How do you know a teen or twentysomething is about to do something risky? If they send a text or tweet with the acronym Y-O-L-O – as in, “you only live once.”

According to the website Topsy - which analyzes Twitter trends – almost 40 million tweets have included the acronym YOLO since it came on the scene in October of 2011.  And a high percentage involved young people doing something risky or dangerous.

For example, it was in the last tweet from aspiring rapper Ervin McKinness. He boasted that he was drunk driving at 120 miles per hour, YOLO – right before he crashed into a wall, and killed all 5 people in his car.

Other YOLO tweets announce when people are binge drinking or skipping an exam at school.  And since the YOLO trend began, about 408,000 YOLO-tagged tweets had something to do with "texting while driving."

Child psychologist Dr. Mary Romano isn’t surprised that young people are announcing their risky behavior online. She points out that the part of the brain involved in reasoning and planning doesn’t fully develop until about age 25.

But the thrill-seeking part of the brain is thriving at that age. Also teens tends to make bad decisions when other people are around to witness them. That’s why teen drivers are more likely to take chances when they’re in cars with other teens – like speeding, making risky lane changes, or running lights.

And even if a teen is alone – thanks to smartphones and Twitter apps - there’s always an audience.

But does a YOLO text or tweet always mean a teen is doing something they shouldn’t? No. Anthropologist Dr. Ilana Gershon says a lot of the dangerous activities kids are tweeting about are lies. She points out that some people brag about risky behavior online to see what kind of responses they get, without actually doing anything.

But remember: You may only live once but you only die once, too.