Halloween trends are telling.

Just ask Robert Thompson, a pop-culture expert and the founding director of the Bleier Center for Television and Popular Culture at Syracuse University.

Here’s an interesting trend: Halloween has fast become the second-most-decorated holiday. Jack-o-lanterns and goblins and lighted trees are all over the place now.

Halloween spending has risen to nearly $5 billion annually — not bad for a non-gift-giving, non-government-sanctioned holiday.

And more adults than ever are dressing up.

“The post-World War II years were the golden age of Halloween for kids,” says Thompson, “a trend that continued into the 1980s. But in the last 20 years, Halloween has been reclaimed by adults.”

Which makes perfect sense! Through most of history, Halloween was for adults.

The origins of Halloween date back to pagan times. During harvest celebrations, the Celts dressed up in costumes to ward off ghosts and demons.

As Christianity spread, the Catholics introduced All Saints’ Day (All Souls’ Day). The holy evening before All Saints’ Day — All Hallow’s Eve — embraced many of the Celtic traditions.

But I think there is another reason why more adults are embracing the Halloween spirit: It’s one of the last bastions of free expression in America.

“It’s the one day where almost anything goes,” says Thompson. “Adults can be a wise guy or do something outrageous they’d never do normally.”

Such as dress like a trollop. Seductive vampire, police officer and French maid costumes are among the most popular for women.

Thompson says adults generally pick costumes that mock or satirize the popular culture.

This year, according to the National Retail Foundation, Kate Gosselin wigs are a hot item and “balloon boy” costumes are sure to be a hit.

But politics are mostly out.

Few will dress as nurses this year due to fatigue over the healthcare debate.

Last year, many wore costumes that mocked John McCain and Sarah Palin, but many more — Thompson says this broke Halloween norms — wore costumes that reflected admiration for Barack Obama.

This year, Obama has become a mere immortal, as demonstrated by this hot-selling vampire mask: “Barackula” Obama.

In any event, as our culture loses its sense of humor — as people are ready to shout or sue at every slight — Halloween is growing in popularity because, for the most part, people can express themselves openly and honestly.

Sure, some groups find Halloween offensive. Some Christians refuse to celebrate it; they say it is a celebration of the dark side. And some witches and warlocks complain that it mocks their religion.

It’s true that more schools are banning Halloween celebrations — one school in Seattle did so, in part, because of its offensiveness to Wiccans. If schools have a party, they call it “Autumn Day” or “Harvest Day.” Costumes are forbidden.

It’s true that some costumes are considered taboo. The “illegal alien” costume — it comes with a space alien mask and orange jumpsuit — has caused a stir.

And you won’t see too many people dressing as hobos or beggars or Klansmen — that would be inconsiderate.

But Halloween is mostly wide open. It’s a widely celebrated secular holiday because it’s the only day of the year when people can freely do or say or be anything they want.

No wonder it’s growing in popularity among adults.

“In an era when getting drunk at the office Christmas party can lead to a harassment charge,” says Thompson, “Halloween remains the one day we can, for the most part, misbehave in peace.”

Tom Purcell is a humor columnist for the Pittsburgh Tribune-Review. Visit him on the Web at www.TomPurcell.com or e-mail him at Purcell@caglecartoons.com.