Part II: Their Reunion

They decided to meet at a neutral spot, a McDonalds parking lot. Tamara brought her ex-boyfriend and her best friend for emotional support.

"I see this girl with a big, black bubble jacket with dark long hair, and I'm like, 'That's Tamara.' And I couldn't look," says Adriana. "We both wore bubble jackets, jeans. Mine was purple, hers was black."

"All we could say to each other was, 'Hi,'" says Tamara. "We didn't cry. Because we didn't really know about each other. We hadn't missed each other. So, it wasn't like we were longing for each other."

The girls barely spoke to each other, occasionally stealing a glance. But for Tamara, nothing seemed real until they got to Adriana's house.

"We walked in and I have my communion pictures over my TV. Tamara was just staring at it, for like 20 minutes," says Adriana. "She was raised Jewish and she was like, 'I didn't have a communion. What is this?' … That got her."

As amazed as Tamara was, her mother, Judy, was also having trouble believing it. When the Rabis were raising Tamara in Manhattan, they had no idea she a sister -- much less an identical twin living just a half hour away on suburban Long Island.

Judy, a therapist, says she had to see Adriana to believe it: "When she walked into my house the first time, that moment, she looked just like Tamara. I wanted to be joyful for her, because she had found a sister and I was conscious intellectually of wanting to feel joyful for her. And yet there was some part of me that felt the sadness."

Just two weeks before the girls met, Judy's husband, Yitzhak, had died of cancer. Judy admits that she was suddenly very worried about being alone: "I did feel frightened of losing her. For that moment, I did."

"She said to me one night, she's like, 'Tamara, you know having a sister is great,'" says Tamara. "She was like, 'But I don't want you to love me any less.' I said, 'You're my mom and I'll always love you no matter what.'"

For Tamara, the reunion was a miracle, a blessing that happened for a reason. Almost 10 years ago, her newfound sister, Adriana, also lost her father to cancer. Both families have come to believe the two fathers brought them together.

"I feel like it's wherever people go when they die, her father met my father and they gave this to us," says Tamara.

Researcher Nancy Segal has spent a lifetime trying to answer these questions. She has interviewed more than 50 pairs of identical twins who were separated at birth.

"Identical twins raised apart are exceedingly rare," says Segal. "they're very, very valuable cases to scientists."

She wants to know how Tamara and Adriana's similarities compare with other identical twins separated at birth. What can they teach us about why we behave the way we do? Why we pick certain jobs? Why we have certain friends?

"To the extent that identical twins are more alike in these things, we can say with a fairly strong degree of confidence that yes, genes do play a role," says Segal.

So, does it really matter what you do as a parent? Long-range studies of identical twins show that parenting is important because it affects how well your child will develop their genetic potential.

"We find that genes play a 50 percent role in fashioning personality, which means that half is also fashioned by the environment," says Segal.

"The twins show some very striking similarities in personality, but when you look at personality development across a broader spectrum of people, you will find some differences."

Whatever the differences between Adriana and Tamara are, it's their similarities that matter most to them.

"I just feel like I've known her my whole life. I just feel so comfortable and there's just so much familiarity with her that it's strange," says Tamara. "Even when we walk together, I just feel like it's right. It's just so strange."