TODAY

A Justin Bieber fan gets hysterical at the concert.

They’d been camping on the streets of New York City for three days -- throngs of teen and tween girls, who had gone without showers (except for the ones that fell from the sky) in the hopes of getting a front row glimpse of their favorite teenage heartthrob.

And when everyone’s imaginary “Boyfriend” Justin Bieber took the stage this morning to play a mini-concert for TODAY, these super-fans did what teen groupies have done for eternity: they screamed, they cried, they quivered. 

So what's behind all the shrieking and sobbing, the shaking and swooning? Is there a physiological explanation for this fan-demonium or is it simply tween and teen girls gone wild?

"It's basically a group response set off by emotions and hormones," says Dr. Sarah Pitts, a specialist in adolescent medicine at Boston Children's Hospital.

TODAY

The crowd surrounds The Biebs.

A lot has to do with what's happening in the heads of tweens and teens. In those early adolescent years from 10 to 14, kids are searching for new people to love outside of their family and they might glom on to a celebrity crush, suggests Pitts.

In addition, being around groups of young people may also make this behavior more "contagious" during a phase when peer pressure rules. For the 15- and 16-year old Bieber fans in the audience, there's also the growing interest in forming relationships and sexual attraction that fuels the emotional excitement of seeing your teen idol.

"The teen brain is still significantly changing and not yet in its final form," points out Pitts. "And that affects how teens respond to the world around them."

Young girls are especially prone to getting emotional and when they get excited, they cry "happy tears."

"I don't know that there's a scientific basis for that," admits Pitts. "It comes down to the chemistry in our brains and nerves."

As for keeling over, Pitts says fainting is really common in teens, more so in females.

If a young Bieber fan has been standing outside for hours, and they're sleep deprived and very excited, and perhaps also hungry and cold, then passing out is a possibility. It's more of an emotional faint, in which falling over is actually protective allowing blood to quickly shunt back to the brain.

Of course, this generation is not the first to weep, screech, and faint at its teenage heartthrobs. These behaviors go way back. If it wasn't the Biebs who was bringing on the waterworks and hysteria, it was the Jonas Brothers, or ‘N Sync; the Jackson 5, the Beatles or Elvis.

So why don't young boys scream and swoon at a Miley Cyrus or Selena Gomez concert? "Boy's brains and hormones are wired differently," says Pitts. And while it's culturally OK for girls to cry, that's not the behavioral norm for guys.

Then again, maybe it's just that chicks are the bigger Bieber "believers." 

More on Justin Bieber:

• Surviving a concert camp-out: 7 tips from Bieber fans
• Matt Lauer joins Twitter (with some help from Bieber)
• Video: Justin and 'Mrs. Bieber' chat with Matt Lauer
• Watch his TODAY performance

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The adolescent brain might not be such a teenage wasteland after all.

Recent research suggests that the activity in teen brains may have some Nostradamus-like qualities when it comes to predicting the hits or misses of popular music.

In a small study, scientists recruited 27 adolescents, ages 12 to 17. They asked each kid to listen to 60 15-second clips of songs from largely unknown artists found on MySpace. The clip included either the hook or chorus of each song, and volunteers only listened to tunes from their three favorite musical categories, which ranged from country, rock and indie to hip-hop, blues and metal.

Researchers recorded the teens' reactions to each song using brain-imaging scans, and they also asked participants to rate how much they liked the music on a scale of one to five stars. By using unfamiliar musicians and vocals, scientists hoped to get a raw response, as if teens were hearing the track for the first time.

For three years after the scanning took place, the scientists gathered data on each song's sales figures to see which ones were fan faves or flops.

Although the teens' tastes in music from their likability ratings showed no link to a song's commercial success, their brain scans told another story: Activity in the ventral striatum -- the brain's reward region -- was predictive of future sales figures and popularity.

"We found that when an area of the brain associated with reward and anticipation was active while listening to the song, chances were greater that the song would eventually go on to sell more than 20,000 units," says Gregory Berns, MD, PhD, a neuroeconomist and director of Emory University's Center for Neuropolicy.

While the teens brains displayed a modest knack for picking out songs that would sell at least 20,000 units -- about one-third of the brain images could predict this -- they were even more accurate at identifying failures: Nearly 90 percent of the songs that showed a weak response in the brain's reward region had tepid sales.

Most of the study songs were duds with dismal sales, but three were industry hits (500,000 units sold) including "Apologize" by OneRepublic and two country cuts, "Don't Laugh at Me" by Mark Wills and "Drink, Swear, Steal, and Lie" by Michael Peterson. But none of these tunes were in the teens' top 10 in eliciting brain activation so they weren't exactly hit-predicting machines.

"The fact that there was any predictive power at all was surprising," says Berns, the study's lead author. "There are so many songs released each year and so few hits, that the odds were stacked against us."

The study appears in the Journal of Consumer Psychology.

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