Accordingly, brain differences related to the experience of loneliness have been reported in visual cortices, as well as visual attention networks, limbic structures, and prefrontal cortex 27, 28, 29, 30, 31. Its cognitive and affective characteristics also include heightened emotional reactivity to social stimuli, often in the context of reduced cognitive control 25, 26. Lonely humans show greater vigilance for, and more rapid detection of, negative social information 22. Research in humans is commonly grounded in the perceptual and attentional sequelae of loneliness. Human studies of loneliness have also observed dampened reward signaling in mesolimbic systems to social cues 23, 24. However, animal studies have so far emphasized differences in subcortical reward systems associated with social isolation 22. Most salient manifestations, we argue, should be expected in brain regions that underwent evolutionary expansion in response to species-specific selection pressures for sociality 1, 21.
Given its central role in everyday life, loneliness is likely associated with specific burdens on the brain. Lonely older adults are 1.64 times more likely to develop clinical dementia than persons who do not self-report as lonely, after accounting for various factors including anxiety and depression 17. Lonely individuals typically have poorer mental health, higher susceptibility to major psychiatric disorders 11 and cognitive decline 16, 17, as well as greater neuropathological load with an increased risk of dementia 18, 19, 20. A sense of loneliness has also been associated with health risks that are equivalent to or exceed that of obesity or smoking 15 cigarettes daily 15. Loneliness is closely related to morbidity, hypertension, and immune system dysfunction 12, 13 as well as increasing risk for suicide 11, 14. The health burden of loneliness is pervasive. Loneliness is estimated to affect 10–20% of adults who lack companionship, consider themselves left out or isolated from others 11. While there is growing evidence that social connectedness may be associated with brain structure and function 7, 8, 9 (and see Bzdok and Dunbar 10 for a review), in the current report we directly investigate the neural correlates linked to trait loneliness, that is, the negative subjective experience of social isolation. One may have few social contacts yet not feel lonely, and vice versa. This concept is distinct from the amount of time spent alone 6 or the frequency of social contact 4. In the present study, we focus on the time-enduring, rather than momentary, nature of this negative sense of an unmet social need, which we henceforth refer to as ‘trait loneliness’. A key concern is the experience of ‘loneliness’: the subjective perception of social isolation, or the discrepancy between one’s desired and perceived levels of social connection 4, 5. Consequently, the absence of sufficient social engagement can impose substantial physical and psychological costs. Our species’ extraordinary reliance on other individuals has led to the characterization of humans as the “ultra-social animal” 2. Social interactions are crucial for survival, and fulfillment 3. Human evolution has been shaped by selection pressures towards enhanced inter-individual cooperation 1, 2. The findings fit with the possibility that the up-regulation of these neural circuits supports mentalizing, reminiscence and imagination to fill the social void. Lonely individuals display stronger functional communication in the default network, and greater microstructural integrity of its fornix pathway. This higher associative network shows more consistent loneliness associations in grey matter volume than other cortical brain networks.
The loneliness-linked neurobiological profiles converge on a collection of brain regions known as the ‘default network’. Using the UK Biobank population imaging-genetics cohort ( n = ~40,000, aged 40–69 years when recruited, mean age = 54.9), we test for signatures of loneliness in grey matter morphology, intrinsic functional coupling, and fiber tract microstructure. Despite severe consequences on behavior and health, the neural basis of loneliness remains elusive. Perceived social isolation, or loneliness, affects physical and mental health, cognitive performance, overall life expectancy, and increases vulnerability to Alzheimer’s disease-related dementias. Yet, social dependency also comes at a cost.
Humans survive and thrive through social exchange.