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This question originally appeared on Quora. Answer by Brad Porter.

This is actually a very important issue - and not just a psychological issue, but a medical one.

In the last decade alone, physicians and researchers have begun looking deeply into the impact of loneliness and social isolation on health, well being, and mortality, and the data on the subject is overwhelming: a lonely person is significantly more likely to suffer an early death than a non-lonely one.

Most of this research is centered around geriatrics, as you might guess, where feelings of loneliness are powerfully predictive of mortality. A few years ago researchers at Brigham Young University conducted an influential meta-analysis of scientific literature on the subject, and found that social isolation increases your risk of death by an astounding ~30%, and some estimates have it as high as 60%! Just to quote the abstract from that paper:

Actual and perceived social isolation are both associated with increased risk for early mortality. In this meta-analytic review, our objective is to establish the overall and relative magnitude of social isolation and loneliness and to examine possible moderators. We conducted a literature search of studies (January 1980 to February 2014) using MEDLINE, CINAHL, PsycINFO, Social Work Abstracts, and Google Scholar. The included studies provided quantitative data on mortality as affected by loneliness, social isolation, or living alone. Across studies in which several possible confounds were statistically controlled for, the weighted average effect sizes were as follows: social isolation odds ratio (OR) = 1.29, loneliness OR = 1.26, and living alone OR = 1.32, corresponding to an average of 29%, 26%, and 32% increased likelihood of mortality, respectively. We found no differences between measures of objective and subjective social isolation. Results remain consistent across gender, length of follow-up, and world region, but initial health status has an influence on the findings. Results also differ across participant age, with social deficits being more predictive of death in samples with an average age younger than 65 years. Overall, the influence of both objective and subjective social isolation on risk for mortality is comparable with well-established risk factors for mortality.

— Loneliness and Social Isolation as Risk Factors for Mortality

To put it another way, loneliness might be a more significant health factor than obesity, smoking, exercise or nutrition.

Why Loneliness May Be the Next Big Public-Health Issue

And, interestingly, this is true whether the person feels lonely or not.

There are a few different factors to think about:

  1. Psychologically, loneliness and social isolation have extreme co-morbidity with a whole host of other issues, as you might expect. Depression, anxiety, dementia, substance abuse, even things like manic-depression and schizophrenia. Loneliness - both its objective state and feelings of loneliness - is also basically THE psychological state most associated with suicide, to the point where it’s safe to say that while not all lonely people are suicidal, all suicidal people are lonely. I imagine most people would hone in on this aspect of it, when thinking about why loneliness might be bad, and for sure the psychological piece is central - but that’s not the end of it.
  2. Loneliness and social isolation also have innumerable practical or circumstantial effects that can contribute to an early death. If you have an accident or are struck by a sudden health event, there may not be anyone around to help. You could have some disease state symptom that’s far more likely to be noticed if you regularly interact with other people (“Too often the first sign of someone who has been left alone is a funny smell and post backing up in the mailbox.” — Loneliness: One of the most serious diseases of our age). Likewise self-maintenance or even just regular old healthy habits are something lonely people are a lot less likely to engage in without the sort of encouragement or accountability others might take for granted; you’re more likely to engage in good hygiene, for instance, more likely to remember to take your pills, get out more, encouraged to adopt healthier habits or guilted into at least moderating bad ones, etc.. Lonely people tend to eat worse, get less exercise, not sleep as well. You may be financially poorer. And on and on and on. Hell, even something as stupidly simple as - “if your house catches fire in the middle of the night, you have a better chance of noticing it quickly if there are more people in there.” On a very practical level, human beings just fundamentally have a better chance of surviving in social and familial groups than in isolation.
  3. And while those two are somewhat intuitive, there is a third factor that we are only just now beginning to understanding - even absent its effect on your emotional state and psychological well-being, even besides its direct practical impact on your life, loneliness itself appears to have a direct physiological impact on the body. In that sense, we are increasingly beginning to look at it similarly to how doctors now view stress - as something that in and of itself detrimentally impacts the body. There are literally hundreds of ways that this happens, from hardening your arteries to depressing your immune system to corroding your brain.

In lonely people who see the world as a threatening place, their immune systems choose to focus on bacteria rather than viral threats. Without the antiviral protection and the body's antibodies produced against various ills, the result means a person has less ability to fight cancers and other illnesses. Those who are socially isolated suffer from higher all-cause mortality, and higher rates of cancer, infection and heart disease.

In addition, loneliness raises levels of the circulating stress hormone cortisol and blood pressure, with one study showing that social isolation can push blood pressure up into the danger zone for heart attacks and strokes. It undermines regulation of the circulatory system so that the heart muscle works harder and the blood vessels are subject to damage by blood flow turbulence.

— Why Loneliness Can Be Deadly

According to University of Chicago social neuroscientist John Cacioppo, the effects of social isolation or rejection are as real as thirst, hunger, or pain. “For a social species, to be on the edge of the social perimeter is to be in a dangerous position,” says Cacioppo, who co-authored Loneliness: Human Nature and the Need for Social Connection. “The brain goes into a self-preservation state that brings with it a lot of unwanted effects.”

— Why You Should Treat Loneliness as a Chronic Illness

So yes, in a very, very real sense: loneliness can kill you.

In my own personal experience, I have tended to find that the lonelier I am, the worse I am doing. And I say that as a guy who, in general, much prefers isolation. I am just a person who is more comfortable and often more content being by myself. I value solitude, and on the flip side, can often get agitated or uncomfortable or even just feel more scattered in social settings. In many respects, this is not really something that bothers me - it’s just a part of who I am.

But, at various points in my life, I have noticed that I might begin drifting from “comfortable being alone” to “uncomfortable NOT being alone,” and from there start slipping into something less like “loner” and more like “isolated.”

The WORST periods of my life tend to be at those times. That’s when I seem most likely to fall into depression or anxiety, or substance abuse, or just generally feeling crappy and not taking care of myself.

It’s a little chicken and the egg - do I start isolating because I feel bad or do I start feeling bad because I’m isolating? - but I have noticed that most often, it’s the loneliness that comes first. It’s like a change in the barometric pressure that you can almost feel as you step outside, and you know a storm is coming.

When that happens, it goes one of two ways - either I let myself slip down into the darkness, or I start talking.

As seductive or even just “path of least resistance” that slide into darkness can be, the older I get, the more I have learned that I much, much prefer the outcome when I start talking instead.

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