Okay, parents, here's some ammunition to use when your 14-year-old daughter falls for the 16-year-old player who just moved into the neighborhood. A national survey of 1,000 teens, released yesterday by Columbia University, confirms what you already suspect: It's dangerous for teenage girls to date older boys, particularly boys two or more years older.

It's also dangerous for girls or guys to spend a lot of time with a boyfriend or girlfriend, and to hang out with a group of sexually active friends.

Why?

Because these pastimes are tightly connected to a slew of risks -- not only early sexual intercourse, but also drinking, smoking and illegal drug use, according to Columbia's National Center on Addiction and Substance Abuse.

The center is careful to say that risky dating patterns do not necessarily cause other negative behaviors. They simply signal that those other behaviors may be present -- if not always visible.

It's what scientists call the cluster effect, or what Mom and Dad mean when they look over the new friend and say, "He [or she] is bad news."

"There is a clear message for parents of 12- to 17-year-olds," says Joseph A. Califano Jr., the center president. "Make sure you are aware of the dating practices of your child and get to know your child's friends."

A striking number of the teens surveyed -- 45 percent -- said they knew friends who regularly use the Internet to view or download pornography. "I haven't seen that before. That's substantial and worth exploring," said Brett Brown, a sociologist and director of Child Trends, a research organization.

If their friends watched a lot of porn, the teens were more than three times as likely to smoke, drink and use illegal drugs than teens who had no such friends.

Other findings included:

* Older boyfriends were common. Among 12-year-old girls who said they had a boyfriend, more than half said their boyfriends were older; among 17-year-old girls, three-fourths said the same thing. (One-third of all the teens polled said they had a boyfriend or girlfriend.)

Girls whose boyfriends were two or more years older were six times as likely to get drunk as girls who dated boys closer to their age. They were also six times as likely to have tried marijuana and more than four times as likely to smoke cigarettes. This finding, like the others, was controlled for age.

* Fifty-six percent of those surveyed said they knew kids in their "close circle of friends" who were sexually active. More than a quarter of 12-year-olds said so; more than three-quarters of the 17-year-olds did.

Other organizations, notably the National Campaign to Prevent Teen Pregnancy, have reported that teens are more likely to have sex if they believe their peers are having sex. The Columbia report goes beyond that to say that teens who believed half or more of their friends were sexually active were more than six times as likely to drink; 31 times as likely to get drunk; 22 times as likely to have tried marijuana; and more than five times as likely to smoke.

Even given the fact that teens tend to exaggerate the bad things their friends do, Columbia's findings are consistent with those of other bodies, including the federally funded National Longitudinal Study of Adolescent Health. That study found that spending time with a bunch of risk-taking friends is one of the most hazardous things a teenager can do, particularly in an unsupervised setting.

But what if those are the only friends a teenager has? Or they're the friends who seem to care the most? Teens long to fit in and to enjoy close relationships with kids their age, as Columbia's report makes clear. Asked to identify the most important problem of their age group, they picked "social issues" (including popularity) over education, drugs and violence.

Although risky behavior took top billing in this survey, there were some positive notes. Fewer teens were identified as "high-risk" this year than last year, when a similar survey was done. This year's teens said marijuana was getting more difficult to buy; the proportion who said they knew users of hard drugs (LSD, cocaine or heroin) has declined.

Regular church attendance -- which studies show is highly beneficial to teens -- was reported by 77 percent of the teens, an unusually large number compared with other recent surveys.

Almost that many said they ate dinner with their parents four or more times a week. Echoing the work of others, Califano said a close, honest relationship between parent and teen is the most powerful protection there is against the risks his center studies. Mealtimes, he said, are "the most convenient, comfortable way" to build those connections.

What if you want to talk about the older boyfriend? Won't that make kids push back from the table?

Have food your kid likes, he said.

Even burgers and fries?

"I'll probably get in trouble saying this," Califano said, "but I'd be a hell of a lot less worried about obesity than drug use, smoking and excessive drinking."

The relationships teens keep are associated with risky behavior -- including smoking and illegal drug use -- according to a survey by Columbia University.