Justin Bieber's Fans: Crazy or Just Babies?

VIDEO: Selena Gomez is threatened after photos surface of her kissing Bieber.PlayABCNEWS.com
WATCH Justin Bieber Kiss Sparks Twitter Firestorm

Who knew a nuzzle could breed so much nastiness.

After Justin Bieber, 16, was photographed getting cozy in the Caribbean with another teen pop star of the moment, Selena Gomez, 18, his fans quickly turned on her via Twitter, even though neither the singers nor their representatives have labeled the relationship as anything more than platonic.

Some of the more vitriolic tweets:

-- roses are red, violets are blue, @selenagomez if you'll break @justinbieber's heart I'm gonna kill you :3

-- @selenagomez If you are the Girlfriend of Justin I will Kill you I HATE YOU :@ !!!

-- @selenagomez whore cancer whore .. like i'mm kill myself cuz i saw you and Justin kissing well thankyou Selena thankyou now i'm killing myself

Amid the cacophony of criticism, some support surfaced:

-- When I see those piccs from @justinbieber & @selenagomez I'm freaking out inside. But then I realize, he likes her so ... I won't stop him :)

-- @selenagomez I love you with all my heart & I always will, no matter what you do or who you date. I'm just happy to see you happy :) xoxo.

A little love, a lot of hate. In any case, a flood of feelings. This isn't the first time it's happened. Last month, the "Baby" singer's fans flew into a frenzy after he and Gomez were snapped leaving an IHOP restaurant together. (At the time, Gomez told ABCNews.com, "It was just pancakes!")

Last May, after Bieber joked on Twitter that Kim Kardashian was his new girlfriend, his devotees came at her, claws out. It begs the question: Why are Bieber's fans so invested in his personal life?

"That's sort of the history of his fan's behavior. He has an intensely loyal fan base," said People magazine reporter Carlos Greer. "He's had to cancel concerts because they've gotten out of control. Any female pop star, actress, whoever, who sort of intrudes, this is pretty much their reaction. He's the king of the current pop boom. He's not only their pop crush, he's their pop idol."

To be sure, rabid fans have ranted and raved since pop music started pumping. Elvis Presley, the Beatles, Michael Jackson and others boasted legions of followers who could probably out-shriek the "Beliebers," as some Bieber-lovers call themselves. But with the Internet, fans can instantly comment on anything and everything relating to their object of affection, often without consequences.

"In past generations, people wrote letters to fan clubs or confided in a diary," said Nadine Kaslow, professor of psychiatry and behavioral sciences at Emory University. "Now, people write it on Facebook, they tweet about it. It's in the public space. Because the forum is different, people feel as if they're more entitled. If one person boos Justin's relationship, other people think they can boo it too. It becomes a competition -- Who Loves Justin More?"

Beyond that, with Twitter, there's the possibility that the star in question might (gasp) talk to a mere mortal, whether by retweeting something a person said or acknowledging praise with a simple ":) thanks." Forget weeks of waiting by the mail for a formulaic fan club letter. Now, direct communication with a pop phenom could be mere moments away.

"It's so easy, it's so quick," said Greer. "If you tweet to Bieber and he responds, it's like, 'Oh my God, he actually talked to me.'"

While it may seem as if Bieber exerts otherworldly influence over his teen and tween fans (Is it the hair? Does it possess magical powers?), the passion he arouses in that demographic has little to do with him and a lot to do with hormones.

"With the adolescent brain, it's sometimes said that the gas pedal -- the neurological parts -- come on way before the breaks do," said Alan E. Kazdin, a professor of psychology and child psychiatry at Yale University. "What you get are more impulsive thoughts and actions."

For some, hormones can turn Bieber into a boyfriend.

"These teenagers develop fantasy relationships in their mind," said Kaslow. "These fantasies feel real to them -- not in a psychotic way, in a teenage way -- and they feel so real that they feel jealous, they feel betrayed. We're seeing this as sort of a microcosm of how people feel when their real boyfriend or girlfriend betrays them. Some people get really mad about it. They get hostile."

Ultimately, Kaslow said, the brain evolves. As teens age, melodramatic reactions to the personal lives of pop stars start to seem silly. But until Bieber's fans grow up or he ceases to be the sensation he is today, watch out for a war of (hopefully, only) words over his every kiss and cuddle.

"There's a nastiness," Kazdin said. "These tweets depict a scary nastiness of our times. You wouldn't want these people to have access to weapons."