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One of the saddest things about loneliness is that it leads to what psychologists call a “negative spiral.” People who feel isolated come to dread bad social experiences and they lose faith that it’s possible to enjoy good company. The usual result, as Melissa Dahl recently noted, is more loneliness. This hardly seems adaptive, but experts say it’s because we’ve evolved to enter a self-preservation mode when we’re alone. Without the backup of friends and family, our brains become alert to threat, especially the potential danger posed by strangers.

Until now, much of the evidence to support this account has come from behavioral studies. For example, when shown a video depicting a social scene, lonely people spend more time than others looking at signs of social threat, such as a person being ignored by their friends or one person turning their back on another. Unpublished work also shows that lonely people’s attention seems to be grabbed more quickly by words that pertain to social threat, such as rejected or unwanted.

Now the University of Chicago’s husband-and-wife research team of Stephanie and John Cacioppo — leading authorities on the psychology and neuroscience of loneliness — have teamed up with their colleague, Stephen Balogh, to provide the first evidence that lonely people’s brains, compared to the non-lonely, are exquisitely alert to the difference between social and nonsocial threats. The finding, reported online in the journal Cortex, supports their broader theory that, for evolutionary reasons, loneliness triggers a cascade of brain-related changes that put us into a socially nervous, vigilant mode.

The researchers used a loneliness questionnaire to recruit 38 very lonely people and 32 people who didn’t feel lonely (note that loneliness was defined here as the subjective feeling of isolation, as opposed to the number of friends or close relatives one has). Next, the researchers placed an electrode array of 128 sensors on each of the participants’ heads, allowing them to record the participants’ brain waves using an established technique known as electro-encephalography (EEG) that’s particularly suited to measuring brain activity changes over very short time periods.

With the apparatus in place, the participants were asked to look at various words on a computer screen and to indicate with keyboard keys, as quickly as possible, what color they were written in. This is an adaptation of a classic psychology test known as the Stroop Test. The idea is that since participants are asked to focus not on the word itself but on its color, any influence that the word’s meaning has on the participant is considered to be automatic and subconscious.

Some of the words were social and positive in nature (e.g., belong and party), some were social and negative (e.g., alone and solitary), while others were emotionally positive but nonsocial (e.g., joy), and others were nonsocial and emotionally negative (e.g., sad). The researchers were specifically interested in when and how the participants’ brains responded to the sight of negative words that were social in nature, compared to those that were nonsocial. To do this, they analyzed the participants’ brain waves to see when, after looking at different word types, their brains entered discrete “microstates,” which are periods of relative stability when a sustained pattern of brain regions are activated. When the brain enters a new microstate, this is a sign that it has initiated a new mental operation — that it’s processing some stimulus in a new way.

For the first 280 milliseconds (about one-quarter of a second) after a word was shown on the screen, lonely people’s brains entered a series of three discrete microstates that were identical whether a negative word was socially relevant or not. After that point, however, their brains entered a distinct microstate in response to socially negative words — with activation particularly notable in neural areas involved in the control of attention — suggesting that they had entered a highly vigilant mode. By comparison, non-lonely people’s brains continued to respond with the same microstates to social and nonsocial negative words for a full 480 milliseconds (nearly half a second). This difference between lonely and non-lonely people’s brains might sound subtle, but this is an important finding because it shows how lonely people’s brains are primed at a basic level to tune into social threats more quickly than is “normal.”

Because these effects occurred so early on in the lonely participants’ response to negative social words — and because this was all done in the context of the Stroop Test (where you focus on the word’s color, not the meaning) — the researchers say this shows lonely people’s vigilance to social threat is an implicit, nonconscious bias. In other words, it’s not something they’re aware of. The participants weren’t even meant to be paying attention to the words’ meaning, yet lonely people picked up on the difference between a socially threatening word like hostile and a negative nonsocial word like vomit more quickly than non-lonely people did.

In a real-world context, this is a troubling finding. When people feel most alone, these results suggest their brains are not tuned in to smiles and laughter, they’re switched on to frowns and snarls — they’re vigilantly looking out for negativity without really knowing it. This might have helped our distant ancestors stay alive back when lacking social ties was more of a direct threat to one’s well-being than it is today, making it evolutionarily adaptive. But in the modern world, it’s a stressful, unhelpful state to be in. It might even help explain why lonely people often have poorer health and shorter lives than people who feel connected and cared for.

Dr. Christian Jarrett, a Science of Us contributing writer, is editor of the British Psychological Society’s Research Digest blog. His latest book is Great Myths of the Brain.