Do They Have a 2-for-1 Break on Membership? : Groups: More than 20,000 twins and multiples are listed with the Twins Foundation. It’s a resource that attracts both researchers and Hollywood.

What do Mario Andretti and Jane Pauley have in common?

Mario Andretti has a twin brother and Jane Pauley is the mother of twins. But they’re also members of the honorary advisory board of the Twins Foundation--a “Smithsonian dedicated to multiples”--created 15 years ago by twins, for twins, about twins.

Before 1983, there was no central clearinghouse for information on twins and there was no such thing as a twin registry. In fact, notes Kay Cassill, the founding president, the United States was the only developed country that had no means of keeping track of who and where the nation’s twins were.

That information, she believed, was important to researchers who could glean vital genetic clues from twins. Furthermore, a twin registry would be a handy resource for modeling agencies and film and TV producers who know the value of a photogenic match.

Today, the Providence R.I.-based Twins Foundation has listed on its rolls more than 20,000 U.S. twins and multiples. Many more are expected to be listed in coming years because of the multiple birth rate explosion due to fertility drugs and in-vitro pregnancies.

Cassill says she got the idea for the organization when she was working on her book, “Twins: Nature’s Amazing Mystery” (Atheneum, 1982). A longtime free-lance writer with friends in the magazine world, she rounded up a group of colleagues who, like herself, were twins.

“I felt we had managed to survive our early years as twins and come away with not too many scars,” says Cassill, now vice president of the foundation. “We had been successful in our work and it seemed like we should be giving something back.”

In May, 1983, the group met for lunch at the top of the Time-Life Building in Manhattan. There was Dick Stolley, who was a founder of People magazine and later managing editor at Life; there was John Mack Carter, the editor of Good Housekeeping, and a number of photographers and writers, including Cassill’s identical twin, Marilyn Holmes. A few doctors and other professionals, all twins, rounded out the founding board.

Of the 11 original board members, eight were identical twins and three were fraternal twins.

The board’s first action was to send notices to more than 4,000 newspapers across the country announcing the creation of the Twins Foundation. They also alerted as many Mothers of Twins clubs as they could that they were on the lookout for twins, triplets, quads, quints and other multiples.

For the foundation’s data base, respondents were asked questions that are particular to twins. For example: If you dressed alike, did it bother you? Was there pressure to separate you in school? If people mistook you for your twin, did it bother you? Has being a twin put stress on your marriage?

Cassill, who has two boys and one girl--”one at a time”--says even the happiest of twins can list a few disadvantages. For example, twins seem to be constantly compared to one another. Or each twin will feel a burden of responsibility for the other. Commonly, twins may have difficulty in developing a self-concept.

To counter the negative aspects of twinship, the foundation’s newsletter, the Twins Letter, takes pains to identify twins who have made an impact on the world, among them: Elvis Presley, whose twin brother died at birth; Norris and Ross McWhirten, who wrote the Guinness Book of World Records, and don’t forget Romulus and Remus, the founders of Rome.

It’s no surprise to Cassill, who lives in Providence, and her sister, who lives in Denver, that they’re both involved in the same enterprise, and that both are accomplished in the arts. Brought even closer together by the death of their mother when they were 3, the girls stuck together throughout their entire school years and even had their own synchronized swimming act.

“We were very close,” Cassill says. “It seemed like a natural thing to do.”

Now, Cassill is part of an all-volunteer staff at the foundation’s office. The size of the staff fluctuates depending on the number of requests received.

On a recent day, volunteers were tracking down twins from a particular part of the country for a cardiovascular risk study. A week earlier, they had located twins in California to be considered for a TV commercial job.

Dr. Nancy L. Segal, director of the Twin Studies Center at California State University, Fullerton, says the registry of the Twins Foundation is invaluable to researchers.

“It’s a wonderful resource because often it’s very difficult to find twins,” says Segal, whose current studies involve people whose twin siblings have died. “It makes them accessible, especially in regard to certain characteristics.”

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Multiple Facts About Twins

* Most “multiple pregnancies” consist of twins.

* Twins occur naturally in about one in 100 pregnancies (triplets, one in 8,000; quadruplets, one in 500,000).

* The likelihood of becoming pregnant with more than one baby increases until about age 40, when there is a decline.

* Assisted reproductive technology such as in-vitro fertilization and the development of fertility medications such as menotropins and clomiphene have artificially increased the number of multiple births.

* Before the availability of ultrasound, twins weren’t discovered until the delivery room.

* The occurrence of fraternal twins--formed from two eggs--is influenced by race, age, family history, past pregnancies and fertility medications. The occurrence of identical twins--formed when a single fertilized egg splits--is not subject to these same factors and is the subject of much study and speculation.

* Fraternal twins are twice as common as identical twins, which occur in about 1 in 250 pregnancies.

* About half of all women who have twins have Cesarean births.

* While most single babies are born at 40 weeks of gestation, twins are born at an average of 37 weeks.

Source: “Mayo Clinic Complete Book of Pregnancy & Baby’s First Year” (William Morrow, 1994)