Elana Warren has learned to spot the lonely teen walking the school hallways or sitting alone at lunch.

She's ready with a kind smile or a soothing word. "After high school," she might say, "life will change and there will be so many more, new and different opportunities."

Sometimes, those are words she tells herself. "I've been lonely," said Warren, a 17-year-old junior at Hopkins High School, "and it's helped me become very empathetic with other teens."

It's not at all unusual for teens like Warren to feel on the outs at times. It would be a rare teen who never saw a Facebook post from a party he or she wasn't invited to, who never hid behind a locked bedroom door in tears.

For most teenagers, those feelings are blessedly short-lived, ebbing more than they flow.

But it's another, smaller group of adolescents who have captured the attention of Belgian research scientist Janne Vanhalst. Her findings offer us a rare look into a sad reality:

Chronic loneliness.

For four years, Vanhalst, along with fellow researchers from Belgium and Duke University, followed 730 teens with an average age of 15 at the beginning of the study.

Most teens in the study did not experience high levels of loneliness, or their sense of isolation was temporary. For about 3 percent, however, loneliness stuck like Superglue year after year until it became a self-fulfilling prophecy.

Vanhalst focused on the social well-being of teens, she said, "because loneliness is a risk factor for many mental and physical health problems. Yet, it is sometimes not taken seriously."

Problems tend to arise when teens, appropriately, pull away from their parents and other adults in favor of their peer group.

"Not all chronically lonely adolescents will become depressed or will experience other health problems," she said, "but as a group, they are at higher risk compared to their peers. We should not wait until their loneliness results in depressive symptoms before helping them."

One part of the study, which was published in the Journal of Personality and Social Psychology, presented teens with short scenarios of social inclusion and social exclusion and asked how they thought they might react.

For example:

"A new lunch place opened in town and they are giving away free sandwiches today. Some of your classmates are going there for lunch and ask if you want to join them."

And:

"You open your Facebook account and see that many of your classmates have been tagged in an album … you notice that the pictures were taken several days ago at the birthday party of one of your classmates. You were not invited."

In the first scenario, an example of social inclusion, the researchers found that chronically lonely teens were the least enthusiastic about being invited, writing off the opportunity as "coincidence." Perhaps they were just standing in the right place at the right time, instead of actually being a desired companion.

Results from the second scenario are equally painful. Ostracized teens felt sadder, angrier, more anxious and more insecure than other teens. They blamed their lack of invitations not on outside circumstances, but on personal failing.

What's normal? What's not?

So, how do we as parents and caregivers differentiate the "normal" stuff of growing up from the "troubling" stuff? How do we identify the chronically lonely child in our classroom, next door or under our own roof?

Vanhalst is candid that she doesn't have concrete answers about how to break "the vicious cycle of chronic loneliness." The research is as young as the subjects are, and more digging is warranted.

Early discussions, however, suggest that lonely teens can be helped most by learning strategies to check and change their negative thinking. This seems to be more effective than helping them improve social skills or increase opportunities for social contact.

Equally important, we must remain vigilant with a young person whose loneliness seems to linger far past its expiration date of normalcy.

"Feeling lonely after a school transition or after moving to a new place is very common," Vanhalst said. "If an adolescent keeps feeling lonely for a long time, for example long after a school transition, and if the adolescent feels lonely in different situations and feels sad about it, I think we need to be alert."

She hopes that her study is a springboard for others to explore the differences between temporary and chronic loneliness, so that teens only experience the former.

Warren, of Hopkins High, hopes for that, too.

"These students seem to be the kindest people when you get to know them, because they know what it's like to feel excluded," she said.

"I would tell them, 'Even if you can't break out in high school, high school is a short part of life.' "