Yeah. She has like a husky, the fluffy dogs more. And I like boxers, dobermans, schnauzers.
NARRATOR: Sharon and Debbie both agree that since reuniting, they have created a special bond.
DEBBIE MEHLMAN: We can't put it into words. We're in each other's head all the time.
SHARON POSET: We could be locked in the closet, and we would have the best time.
DEBBIE MEHLMAN: And it's not like one and one makes two. It's like one and one makes--
SHARON POSET: Like 20.
DEBBIE MEHLMAN: --20, yeah.
SHARON POSET: --when we're togther.
DEBBIE MEHLMAN: I just hope we're healthy and we can see each other and be together. This must be a dream. This is too much. It's too wonderful. I must have dreamt that I found my twin.