Dec. 21, 2012 -- When 21-year-old rap artist Ervin McKinness died in a car accident this fall, his fans were quick to notice one final message from his Twitter account: "Drunk af going 120 drifting corners #F***It YOLO."
Less than an hour later, the driver of the car ran a red light, lost control and slammed into a wall, according to Ontario, Calif. police. Four passengers were pronounced dead at the scene. A fifth died at the hospital.
"What is YOLO?" was among the top searches in the United States this year, according to Google, but Twitter users certainly know what it is. It stands for You Only Live Once. According to Topsy, a Twitter analytics company, about 36.6 million tweets have included the YOLO acronym since it first appeared in October 2011 -- and a good percentage of them involved young people doing something dangerous or risky.
"There's something about the way the teenage brain functions that may tend to make poor decisions when other people are present," said child psychologist Dr. Mary Romano, who works at Monroe Carell Jr. Children's Hospital at Vanderbilt University in Nashville, Tenn. "Tie that into social media and even when you're alone, it almost artificially creates the presence of an audience because you sort of assume your actions are on display."
Over roughly three hours one morning last week, 34 people tweeted about the prior night's drunken antics with the YOLO acronym, an ABC News search found. A handful of YOLO-ers asked whether it was too early in the day to start drinking. Another 10 woke up that morning and tweeted that they didn't study for tests they had to take. Their tweets all included YOLO, or, as a Twitter hashtag, #YOLO.
"It was probably the most popular phrase on Twitter, I think," said Jamie de Guerre, the vice president of product at Topsy. He said Twitter users tweeted the #YOLO hashtag 388,000 times a day at the peak of its popularity in March. It's still used tens of thousands of times a day
Since the YOLO trend began, about 408,000 YOLO-tagged tweets had something to do with "texting while driving," according to Topsy.
Trumpeting dangerous behavior is part-biological, part-environmental, Romano said.
Romano said the part of the brain called the prefrontal cortex, which is involved in reasoning and planning, doesn't fully develop until someone is about 25 years old. And while the reasoning part of the brain is undeveloped, the thrill-seeking part of the brain thrives.
"In terms of why teens make risky decisions, there's biological evidence," Romano said.
This is a chart of YOLO and #YOLO on Twitter over time, created by Jamie de Guerre, vice president of product at Topsy.
A 2010 study showed that teens were more likely to make poor driving decisions when they were in cars with other teens, proving what mom, dad and insurance companies have known for years, Romano said. And the near-constant connectedness that comes with Twitter, which many users have installed on their phones, means that teens are rarely alone anymore.
No one has researched whether Twitter causes risky behavior, so it's not possible to conclude that YOLO has actually contributed to risk-taking, said anthropologist Jordan Kraemer, who recently earned her doctorate from the University of California, Irvine.
"Generally, social media provides a new venue for actions that are often the same kinds of actions both young people and adults are already engaging in," Kraemer said."It's hard to know if what we're seeing online is causing new or more risky behavior or if we're just seeing it more because it's more visible."
However, social media do help spread tastes when it comes to movies and music, she added.
It's also possible that people tweeting about dangerous behavior are lying, Indiana University anthropologist Ilana Gershon said. Some users create Twitter personas as a sort of experiment, she added.
"It's not just the act of doing it, it's the act of announcing it publically," Gershon said. "For some people, it's really fun to lie about risky behavior. They get to see what a certain kind of response would mean without actually doing it."
Gershon wrote a book about how Facebook affects relationships. She is now researching how people create personas on Twitter tailored to landing certain jobs.
"People constantly believe other people's profiles, and admitted to me a great deal to which they lie on their own," she said.
Canadian pop artist Drake coined the acronym in a song called "The Motto." YOLO's popularity online began in California last October before spreading to Florida and Texas, up the East Coast and then to the rest of the country, according to Google Trends, which maps search term popularity over time.
According to Topsy, other YOLO Twitter trends included dialing a random telephone number and wearing or not wearing knee pads.
Eventually, Twitter users started reminding the YOLO crowd that they didn't have horcruxes, the soul fragments that Voldemort thought would make him immortal in the Harry Potter series. About 304,000 tweets contained both the terms YOLO and horcruxes.
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