The lonely brain: The neuroscience of social isolation

March 23rd, 2023

Written by: Andrew Nguyen

Since the beginning of the COVID-19 pandemic, the world has experienced an abrupt, unprecedented mass social isolation due to quarantine and social distancing guidelines put into place. During major peaks of social distancing, many people, myself included, felt overwhelmed by profound loneliness; daily schedules were out of whack, mental health was pushed to the limits, and we were deprived of social interactions. The onset of this mass social isolation triggered an emergence of interest in understanding how isolation acts on our brains to change our behaviors and how social isolation influences human health. 

What is social isolation?

Social connections and our social environments actually play a huge role in our physical and mental health. When we are physically separated from others is termed social isolation and when we feel separated from others, but are not necessarily physically separated, is termed loneliness. Social isolation and loneliness are both seen as public health risk-factors for a number of psychiatric disorders such as major depressive disorder, generalized anxiety, sleep disturbances, cardiovascular disease, and even early mortality^1,2,3,4. The COVID-19 lockdown and social distancing measures were an important public health intervention, but in doing so facilitated mental health-related issues across many communities due to forced social isolation^5,6. There are undoubtedly profound effects of social isolation that can influence both physical and mental health.

How social isolation changes our behaviors

COVID-19 lockdown created an unprecedented and abrupt transition from socially connected to socially disconnected, either left by ourselves or with a small group of people. We found ways to cope with the isolation: adapting to working from home with our Zoom happy hours, clapping out of our windows to support the healthcare workers, and finding ways to establish a daily schedule within the confines of our homes. These kinds of coping mechanisms are an example of behavioral changes that are a major hallmark of social isolation, and adapting our behavior to isolation is important for our physical and mental health. 

Social isolation also produces profound behavioral and physiological changes across the animal kingdom. Monkeys that are chronically socially isolated during the first six months of life exhibit fewer cooperative behaviors and many other social deficits into adulthood⁷. Beyond primates, cichlid fish are less cooperative and socially oriented after a chronic social isolation period⁸, and a recent study modeling chronic social isolation in fruit flies demonstrated that social isolation leads to sleep loss and excessive feeding⁹. In humans, similar behavioral changes are implicated in a number of mental health disorders. Characterizing how social isolation changes our behaviors is foundational to our understanding of why social isolation is a major risk-factor for both mental and physical health. 

Social isolation and the brain

Scientists are working to combine these findings about behavioral changes, mental and physical health, and longevity to understand the biology of social isolation. Studying social isolation faces many ethical concerns due to harmful psychological consequences to the subjects. Animal models have been important in facilitating this understanding of the relationship between the brain and social isolation. The brain regions critical for learning and memory of socially-isolated ants were shown to have developmental impairments compared to their socially-connected peers¹⁰. Fruit flies are a commonly used model organism scientists use to understand everything from genes to neural circuits to behavior, and importantly are more flexible in terms of research protocols. Studies using fruit flies have linked social isolation to specific neuronal populations that are involved in complex behaviors like sleep, feeding, aggression, courtship, learning and memory, and sociability¹¹ (Figure 1). A recent study demonstrated that socially isolated fruit flies eat an excessive amount and sleep less following chronic, but not acute, social isolation⁹. Public health research reports that the duration of loneliness is a stronger predictor of depression than the intensity of loneliness¹². 

Many organisms have evolved in social environments and adapted social behaviors to promote survival. Humans are one such organism, and our brains have developed for us to respond to being taken away from social environments. As scientists continue to consider the impacts of social isolation, new findings help us understand why we evolved to be social, the evolutionary fitness to being social, and why social connections are so important to our everyday lives and overall health. The stress response many of us experience following extended periods of social distancing, like the COVID lockdown, may be an evolved biological reaction to social isolation and we should think about why that is.

References

1. Taylor HO, Taylor RJ, Nguyen AW, et al. Social isolation, depression, and psychological distress among older adults. J Aging Health 2018, 30(2): 229–246.
2. Holt-Lunstad J, Smith TB, Layton JB. Social relationships and mortality risk: a meta-analytic review. PLoS Med 2010, 7(7): e1000316.
3. Stickley A, Koyanagi A. Loneliness, common mental disorders and suicidal behavior: findings from a general population survey. J Affect Disord 2016, 197: 81–87.
4. Vigorito C, Giallauria F. Loneliness, social isolation and risk of cardiovascular disease in the English Longitudinal Study of Ageing. Eur J Prev Cardiol 2018, 25(13): 1384–1386.
5. Killgore WDS, Cloonan SA, Taylor EC, et al. Loneliness: a signature mental health concern in the era of COVID-19. Psychiatry Res 2020, 290: 113117.
6. Lee CM, Cadigan JM, Rhew IC. Increases in loneliness among young adults during the COVID-19 pandemic and association with increases in mental health problems. J Adolesc Health 2020, 67(5): 714–717.
7. Harlow HF, Suomi SJ. Social recovery by isolation-reared monkeys. Proc Natl Acad Sci USA 1971, 68(7): 1534–1538.
8. Hesse S, Anaya-Rojas JM, Frommen JG, et al. Social deprivation affects cooperative predator inspection in a cichlid fish. R Soc Open Sci 2015, 2(3): 140451.
9. Li WH, Wang ZK, Syed S, et al. Chronic social isolation signals starvation and reduces sleep in Drosophila. Nature 2021, 597(7875): 239–244.
10. Seid MA, Junge E. Social isolation and brain development in the ant Camponotus floridanus. Naturwissenschaften 2016, 103(5/6): 42.
11. Vora A, Nguyen AD, Spicer C, Li W. The impact of social isolation on health and behavior in Drosophila melanogaster and beyond. Brain Science Advances. 2022;8(3):183-196.
12. Loades ME, Chatburn E, Higson-Sweeney N, et al. Rapid systematic review: the impact of social isolation and loneliness on the mental health of children and adolescents in the context of COVID-19. J Am Acad Child Adolesc Psychiatry 2020, 59(11): 1218–1239.e3.

Cover Photo by Sasha Freemind on Unsplash