Double Life: Identical - Or Not?

Erin Moriarty Reports

All their lives, 48 Hours Correspondent Erin Moriarty and her sister Sheelah have believed they were identical twins.

Then they signed up for a simple test to find out for sure if they are, or if the doctor who delivered them more than 40 years ago got it wrong.

They don't look alike; Sheelah tans better than Erin and has smoother hair. But twin expert Nancy Segal says some identical twins don't look alike, while many fraternal twins do. Everything from nutrition to how twins are positioned inside the mother before birth can cause noticeable differences. Indeed many sets of twins are uncertain whether they are identical or fraternal, Segal says.

The only one way to know definitively is with a DNA test. Sheelah took a test in Chicago. Erin had one in New York. The samples were evaluated in, of all places, one of the Twin Cities, Minneapolis. Segal, Sheelah and Erin met there to find out the results.

The answer: They are fraternal twins! Segal's own study found that 67 percent of delivering doctors have been wrong in their determination of whether twins were identical or fraternal, she says.

But Sheelah needed more convincing. So the twins decided to take a more accurate test - which also indicates they are indeed fraternal. And now they know the truth. Finally, there's an explanation for all of the twins' differences. There's only one thing left to do. "You have to tell Mom," says one, laughing.