The reason loneliness could be bad for your health

SCIENCE has many uses, but it doesn't often produce handy pick-up lines. Recent work on the genetics of disease, however, suggest a way of opening a conversation with that solitary attractive stranger in a bar: loneliness can make you ill.

Lonely people, it seems, are at greater risk than the gregarious of developing illnesses associated with chronic inflammation, such as heart disease and certain cancers. According to a paper published last year in the Public Library of Science, Medicine, the effect on mortality of loneliness is comparable with that of smoking and drinking. It examined, and combined the results of, 148 previous studies that followed some 300,000 individuals for an average period of 7.5 years each, and controlled for factors such as age and pre-existing illness. It concluded that, over such a period, a gregarious person has a 50% better chance of surviving than a lonely one.

Steven Cole of the University of California, Los Angeles, thinks he may know why this is so. He told the AAAS meeting in Washington, DC, about his work studying the expression of genes in lonely people. Dr Cole harvested samples of white blood cells from both lonely and gregarious people. He then analysed the activity of their genes, as measured by the production of a substance called messenger RNA. This molecule carries instructions from the genes telling a cell which proteins to make. The level of messenger RNA from most genes was the same in both types of people. There were several dozen genes, however, that were less active in the lonely, and several dozen others that were more active. Moreover, both the less active and the more active gene types came from a small number of functional groups.

Broadly speaking, the genes less active in the lonely were those involved in staving off viral infections. Those that were more active were involved in protecting against bacteria. Dr Cole suspects this could help explain not only why the lonely are iller, but how, in evolutionary terms, this odd state of affairs has come about. For inflammation is an antibacterial response.

The crucial bit of the puzzle is that viruses have to be caught from another infected individual and they are usually species-specific. Bacteria, in contrast, often just lurk in the environment (like tetanus), and may thrive on many hosts (as does bubonic plague, for example). The gregarious are therefore at greater risk than the lonely of catching viruses, and Dr Cole thus suggests that past evolution has created a mechanism (the details of which remain unclear) which causes white cells to respond appropriately. Conversely, the lonely are better off ramping up their protection against bacterial infection, which is a bigger relative risk to them.

What Dr Cole seems to have revealed, then, is a mechanism by which the environment (in this case the social environment) reaches inside a person's body and tweaks its genome so that it responds appropriately. It is not that the lonely and the gregarious are genetically different from each other. Rather, their genes are regulated differently, according to how sociable an individual is. Dr Cole thinks this regulation is part of a wider mechanism that tunes individuals to the circumstances they find themselves in. Where it goes wrong is when loneliness becomes chronic, and the inflammatory response becomes chronic at the same time.

Before civilisation intervened, such chronic loneliness would have been so rare (because isolated individuals are so vulnerable to predation) that evolution would have ignored it. Now, paradoxically, the large population that civilisation makes possible means loneliness is commonplace—and with it consequences that natural selection, which is blind to the future, has not yet had time to deal with.