12-year-old twins Adam, left, and Evan Bogart

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Two years ago, I entered Twins Nation.

I don't just mean I gave birth to twins. I mean my obstetrician had twins, I attended a childbirth class exclusively for parents of twins, and not long after I came home to my Chicago three-flat, a family with twins three months younger than mine moved in downstairs.

A co-worker bought a condo a few doors down; now his wife is pregnant with twins.

This past fall I attended one birthday party with three sets of twins and another party with two sets and a woman with twins on the way.

Had I somehow slipped into a Twins Time Warp, a "Twilight Zone" episode populated solely by chubby-cheeked duos with a taste for mischief and mac 'n' cheese?

As it turns out, the truth is almost as strange.

The twin birthrate, which stood at about 1 in 60 in 1971, has risen rapidly because of fertility treatments and an increase in the number of older women giving birth, with almost 1 in 30 American babies now being born as part of a pair.

In Arizona, the number of babies born in twin deliveries increased 59.4 percent, from 1,568 in 1994 to 2,500 in 2004 — a year with a record 2,663 multiple-birth events (which includes triplets and more), according to the Arizona Department of Health Services' most recent data.

Multiple-birth deliveries accounted for 2.9 percent of the state's total births in 2004, up from 2.3 percent in 1994 and 1.8 percent in 1984, department records show.

The increases in multiple births don't surprise De Gaddis, who joined the Tucson Mothers of Multiples Club soon after her daughters were born 11 years ago.

It's more common to see twins around Tucson now, she said, particularly within the last five years. "I've noticed that they pop up everywhere. I go to the grocery store, and there's twins. At the kids' school, there's a couple sets of them," said Gaddis, who learned that twins run in her family.

Dr. Louis Keith, a professor emeritus at Northwestern University's medical school, says the increase in the nation's "twin birthrate is unprecedented anywhere else in the world."

"The real epidemic of twins didn't begin until the mid-1990s, so we are now in the epidemic," says Keith, president of the Center for the Study of Multiple Birth in Chicago.

Keith says it's too early to know what that might mean in the long term, but some experts say that the increase could have an impact on facets of society ranging from athletics to politics.

Already, the parents of twins have made their mark in the field of education, where schools have traditionally separated twins entering kindergarten. Minnesota recently became the first state to pass a law guaranteeing parents a say in separation decisions, and in December a similar bill was introduced in the Illinois General Assembly.

Parents of twins have turned to experts such as Nancy Segal, a psychology professor at California State University-Fullerton, who says that there is no scientific evidence to support always separating twins. In fact, says Segal, there are relevant data, regarding the effect of entering school with a close friend, that suggest that kids actually adjust faster in the company of a close companion.

Among the implications for the general population: If parents of twins can now increasingly request that their kids enter school together on the grounds that this will improve their academic performance, what's to stop parents of singletons from requesting a class placement with a best friend on the same grounds?

Other potential societal effects could spring from the unique characteristics of the twins themselves.

Twins tend not to be the very top achievers in their fields, many observers have informally noted, although no one has actually studied this. We have had no twin presidents, for example. Bill Gates isn't a twin; Picasso wasn't a twin, nor was Bach or Marie Curie.

On the other hand, twins do excel in athletics, perhaps even beyond what their numbers would indicate, with well-known examples such as gymnast Paul Hamm, an Olympic gold medalist, and his brother, Morgan.

"It's helped us a lot," Paul Hamm says of being a twin. "Just the fact that you have someone else there to kind of one-up each other, push each other to do new things."

There is also anecdotal evidence, according to Segal, that twins, because of their unusual side-by-side upbringing in which so much is shared, tend to be concerned with fairness and sensitive to the needs of others. These are qualities that at least one political scientist, Harvard professor Barry Burden, associates with support for welfare and education programs.