(CNN) - "Have smartphones destroyed a generation?" Jean Twenge, a professor of psychology at San Diego State University, asked in an adapted excerpt of her controversial book, "iGen."
In the book, she argues that those born after 1995 are on the "brink of a mental-health crisis" -- and she believes it can be linked to growing up with their noses pressed against a screen.
Her newest study provides more backing to that connection, showing that teens who spent more than an hour or two a day interacting with their gadgets were less happy on average than those who had more face time with others. The research was published Monday in Emotion, a journal by the American Psychological Association.
The study -- which drew from a survey of hundreds of thousands of teens across the US -- also found that roughly 13% of eighth- and tenth-graders who spent 1 to 2 hours a week on social media said they were "not happy."
For those who responded 10 to 19 hours per week, that number was about 18%. For those who spent 40 or more hours a week using social media, that number approached 24%.
By the twelfth grade, however, the negative correlations between screen time and teen psychology had somewhat dissipated. In addition, less is not always more: Teens with zero hours of screen time had higher rates of unhappiness than their peers who logged in a few hours a week.
The study comes two weeks after two major investors urged Apple to do more to combat iPhone addiction among young people.
Twenge's conclusions have come up against criticism in the past. Some have accused her work of oversimplifying or overlooking data that may tell a slightly different story.
"The bottom line with this project is that they're asking the data to do things that the data is not set up to do," said Amanda Lenhart, deputy director of the Better Life Lab at New America, a DC-based think tank. Lenhart, whose own work examines technology use among children and families, was not involved in the new study.
Twenge recognizes that her study shows only a correlation between screen use and "psychological well-being," which is measured using survey questions about self-esteem, life satisfaction and happiness. The surveys can't say whether screen time directly changes teens' mental health, the research states.
"The other possible interpretation is that I'm an unhappy adolescent, and I'm running to my screens to escape from the things in my life that are making me unhappy," Lenhart said. "What are all the factors that are at work here?"
But Twenge is particularly concerned by a drop in happiness and "psychological well-being" that she identified in the survey data, which largely took place between 2012 and 2015.
"The question here isn't what are all the causes of unhappiness," said Twenge. "We're asking what changed in that three-year period that could have possibly caused teens' happiness and life satisfaction to fall so suddenly."
"I spent my career in technology. I wasn't prepared for its effect on my kids," philanthropist Melinda Gates, whose three children were also born after 1995, wrote August in the Washington Post. "Phones and apps aren't good or bad by themselves, but for adolescents who don't yet have the emotional tools to navigate life's complications and confusions, they can exacerbate the difficulties of growing up."
At the same time, she said, kids are learning on their devices and connecting in novel ways. "Marginalized groups such as gay and lesbian students (are) finding support they never had before through social networks," said Gates.
In a 2015 report, 92% of over 1,000 teens surveyed said they went online daily, according to Lenhart's previous research at the Pew Research Center. This includes 24% who were online "almost constantly."
But Pew never asked teens how many hours they spent on social media or texting because "people are remarkably bad at determining how long they spend doing things on screens," Lenhart said.
"I look a little bit out of the side of my eye at that data," she added.
Twenge found that happiness correlated most strongly with sports, followed by socializing in person and religious services. On the negative side were online computer games and social media.
Curiously, the study also found that teens who spent more time face-to-face with friends also spent more time communicating with them online. Twenge said she hopes to explore this trend further in upcoming research.
Other studies have explored the connection between social media and isolation and how "likes" activate the brain's reward center. Some analyses have found that moderate use of these technologies is "not intrinsically harmful" and can even improve social skills and develop resilience.
Lenhart doesn't doubt the premise that people are spending more time on these devices and that it's having a major impact on kids and adults alike.
"These are really important devices that have changed our lives in so many ways -- not just for the worse but for the better," she said. But the latest research "is looking straight at technology and wanting it to be the scapegoat."
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