Start with the fact that many of us feel anxious if we’re not making use of every tiny slice of time. The effort required for a single online “transaction” — a click, a view, checking Instagram or Facebook — is minuscule, so much so that not texting or reading your smartphone screen feels like a greater burden than doing so. “The timescale on which you work with online technology is central to making it compelling,” said psychologist Tom Stafford of the University of Sheffield in England. “What else can you do in five seconds that’s interesting? Why not check your phone?” This is a large part of why “using the internet can be compulsive,” he said.

One reason we often feel anxious if we’re not using every tiny slice of time is that we find it hard—even unpleasant and anxiety-producing to be alone with our thoughts, as a 2014 study showed. Researchers led by social psychologist Timothy Wilson of the University of Virginia gave volunteers two options: do “nothing” for 15 minutes or give themselves a small electric shock (which three-quarters had previously told the researchers they’d pay money not to experience). Two-thirds of the men and one-quarter of the women chose the latter, so anxious were they for “something to do.”

“The untutored mind does not like to be alone with itself,” Wilson concluded.

Especially when the mind encounters payoffs structured like a slot machine’s. Like those one-armed bandits, the digital world offers what are called intermittent/variable rewards: An action — pulling the slot machine’s arm, checking for texts — can bring either a payoff or nothing. Most of what fills your Twitter feed or Facebook updates is digital dross. (“Barbara changed her Facebook picture!”) Payoff: zero. But every so often, you find a gem — a friend offering Bruce Springsteen tickets, an acquaintance posting a link to the morning’s must-read Trump story.

“If I give you a treat sometimes, you have to keep checking all the time: You don’t know when it will come,” said Stafford. “No matter how frequently you check, even if you checked only a second ago, a brilliant email might have just come in. You feel anxiety about possibly missing something.” Such low-cost, occasionally high-reward activities are catnip to the brain.

If we’re prevented from compulsively checking for texts, the anxiety that the compulsive behavior alleviates comes roaring back. Psychologists have found that people who are separated from their smartphone experience an elevated heart rate and other signs of anxiety.

In one 2016 study, volunteers who filled out a standard questionnaire about their smartphone use and emotions told researchers that they turn to their phones “to avoid negative experiences or feelings” and “to cope [with] or escape from feelings related to an anxiety-inducing situation.” Psychologist Alejandro Lleras, of the University of Illinois at Urbana-Champaign, who led the study, described it as a security-blanket effect, absorbing our bubbling-over anxiety.

That fits with studies finding that people text as a way to escape anxiety; something like 70 percent of study participants say smartphones and texting help them overcome anxiety and other negative moods. It’s become a stereotype that people in awkward (read: anxiety-provoking) situations “turn to their mobile phones as a way to disengage,” the Illinois researchers wrote, especially “during times of more intense distress.”

Lleras and a colleague gave volunteers a short writing assignment that, they explained (falsely), would be evaluated by two experts. To ratchet up the stress further, the researchers said the experts would also interview them about their essay. While waiting for that, half the volunteers had access to their mobile phones and half didn’t. Among those who were able to text and surf to their anxiety-ridden-heart’s content, half felt intense anxiety. In comparison, three-quarters of those who were deprived of their phones did, the researchers reported.

By giving in to a compulsion to use their phone, many of the study volunteers were able to defuse much of their anxiety. “People seem to be less vulnerable to becoming stressed in anxiety-provoking situations when they have access” to their mobile phone, the researchers wrote.

It’s not only being deprived of those variable-interval rewards that makes ditching their smartphone unthinkable for many people. Because it has become our main connection to other people and the world at large, the anxiety that comes from not being able to check it arises, too, from the feeling of missing something, as if everyone else is plugged in, connected, on top of things, and you aren’t.

“There are people who feel, if I’m not there, if I’m not on that site, I’m missing something — something about my friends, or my health, or anything else,” said psychiatrist Dr. David Reiss, who practices in San Diego.

The internet exploits this FOMO, or fear of missing out. Being disconnected is synonymous with missing out.

By making us feel we are always connected to the world, smartphones also alleviate the anxiety that otherwise floods into us from feeling alone and untethered. A character in the 2014 New York City production of Laura Eason’s play “Sex with Strangers,” learning there’s no cellphone service at a bed-and-breakfast, says, “People will think I’m dead.” People don’t like feeling dead.

A 2010 study by the International Center for Media and the Public Agenda at the University of Maryland showed how profound an existential dread engulfs people cut off from the online world. The researchers asked 200 students to abstain from using their phones and computers (and all other media) for 24 hours. Describing their experience, the students said they felt disconnected and anxious that they were missing out on something, using terms evoking compulsion: Frantically craving. Very anxious. Extremely antsy. Miserable. Jittery. Crazy.

It’s worth reading some of their comments:“Texting and IM’ing my friends gives me a constant feeling of comfort … the fact that I was not able to communicate with anyone via technology was almost unbearable.”

“I feel so disconnected from all the people who I think are calling me, but really they aren’t half the time.”

“I couldn’t take it anymore being in my room … alone … with nothing to occupy my mind so I gave up.”

And if we do miss out? If we’re not connected? If existence is defined these days by an online presence, then not being online is not to exist. Human history knows no greater motivation for action than the existential one of raging against the dying of the light. When we are not online, when we are not connected, when we miss out, we do not exist, and that causes the most unbearable and existential anxiety there is.That is how we should understand the digital compulsion: not as a pathology, but as the result of the online world’s ability to tap into something deep in the human psyche and make many of us digital casualties.

This story was originally published by STAT, an online publication of Boston Globe Media that covers health, medicine, and scientific discovery.

It has been adapted from Sharon Begley’s book “Can’t. Just. Stop.: An Investigation of Compulsions,” published by Simon & Schuster on Feb. 7.

Sharon Begley can be reached at sharon.begley@statnews.com

smart phonessmart phones

Why We Compulsively Check Our Phones

All That Multitasking is Harming, Not Helping Your Productivity. Here's Why

San Francisco, New York Prosecutors Wait to Judge Apple 'Kill Switch'

San Francisco Muni to Enable Taxi App for Smart Phones

San Jose Council Must Disclose Lobbyists' Texts

Cal. Academy of Science Releases Penguin App; Watch Live Penguin Video Feed