Two of Apple's largest investors called on the tech giant to take smartphone addiction more seriously. Research shows they're right to be concerned.

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On Monday, two of Apple's largest investors, the California State Teachers' Retirement System and Jana Partners LLC, issued an open letter to the tech giant, calling on the company use its dominant market position "to offer parents more choices and tools to help them ensure that young consumers are using your products in an optimal manner."

The authors, citing the work of professors at Harvard University and San Diego State University, offer a laundry list of statistics that should keep the parent of any teenager up at night.

Teens who spend five hours per day on a device are 71 percent more likely to have a risk factor for suicide, they write, while eighth graders who use social media heavily have a 27 percent higher risk of depression. In recent years, as smartphone technology has become ubiquitous and entered the classroom, more than 85 percent of teachers report an uptick in students' social and emotional challenges. Phone-addicted teens get less sleep and have less empathy than their disconnected peers. Half of parents describe being locked in a "constant battle" with their children over screen time.

Add to all that the fact that "the average American teenager who uses a smart phone receives her first phone at age 10 and spends over 4.5 hours a day on it (excluding texting and talking)." Three-quarters of teens admit to checking their phones every hour; half describe themselves as "addicted." This squares with a 2015 Microsoft Advertising Canada report, which found that 77 percent of young adults agreed with the statement "When nothing is occupying my attention, the first thing I do is reach for my phone."

"As one of the most innovative companies in the history of technology," the investors write, "Apple can play a defining role in signaling to the industry that paying special attention to the health and development of the next generation is both good business and the right thing to do."

Research into smartphone addiction is still in its infancy. Studies tend to be retrospective, anecdotal, qualitative, or reliant on self-reported data. (The investors, in their letter, cite "common sense" as well as peer-reviewed journal articles.) Part of the problem is that psychologists haven't agreed upon what qualifies as smartphone addiction—or even that it's a disorder at all.

One rather clever study, published in 2015 in the Journal of Computer-Mediated Communication, found that adult iPhone users who were separated from their smartphones but could hear them ringing experienced spikes in blood pressure and heart rate, as well as increased feelings of anxiety and unpleasantness. (The participants thought they were testing a wireless blood-pressure cuff, and had their phones confiscated under the pretense that they were interfering with the cuff's Bluetooth technology.)

That same year, researchers at the University of Iowa went so far as to coin a neologism for the disorder, nomophobia—"no mobile phone" phobia—and developed a 20-point questionnaire to gauge users' dependence on their devices.

A more recent study, presented in December at the annual meeting of the Radiological Society of North America, reported finding chemical imbalances in the brains of smartphone-addicted teens.

Sung Suk Seo, a professor of neuroradiology at Korea University, in Seoul, used magnetic resonance spectroscopy—a kind of MRI—to examine levels of gamma-Aminobutyric acid (GABA) and other neurotransmitters in the brains of 19 smartphone-addicted teenagers, as well as 19 non-addicted control subjects. Suk and his colleagues found elevated ratios of GABA—an inhibitory neurotransmitter, or "downer," thought to be linked to anxiety—in the brains of addicted teens. (Other studies have shown that heavy smartphone users demonstrate "increased impulsivity, hyperactivity and negative social concern.") After the smartphone-addicted teens completed nine weeks of cognitive behavioral therapy, based on a protocol for Internet addiction, their GABA ratios decreased significantly.

Studies with so small a sample size should be taken with a hefty grain of salt. Their conclusions are preliminary, at best. Their real value lies in pointing the way for future research.

Which is just what Apple's investors are calling for. In their letter, the investors urge the company to convene an expert committee of child development specialists, offer their vast trove of user data to researchers, build out a menu of parental controls for a variety of ages, educate families to make informed choices, and report on its progress annually, in the same way it reports on sustainability and corporate responsibility.

Apple published a statement late Monday night detailing the parental controls already on offer in iOS, adding: "We think deeply about how our products are used and the impact they have on users and the people around them. We take this responsibility very seriously and we are committed to meeting and exceeding our customers' expectations, especially when it comes to protecting kids."

A more interesting—and less sanitized—response came from former Apple executive Tony Fadell, one of the creative minds behind both the iPod and the iPhone. "These things can be incredibly addictive," he told the New York Times. "It's amazing, but there are a lot of unintended consequences."

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