The elementary school daughter of Jean Twenge, a psychology professor in California, feels a direct -- and probably unwelcome -- impact from her mom's research and latest book, "iGen." It's about the impact of handheld digital devices and social media on young people born between 1995 and 2012. "Put off getting teen a smartphone as long as you can," ideally until high school, the author advises other parents, noting that heavy phone use can bring a sense of isolation and other drawbacks. Her daughter "said half of the kids in her [fourth-grade] class have a phone," she tells an interviewer. "The mental health effects are stronger for the younger kids. So, put that off."

This is the sixth book by Twenge, who studies the behavior of different generations. Her new work, published in late August, has a long, colorful subtitle: "Why Today's Super-Connected Kids Are Growing Up Less Rebellious, More Tolerant, Less Happy – and Completely Unprepared for Adulthood." Here's how she summarizes her findings from surveys and conversations: "The arrival of the smartphone has radically changed every aspect of teenagers' lives, from the nature of their social interactions to their mental health. These changes have affected young people in every corner of the nation and in every type of household. . . . The impact of these devices has not been fully appreciated, and goes far beyond the usual concerns about curtailed attention spans." She says iGen'ers are immature for their age, obsessed with their phones and more comfortable texting than talking. Alarmingly, she adds: "Rates of teen depression and suicide have skyrocketed since 2011. It's not an exaggeration to describe iGen as being on the brink of the worst mental-health crisis in decades. Much of this deterioration can be traced to their phones."

There is some bright news too. Students in iGen tend to delay risky behaviors. Compared to the preceding generation, they generally drink less alcohol, start driving later and even put off sex. "Maybe you don't have to go out with your friends and thus have sex and drink, because the party is on Snapchat you can just stay at home," Twenge says. The professor advocates moderate use of smartphones, tablets and social apps: "An hour, even an hour-and-a-half a day of use doesn't seem to have any negative mental health effects. But two hours and beyond -- that's where we start to see the effects. And most teens are on their phones a lot more than two hours a day."

Author says: "Girls spend more time with their smartphones and more time on social media, and their interactions on social media are often more negative. So that might be one of the reasons why that mental health trend is more negative for girls." – Jean Twenge on "PBS News Hour"

Tech blogger says: "For as much good as the smartphone has introduced, Twenge's report underscores the fact that too much of a good thing can often result in unforeseen consequences." – Yoni Heisler at bgr.com

Book reviewer says: "Her findings are riveting, her points are compelling, her solutions are invaluable. -- Michele Borba, Los Angeles educator and author