News of an alleged plot to kill Canadian pop star Justin Bieber was devastating to his millions of fans — affectionately called Beliebers — many of whom took to social media this week to express their shock and disbelief.

But the convicted killer who hatched the murder scheme was a Bieber superfan himself, one so infatuated with the singer that he had a tattoo of the young star inked onto his leg.

Dana Martin, 45, told investigators in New Mexico that he hired two men to kill the young star after the 18-year-old pop phenomenon did not return his messages. It was this perceived snub that pushed Martin to plot to kill the object of his obsession, police say.

As bizarre as the plot seems, it is far from the first case where a fan's adoration turns into a fanatic's bloodlust.

In 1981, John Hinckley Jr. shot U.S. President Ronald Reagan to impress actress Jodie Foster, and wrote a letter to the Taxi Driver star expressing how he hoped the assassination attempt would get her attention.

It was the gruesome murder of American actress Rebecca Schaeffer in 1989, however, that shone the spotlight on the deadly tactics some fans could resort to. The star of '80s sitcom My Sister Sam was shot and killed at her apartment by Robert Bardo, who had been obsessed with her for years.

Celebrity fascination 'tied to evolution'

What is it about celebrities that fascinates people? And what turns that initial admiration into obsession, and sometimes pushes people to a murderous rage?

The tendency to dote on celebrities is "tied to our evolution" in many ways, says Jim Houran, a clinical psychologist and author of books and academic studies exploring the psychology of fandom.

"We've always as a species seemed to be hard-wired to worship something," he said from New York. "Whether that's gods, a god, or whether that happens to be successful people in our society that we look up to naturally. In our history that used to be the best hunters, the best gatherers, the best athletes — the people who truly stand out because of their ability."

Now, with a barrage of celebrity news on television, online, in print and on social media, that fascination happens faster, and with a wider range of people, said Park Dietz, a criminologist and forensic psychiatrist who has testified as an expert witness in the trials of Jeffrey Dahmer and John Hinckley Jr.

"Everyone famous since Moses has had a unusual amount of focus on them as a person," said Dietz. "How they got to be what they are has been a source of vicarious identity. Today, because of the changes in the technology of communication, this can happen quite rapidly."

'Sweet' figures draw more stalkers

That being said, the cult of celebrity isn't necessarily bad, said Houran. People chat amicably about the latest show, or celebrities can use their power to mobilize people and raise funds for a good cause, such as the 12/12/12 concert to help victims of superstorm Sandy, he said.

"We use celebrities as a form of entertainment, but also escapism. So actually, celebrity worship in its low levels is a very healthy thing. It can actually bring people together," he said.

But celebrities who are perceived as "sweet, pleasant and non-threatening," draw many more stalkers than equally famous people who are regarded as nasty or sarcastic, said Dietz, based on his research conducted in the 1980s.

"Justin Bieber has a public image that virtually guarantees that he's got 20,000 active stalkers at a time.… He attracts so much positive attention, and so many wishes get pinned on him by both men and women, that within that mix there are going to be people with every kind of personal problem and psychopathology."

Affection for celebrities is a "psychological phenomenon" and the shift from a friendly fan to a deadly stalker progresses on a continuum, said Houran.

Identity issues, mental illness key factors

Most people start off in the first stage, with an affinity for public figures that is voluntary. However, a major disruption in a person's life — possibly a job loss or a divorce — can push them to the next level of celebrity worship, said Houran.

Fans may begin to feel a sense of personal attachment with the celebrity on a level typically reserved for family and friends,

"I'm not saying they turn into a stalker, but it stops being voluntary," he said. "They start withdrawing from family and friends, instead devote time, attention and loyalty to their favourite celebrity. That gives them a sense of fulfilment, a sense of identity."

In the next stage, the fan will compulsively try to get closer to their crush of choice. Houran likens the behaviour to the desperate acts of a drug addict. They may use funds earmarked for bills on memorabilia and likely try to make contact with the celebrity, said Houran.

"It's almost like what used to give that person a fix, a sense of fulfilment, has reached a tolerance," he said. "And now the person has to endorse more intense feelings and has to go through more intense behaviour to get the same level of connection. That's where you start seeing stalkers."

Perceived threat to relationship prompts drastic actions

When that personal relationship with their celebrity of choice is perceived to be at risk, that's when some resort to violence, he said.

It was unreciprocated messages that prompted Martin, who is in a New Mexico prison for the murder of a 15-year-old girl, to hatch the plot against Bieber. The same emotions involved in a nasty breakup can be at play, said Dietz.

"It's all of the usual suspects — it can be jealousy, rejection, humiliation, anger, frustration. Literally any human emotion, when powerful enough, can turn a real or fictitious relationship to the negative."

Mental illness, of course, is likely a key factor that leads a superfan to be an obsessive, even violent, stalker.

Research examining cases involving fans who managed to hurt, or fatally wound, their celebrity targets showed that the majority of the perpetrators had some kind of mental affliction, said Dietz. The study examined letters written by the aggressors to their victims from the 1960s to the 1980s, he said.

"We found that a very large proportion of the people who communicated inappropriately suffered from an obvious mental disorder that we could accurately categorize from their written communications," he said. "Many had serious mental illnesses, personality disorders."

Mostly 'harmless' fans

But the two extreme types of fans are at opposite ends of the same continuum, Houran said, and factors in an individual's personal life can propel them toward more intense activity.

"Well over a decade of research suggests that there are not two groups of people: normal healthy fans and crazy fanatics. No. There's a continuum," he said.

But on the whole, celebrity gossip and fascination with the lives of rich and famous people are harmless, said Jason Anderson, arts writer for The Grid newspaper.

"There's always going to be people that have what we might consider an unhealthy fixation on celebrities," he said. "But a lot of the time, it's just going to harmless. You look at N'Sync fans or Bay City Roller fans. There is a degree that many people might see as extreme, or too much, but it's usually something pretty harmless."

In fact, the entertainment world relies on these highly energetic devotees to drive demand for everything from the latest instalment of Twilight films or the newest celebrity perfume, he said.

"The industry wants superfans, these are the people that drive demand, the people buying the stuff," he said.

"The trouble is if you foster this, if you make Kristen Stewart really important in all these people's lives, there are going to be the people with predispositions and troubles that go that way. But you can't not do it because there are people that are potentially dangerous."

With files from The Associated Press