Whether it's Luke and Leia, Romulus and Remus, Mary-Kate and Ashley or those adorable kids down the block in their matching outfits and their double stroller, twins turn heads.

Doctors and scientists have been studying them for centuries. Psychologists have been at them for years. Hollywood loves them. Everyone who has ever had to do a double-take to tell one from another understands that, without a doubt, twins are cool.

What is it about twins that fascinates us? Here are six reasons (or, if you prefer, twin lists of three) why twins are so (awesomely) awesome:

1. You don't see them every day.

Twins gather for a group photo during the final day of Twins Days 2007

Twins gather for a group photo during the final day of Twins Days Aug. 5, 2007 in Twinsburg, Ohio. (Photo: Rick Gershon/Getty Images)

Twins are — relatively speaking (ahem!) — fairly rare. In 2011, according to the Centers for Disease Control and Prevention (which keeps track of birth rates and the such), the twin birth rate in the U.S. was 33.2 births for every 1,000 total births.

Now, they're not as rare as they used to be. Over the past three decades or so, from 1980 until 2011, the rate of twin births in the U.S. rose about 76 percent. (The Atlantic estimated last year that the surge resulted in more than 1 million more twins in the U.S.)

The theories on the explosion of twins: Women are giving birth later now, and older women, because they produce more of certain hormones, are more likely to have twins. An increase of infertility drugs is also cited as a major reason.

Still, even with the increase in twins — and it has leveled off since 2009 — a little math shows that they’re rare in the population as a whole, which accounts for all the head-turning.

2. They look like each other!

Identical twins Edgar and Gabriel pose for photographs with two version of Dutch artist Vincent van Gogh's Sunflowers paintings at the National Gallery in London

Identical twins Edgar and Gabriel pose for photographs with two version of Dutch artist Vincent van Gogh's Sunflowers paintings at the National Gallery in London on Jan. 24, 2014. (Photo: Leon Neal/AFP/Getty Images)

Well, of course they don't. Not all of them, anyway. Fraternal twins develop from two different eggs (they are dizygotic), which makes them little more than regular siblings.

Now, sure, regular siblings can share many physical characteristics, thanks to the genetics supplied by their parents. But to say twins automatically look alike just because they were born at the same time?

Ah, no. But, when you think about it, that's pretty amazing, too.

3. Okay, then. They look so different!

Fraternal twins

Photo Beth Shepherd Peters/Shutterstock

Well, yes. Twins can look really different. That's the point. Take this pair, for instance. One is light-skinned with straight red hair. The other is dark-skinned with dark curly hair. They are, according to the post on iflscience.com, the 18-year-old offspring of a white father and a mother who is half-Jamaican.

Both great-looking girls who look nothing alike. That's the genetic luck of the draw.

4. But wait. Some are identical. Right?

Twins Lisa and Julie York at the 2007 Twins Day festival in Twinsburg

Twin sisters Lisa and Julie York pose for a picture during the final day of Twins Days Aug. 5, 2007 in Twinsburg, Ohio.  (Photo: Rick Gershon/Getty Images)

Absolutely. Identical twins come into being when a single fertilized egg (they are monozygotic) splits in two. They account for about only 4 percent of twin births. So these twins are really rare. And they often look very much alike.

Check out this National Geographic photo gallery and just try to tell those identical twins apart.

5. And they have some weird mental bond that nobody else has, right?

Pfc. Lane Higson and Pfc. Casey Higson, identical twins serving in Iraq

Pfc. Lane Higson and Pfc. Casey Higson, identical twins serving in Iraq. (Photo: Spc. Roland Hale/DVIDSHUB/flickr)

Slow down, now. Twins, and many family members of twins, swear this is true. But the science remains fuzzy.

"Twins do sometimes say they can sense when the other is in trouble," psychologist Nancy L. Segal, director of the Twin Studies Center at California State University-Fullerton, told National Geo for the magazine's look at twins a couple yeas ago. "Of course, when you worry about somebody 20 times a day, chances are that one of those times they are going to really need your help. That doesn't make it telepathy."

6. Twins are, to many scientists, the key to answering the nature vs. nurture question.

A man aims a paper ball at his sleeping twin brother

Photo: bikeriderlondon/Shutterstock

Yes. If there's ever going to be an answer to nature vs. nurture — the ages-old debate over the role that genetics play in the people we become, vs. the role that our environment plays — twins almost undoubtedly will have a huge say in it. Especially identical twins.

The theory that identical twins could be crucial to science, first espoused in 1875, goes like this: Because identical twins come from one egg, they share the same DNA. So any differences that arise — whether physical or, perhaps, psychological — are likely due not to who they are at birth, but what they face after it. In other words, what they eat, how they're raised, the air they breathe and what they watch on TV (for just a few) play more of a factor in what differences they have, or what differences pop up, than their genetic makeup. That's the working theory.

The University of Utah Health Sciences explains how the studies work. "For example, when just one twin gets a disease, researchers can look for elements in the twins' environments that are different. Or when both twins get a disease, researchers can look for genetic elements shared among similar twin pairs."

(Spoiler alert: Nature vs. Nurture is a trick question. It's really about nature and nurture and their different roles.)

One recent study looked at twins in Finland who shared exercise habits as kids but diverged in their exercising as adults. The study, published in the March issue of Medicine & Science in Sports & Exercise, found that the twins developed different body styles and mental capacities over time. The ones that didn't exercise had higher body fat and exhibited less endurance in tests. The active twins showed much more brain activity.

There are many, many more studies that examine how genetics shape identical twins and the role that the environment plays after birth. Whole departments of many colleges are devoted to the study of twins. The University of Minnesota is a big one in in the U.S. and Kings College London is an important one in the U.K.

All that research may not help us tell Harry Potter friends Fred and George Weasley apart (get on that, Kings College!), or give us hints on how to play tennis like three-time Wimbledon doubles champs Mike and Bob Bryan (identical American twins).

But the more we look at twins, the more fascinating they become.

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