Hang around Harvard Square for a while and you just might see 855 smiley coeds skipping around the quad.

Pills? Booze? Wacky tobacco? Hey, this is Harvard — these kids are high on learning.

They're devout followers of Tal Ben-Shahar, a psychology lecturer who created Positive Psychology, now the most trendy class on campus.

The course is so popular, it now trumps Introductory Economics, the previous enrollment champ (which currently boasts 669 students).

And lest you think this is some New Age mumbo jumbo, Ben-Shahar is quick to point out that the search for contentment has old and illustrious roots.

"Aristotle talked about being happy and so did Confucius," he says. "I think my students love my class because they want to be happier."

And how do you learn how to be happy? Ben-Shahar believes happiness has a lot to do with simplifying your life.

"We live in this rat-race culture and we want to do more and more stuff or fit in more and more things in less time," he says. "I talk to my students about identifying what they really want to do. Then they need to do it, which means sometimes saying no to other things."

This is serious stuff. High up on the syllabus: Learning to be grateful. "There's research that shows that keeping a gratitude journal leads to a high-level feeling of well-being and health," he says. "That's fascinating."

The class, which meets for 90 minutes twice a week, even includes time for meditation — not exactly standard ivory tower fare.

And how about that homework? Assignments can include writing about a difficult experience and the lessons learned from it or penning a "gratitude letter." (Talk about higher learning — taking a class to learn how to write thank-you notes!) Students learn how to overcome procrastination and discuss the "underprivilege of privilege."

If it sounds like fun, it is. Only according to Ben-Shahar's introductory lecture, it's "rigorous fun."

And more important, it's necessary. In his class, the happy professor notes that rates of depression are 10 times higher today than in 1960, and that the mean age of depression has plummeted to 14.5, down from 29.5 in 1960.

But it's not just Harvard students. At a time when self-help books fill bookstore shelves, positive psychology courses are being offered at more than 100 universities nationwide.

And your Great Aunt Hortense isn't the only one who thinks gratitude and optimism are fine subjects of study.

"I think it's terrific that positive psychology is being taught in colleges because it presents a healthier, proactive way of thinking, analyzing and approaching life," says Jane Greer, a psychotherapist in private practice in New York City.

"The fact that it's being taught at Harvard means that these students are going to be empowered. Coming out of Harvard, these students have the potential for great leadership. If they're being schooled in the power of positive thought, then sky's the limit."

But even though the premise of the class is to tap into your inner optimist, Crimson students still have to face the unhappy prospect of midterms and finals. The good news: Being grumpy doesn't earn you an automatic F.

"It's a class like any other and you fail if you don't do the work," Ben-Shahar says.

And, he points out, there is real work. Students must develop presentations based on academic literature, hand in written assignments and corresponding PowerPoints and take exams based on literature as it relates to studies and experiments within positive psychology.

Sounds like a real class after all. So, with all that work, why is it so popular?

"Students feel that they can take the material and directly apply it to their lives," Ben-Shahar explains. Or at least, for the sake of their mood, they hope so.