The vast majority of teenagers who hosted underage drinking parties in their California homes said their parents knew their guests were illegally consuming alcohol, researchers report.

“When we typically think about teen parties, we think about the kids drinking secretly,” lead researcher Bettina Friese told Reuters Health.

“I was quite surprised how many parents knew about the alcohol and also that parents were willing to take that risk,” said Friese, a sociologist at the Prevention Research Center in Oakland, California.

Researchers conducted telephone interviews with 1,121 teens living in 50 mid-sized California cities in 2011 and 2012.

Nearly a quarter, or 272, of the 15- to 19-year-olds reported hosting parties in their homes in the past year. And more than a third of them - 106 teens, or 39 percent - said liquor had flowed at their last party.

Seventy percent of the teens who hosted parties with alcohol said at least one of their parents knew that kids under the legal age of 21 had been drinking. Another 24 percent said their parents probably knew about the illegal drinking.

That left only 5 percent of the youth who hosted underage drinking parties to report that their parents knew nothing about the booze, the authors write in the Journal of Primary Prevention.

“We need to do a better job,” Friese said. “The parents know about it. So that’s where we need to target our efforts.”

Of the 50 cities in the study, 24 had adopted social host ordinances. Under the laws, parents can be fined for serving alcohol to teenagers on their property, even if the parents did not supply the drinks and even if they were not present.

But social host laws did not appear to deter underage drinking in the new study.

Previous research, however, did find that when laws hold adults responsible for kids drinking in their homes, teens are less likely to spend their weekends getting drunk (see Reuters Health story of Oct. 28, 2014 here: reut.rs/1EOANu2)

In the prior study, teens were 20 percent less likely to drink at parties in cities with the strictest social host laws compared to youth in cities without the laws.

In another recent study, Friese found that few parents living in cities with social host laws knew about them. She wonders how strictly police enforce the laws.

The new study highlights the need to educate parents about social host ordinances, which have been enacted in cities across the U.S., she said.

“We need to communicate with parents – not teens, about social host ordinances because the teens don’t care,” she said. “We need to get parents to care.”

Parents have told Friese they allowed their teens to drink at home for fear they would otherwise drive drunk or get a ride with another drunken driver. But underage drinking carries other risks, including increased risks of unprotected sex, sexual assaults and alcohol dependency, she said.

Kimberly Wagoner, a researcher at Wake Forest Baptist Medical Center in Winston-Salem, North Carolina, also believes parents need to be educated about the consequences of underage drinking and social host laws. Wagoner does similar research but was not involved in the current study.

“This study shows that parents are providing the place, and the parents are allowing underage drinking to occur in their homes,” she told Reuters Health.

Parents should not only urge their children to obey drinking laws but should also talk to other parents about not supplying alcohol to their children, she said.

“Equally important is for communities to set up expectations and to use social host ordinances,” she said. “There are things communities can do to try to change the community norm.”