Four out of five New Zealand teenagers now have a smartphone, but this super-connected generation is less happy and more isolated than those who've gone before, according to a new book by American psychologist Jean Twenge.
Twenge, a professor of psychology at San Diego State University, has studied studied data on teenage mental health for many years, says she's never seen a trend like this.
The smartphone is a pox on those born between 1995 and 2012 (the iGen), she claims in a new book.
The iGen spans children who are now five through to 22-year-olds at university and entering the workforce.
They are the first generation to spend their entire adolescence with access to a smartphone.
"In 2012, I noticed some big shifts in behaviour and mental health that really distinguished iGen from the millenials."
This generation, unlike its predecessors, is growing up slowly, Twenge says.
"They aren't hanging out, driving around in cars and going to parties," she says.
Consequently, some of the traditional rites of passage are happening much later.
"They are less likely to have their drivers' licence, to work a job, to go out without their parents, to date, have sex and drink alcohol compared to teens just ten years ago, but especially to baby boomers 30 and 40 years ago."
The teens of today are safer, have fewer pregnancies, fewer car accidents when they do drive and are less likely to drink. In fact, they are the safest generation in history, she says.
"Not just physically safe, but much more concerned with being emotionally safe."
Yet they are actually more emotionally vulnerable, she says.
Data from 2011/2012 shows a spike in teenagers reporting feelings of depression and a 50 percent increase in those being diagnosed with a clinical level of depression, she says.
"For young teens, a doubling, tripling in the suicide rate; for older teens a 50 percent increase."
The amount of time spent in front of screen is the "worm at the core of the apple" of iGen unhappiness, she says.
Long stretches alone with a screen is shown to increase anxiety and depression, while face-to-face contact has been proven to benefit mental well-being.
"They're getting this double whammy with this social interaction moving from in-person to screen, and that might be one of the reasons why those mental health effects are showing up."
Girls are more affected than boys, she says.
"That makes sense because girls tend to spend more time on social media and social media has particular challenges around mental health hand anxiety."
Add to this the fact iGen is not getting the sleep it needs, either.
Rates of healthy sleep started to decline for teenagers in the late 1980s then was stable for most of the 2000s, but in 2007 stated to deteriorate again, Twenge says.
"More teens now say they get fewer than seven hours a night and teens need about nine a night.
"You can see how this all works together. The lack of sleep, that might be one of the key reasons why the age of the smartphone has produced these issues of teen mental health."
Twenge's advice is to limit smartphone use - children under 13 should not have a smartphone, and those older should be limited to no more than two hours on it a day.
"Two or more hours a day on social media is linked to unhappiness and more depression."
Her new book is called iGen: Why Today's Super-Connected Kids Are Growing Up Less Rebellious, More Tolerant, Less Happy - and Completely Unprepared for Adulthood - and What That Means for the Rest of Us.