Chloe Grace Moretz plays a sorority sister who wants to party in her own house, for once. (YouTube)

In the new trailer for Neighbors 2, a sorority leader played by Selena Gomez informs a group of potential sisters, “In the United States, sororities are not allowed to throw parties in their own houses. Only frats can.”

Chloë Grace Moretz, the hard-partying protagonist, rejects the idea. “This is a sexist and restrictive system,” she says to her two pals (before chest-bumping one). “We’re going to start a sorority where we can party the way that we want to.”

So, the young dreamers set off to open their own house off-campus, where the rules can’t touch them and the liquor flows. This is how Hollywood highlights a decades-old norm that critics say is outdated and possibly dangerous. The film, also starring Seth Rogen and Zac Efron, hits theaters in May.

Gomez’s character is almost right. No law actually prohibits sororities from throwing parties in the United States. However, the National Panhellenic Conference, which governs the country’s 26 major sororities, maintains that sisters can’t swig booze in sorority houses — even as the fraternity down the street throws a keg party. (A 1998 decision by the NPC stressed that member-sororities could only co-host social events with fraternities if they lack substances.)

“There is an expectation that all chapter housing facilities will be alcohol-free," said NPC executive director Dani Weatherford in an email. "Each member organization has a different structure in place to manage the chapter facilities and enforce policies set by their housing corporations.”

She declined to explain where the mandate came from or why it persists. Sororities caught breaking the rule receive fines or risk probation.

A common urban legend contends that the rule was necessitated by old laws that defined dozens of women living and drinking under one roof as a brothel.

The real reason is more practical: It’s way, way cheaper to insure dry sororities.

Cindy Stellhorn, a broker at MJ Insurance in Indianapolis (who handles policies for more than a dozen national sororities), said the party rules emerged at a time when society’s attitudes about women were more “provincial.”

As the broader culture changed, economics didn't. Insurance policies cost $25 to $50 a year per sorority member, Stellhorn said, but fraternity brothers, she estimates, pay up to $180 each.

“They need more dollars to pay for the usual claims you can expect from young men who are drinking,” she said.

But the alcohol ban can be interpreted to mean a 21-year-old woman cannot sip a glass of chardonnay in bed while watching "The Bachelor."

Some students in the Greek system argue that’s a basic injustice and, on principle, women should have equal rights to men.

Dartmouth College’s Sigma Delta, for example, broke away from the national organization in 1988. According to the house's website, it split because of various “irreconcilable differences ... specifically religion in rituals and an emphasis on men in national songs and overall attitudes.” The alcohol ban isn't listed among the concerns, but the house does now host parties.

Author Alexandra Robbins, who went undercover as a sorority sister for her book "Pledged," said control of the party scene gives fraternity brothers disproportionate social power.

“Many sisters feel that because the fraternity brothers control the parties, the brothers determine sororities' tiers on campus and perpetuate a culture of exclusivity,” she said. “As in, ‘That chapter is hot — they're invited; that chapter's prude, so they can't come in.’ It's completely disempowering that the women are strongly encouraged to interact with the fraternities by attending their parties.”

Others say avoiding fraternity houses altogether might be the safest route. Two separate longitudinal studies have found that fraternity brothers are three times more likely to commit sexual assault than their college peers.

Last year, the New York Times ran a story framing sorority-hosted parties as an anti-rape measure, citing a 2007 study from the Department of Justice that suggests women who attended fraternity parties were significantly more likely to be sexually assaulted.

“I’ve been to parties run by girls, and they’re much more protective — they keep an eye on each other,” a George Washington student told the reporter. “At frat parties, it’s more of a hunting ground. Not all guys are like this, of course, but sometimes it feels like the lions standing in the background and looking at the deer. And then they go in for the kill.”

Sisters elsewhere, who prefer more quiet and private living quarters, are relieved the party onus (and bill) falls on the brothers’ shoulders.

“Adding alcohol to a sorority chapter only increases the safety issues within the home, and encourages girls to drink even more because of the comfort with their surroundings,” wrote University of Arkansas student Lauren Randall last year for the Odyssey, a national Greek life blog.

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