Mr. Sollars, 51 years old, who owns a workplace-security consulting firm in Mesa, Ariz., hates being away from his wife even when she is just going to work, as an intensive-care nurse on the night shift at a local hospital. When she is away for a longer stretch, Mr. Sollars feels nauseated and finds it hard to concentrate.

As his wife packed for vacation, he hovered anxiously. She eventually snapped, and they argued for hours, he says. That night, after she'd gone to the airport, Mr. Sollars couldn't sleep. Among his thoughts: She will have a car accident. She will get sick or hurt. She will find someone else. 'I firmly believe that my worry is based in fantasy land,' Mr. Sollars says. 'But I am still deathly afraid of losing the woman I love.'

To most people, 'separation anxiety' is what young children feel while crying on the first day of preschool. But adults also can experience it when they are separated from the people who matter most to them. They may be unable to contain their worry and end up pushing away the very person they need so desperately.

You'd think pining for someone would be rare at a time when everyone is hyperconnected. But all this effortless connectivity has spoiled us. We expect to be able to reach everyone immediately, and when we can't, we're losing the ability to cope.

Researchers at Haverford College, in Pennsylvania, found people who missed their partners when apart from them were more committed to the relationship, worked harder to take care of it and avoided damaging behavior such as cheating. 'Missing prompts you to maintain your social connection,' says Benjamin Le, associate professor of psychology at Haverford and lead author of the study, published last year in the Journal of Social and Personal Relationships.

The way we cope with separation is determined by something psychologists call our attachment system. Scientists believe the attachment system is an evolutionary process that humans developed to survive. Early hunter-gatherers learned to work together, and children perished without the care and protection of an adult. Although it's partly genetic, much of our lifelong 'attachment style' is determined by how as young children we learned to relate to our parents.

There are three attachment-style types: secure, anxious or avoidant, according to Hal Shorey, a psychologist and assistant professor for the Institute for Graduate Clinical Psychology at Widener University, in Chester, Pa. Secure people, roughly 55% of the population, typically are warm, loving and comfortable with intimacy. They were raised, most likely, by a consistently caring and responsive mother or parental figure. The other 45% has a sometimes problematic attachment style, meaning they are anxious, avoidant or a combination, Dr. Shorey says.

Anxious people who worry about whether their partner loves them often had parents who were inconsistently nurturing. Avoidant people, whom psychologists also call 'dismissive,' try to minimize closeness and often had parents who didn't tolerate neediness or insecurities.

When we are scanning for signs of danger in a relationship such as abandonment our brain often can't distinguish between a real or imagined risk, Dr. Shorey says. The brain structure that picks up on threats, the amygdala, triggers the release of adrenaline faster than the thinking part of the brain, the cortex, can analyze the threat.

The way we learn to respond to the threat of abandonment as young children actually changes the wiring of our brains, Dr. Shorey says. 'This is an automatic response that will trigger even if you know you should not feel this way.'

When he was a child, Mr. Sollars says, his mother left him to be raised by his grandmother, who died when he was 16. 'I started being a worrier then,' Mr. Sollars says. His separation anxiety worsened a few years ago. He has diabetes and lost his eyesight; his wife had knee surgery and a procedure to correct a throat stricture. Now, Mr. Sollars is troubled by thoughts of becoming a burden to her. To distract himself while she is away, he plans to work on a book he is writing about preventing workplace violence.

Rosita Alvarez, a 45-year-old artist in Spicewood, Texas, grew up as a so-called Air Force brat, moving with her family every two years. She has vivid childhood memories of writing letters to old friends and never hearing back. 'I learned at 9 that you can lose people you love,' she says. She was 36 and married for 10 years with four children younger than 10, when her husband died of a heart attack.

She keeps voice messages from each of her children on her cellphone. 'I want to have their voice with me as long as I can, in case they die,' she says. She keeps her phone with her, day and night. When it rings, she often imagines the worst.

A few weeks ago, her 14-year-old daughter went to boarding school. Ms. Alvarez says the anxiety she feels now is like 'a hand on the back of my body, gripping my spine.' She often worries that her daughter will simply grow distant. 'I am afraid that because she is removed from me physically, she will remove herself emotionally,' Ms. Alvarez says.

When she gets stuck in a worry loop, Ms. Alvarez makes herself stop and notice her surroundings the couch, the carpet, the breakfast dishes in the sink and tells herself that all is well. 'I remind myself that my daughter and I have a loving connection, she is in a safe environment and I can control my thoughts and choose to imagine her happy,' she says. 'This breaks the spell.'

Ms. Alvarez also has anxiety when parting from her boyfriend, who lives an hour away. The two have developed a 'separation ritual' for Sunday evenings, after a weekend spent together. First, they agree in advance on the time he will leave. For their last hour or so together, they talk and relax. When it's time, they go outside together. Ms. Alvarez gives him a hug but doesn't watch him get in the car. Instead, she goes immediately back in the house with her paintings. 'I choose where I place my attention,' she says.