The fact that we see such high levels of loneliness, with substantial divergence across countries, explains why this is an important and active research area. Indeed, there are literally hundreds of papers that have used survey data to explore the link between loneliness, solitude, and health. Below is an overview of what these studies find.

The link between loneliness and physical health

Most papers studying the link between loneliness and health find that both objective solitude (e.g., living alone) and subjective loneliness (e.g., frequent self-reported feelings of loneliness) are correlated with higher morbidity (i.e. illness) and higher mortality (i.e. likelihood of death).

The relationship between health and loneliness can, of course, go both ways: lonely people may see their health deteriorate with time, but it may also be the case that people who suffer from poor health end up feeling more lonely later down the line.

Because of this two-way relationship, it's important to go beyond cross-sectional correlations and focus on longitudinal studies – these are studies where researchers track the same individuals over time to see if loneliness predicts illness or mortality in the future after controlling for baseline behaviors and health status.

The evidence from longitudinal studies shows that people who experience loneliness during a period of their lives tend to be more likely to have worse health later down the line. In the Netherlands, for example, researchers found that self-reported loneliness among adults aged 55-85 predicted mortality several months later, and this was true after controlling for age, sex, chronic diseases, alcohol use, smoking, self-assessed health condition, and functional limitations. 3

Most studies focus either on subjective loneliness or on objective isolation. However, some studies try to compare both. In a recent meta-analysis covering 70 longitudinal studies, the authors write: “We found no differences between measures of objective and subjective social isolation. Results remain consistent across gender, length of follow-up, and world region.” In the concluding section, they highlight that, in their interpretation of the evidence, “the risk associated with social isolation and loneliness is comparable with well-established risk factors for mortality”; which include smoking and obesity. 4

The link between mental health and subjective well-being

In another much-cited review of the evidence, Louise Hawkley and John Cacioppo, two leading experts on this topic, concluded that “perhaps the most striking finding in this literature is the breadth of emotional and cognitive processes and outcomes that seem susceptible to the influence of loneliness”. 5

Researchers have found that loneliness correlates with subsequent increases in symptoms related to dementia, depression, and many other issues related to mental health, and this holds after controlling for demographic variables, objective social isolation, stress, and baseline levels of cognitive function.

There is also research that suggests a link between loneliness and lower happiness, and we discuss this in more detail here.

Mechanisms

Experiments with social animals, like rats, show that induced isolation can lead to a higher risk of death from cancer. Humans and rats are, of course, very different, but experts such as Hawkley and Cacioppo argue that these experiments are important because they tell us something meaningful about a shared biological mechanism.

In a review of the evidence, Susan Pinker writes: “If our big brains evolved to interact, loneliness would be an early warning system-a built-in alarm that sent a biological signal to members who had somehow become separated from the group”. 6

Indeed, there's evidence of social regulation of gene expression in humans: studies suggest perceived loneliness can switch on/off genes that regulate our immune systems, and it is this that then affects the health of humans, or other animals that evolved with similar defense mechanisms. 7