As young people spend more time on computers, smartphones and other devices, researchers are asking how all that screen time and multitasking affects children’s and teenagers’ ability to focus and learn — even drive cars.

A study from Stanford University, published Wednesday, wrestles with a new question: How is technology affecting their happiness and emotional development?

Michelle Litvin for The New York Times

The answer, in the peer-reviewed study of the online habits of girls ages 8 to 12, is that those who say they spend considerable amounts of time using multimedia describe themselves in ways that suggest they are less happy and less socially comfortable than peers who say they spend less time on screens.

The research raises as many questions as it seeks to answer, as the scientists readily acknowledge. That is because the research was based on an online survey taken by more than 3,400 girls, a sample that may well not be representative of the larger population and, because the responses are self-reported, are not subject to follow-up or verification by the researchers.

Among the crucial questions that the researchers were not able to answer is whether the heavy use of media was the cause for the relative unhappiness or whether girls who are less happy to begin with are drawn to heavy use of media, in effect retreating to a virtual world.

But the researchers hypothesize that heavy use of media is a contributing factor to the social challenges of girls.

The reason, say the researchers, is that on a basic, even primitive level, girls need to experience the full pantheon of communication that comes from face-to-face contact, such as learning to read body language, and subtle facial and verbal cues.

“Humans are built to notice these cues — the quavering in your voice, perspiration, body posture, raise of an eyebrow, a faint smile or frown,” said Clifford Nass, a Stanford professor of communication who led the study. Social media, he added, leaves the conversation two-dimensional. “If I’m not with you face to face, I don’t get these things. Or, if I’m face to face with you and I’m also texting, I’m not going to notice them.”

The peer-reviewed study appeared Wednesday in Developmental Psychology, a journal published by the American Psychological Association, as part of a series of articles on interactive technology and human development. There is no analogous study about how screen time affects boys.

The fact that the study was based on an online survey gave pause to some academics. While they said the paper raised good questions, they also expressed concern about giving it too much weight, given that the researchers were not able to follow up with the survey subjects to get important context, including their family circumstances, income or ethnicity.

Moreover, the limitations of the online survey did not even allow the researchers to verify the ages of the girls.

Lyn Mikel Brown, an education professor at Colby College who studies girls’ behavior, cautioned against reading too much into the research because so little is known about the survey subjects.

It may well be, she said, that girls who seek out online relationships are girls who otherwise might not feel social at all.

“Finding like-minded people online and issues they can relate to and work on with others can be incredibly important,” she said.

But she also said the research should provoke further study about the connection between time spent online and social development, and should provoke conversations in families.

“The clear message is also how important it is for parents to create opportunities for girls to unplug, to live a balanced life, and increase quality face-to-face time with the people important to them,” Ms. Mikel Brown said.

Patricia Greenfield, a distinguished professor of psychology at the University of California, Los Angeles, who edited the article, said that that even though the authors did not find that Internet use causes unhappiness among girls, the correlation that the authors did find was very significant.

The research was based on an online survey of about 80 questions answered by 3,461 girls whom the scientists found by advertising in Discovery Girls magazine. The researchers found that the average amount of media use by the girls surveyed was 6.9 hours per day, a figure that included reading as well as screen time. The average amount of time spent in face-to-face social settings was 2.1 hours, a figure that did not include classroom time.

Some parents of girls who are heavy Internet users said the research addressed questions that they had been concerned about.

Lucy Gray, 45, who lives in Chicago and helps schools integrate technology, said her daughter, Julia, 13, has for several years been a heavy consumer of media — she watches movies on her laptop, has an iPad, iPhone and a Nintendo DS portable game machine. Ms. Gray said that Julia can have trouble picking up on subtle social cues in face-to-face interactions, but she is not ready to blame her daughter’s heavy use of technology.

In fact, she said, she thinks that, on the whole, the technology has helped her daughter navigate the world socially.

“She’d be missing out on an opportunity if she wasn’t connected.” Ms. Gray said.

At the same time, Ms. Gray said, she worries that her daughter, who is using Facebook more, is playing out her social life online sometimes without the benefits of the full emotional range that comes from face-to-face interaction.

“It’s a double-edged sword,” Ms. Gray said of the social implications of social media.

Lena Garzarelli, 13, an eighth grader in Asheville, N.C., who spends as much as two hours each day on Facebook, video chatting with friends and using other multimedia, said that the technology, on the whole, has helped enrich her social life. But she said that she felt it could be a poor substitute for face-to-face interaction.

She has had instances, she said, of serious miscommunication because her real meaning was lost in text-based communications.

When people converse online, she said, they “may not understand how I feel because they can’t see the emotion in my face and can’t hear my voice.”