Loneliness across phylogeny and a call for comparative studies and animal models

Loneliness and Health in Older Adults: A Mini-Review and Synthesis

Increasing evidence suggests that perceived social isolation or loneliness is a major risk factor for physical and mental illness in later life. This review assesses the status of research on loneliness and health in older adults. Key concepts and definitions of loneliness are identified, and the prevalence, correlates, and health effects of loneliness in older individuals are reviewed. Theoretical mechanisms that underlie the association between loneliness and health are also described, and illustrative studies examining these mechanisms are summarized. Intervention approaches to reduce loneliness in old age are highlighted, and priority recommendations for future research are presented.

Loneliness, eudaimonia, and the human conserved transcriptional response to adversity

BACKGROUND

Chronic social adversity activates a conserved transcriptional response to adversity (CTRA) marked by increased expression of pro-inflammatory genes and decreased expression of antiviral- and antibody-related genes. Recent findings suggest that some psychological resilience factors may help buffer CTRA activation, but the relative impact of resilience and adversity factors remains poorly understood. Here we examined the relative strength of CTRA association for the two best-established psychological correlates of CTRA gene expression-the risk factor of perceived social isolation (loneliness) and the resilience factor of eudaimonic well-being (purpose and meaning in life).

METHODS

Peripheral blood samples and validated measures of loneliness and eudaimonic well-being were analyzed in 108 community-dwelling older adults participating in the longitudinal US Health and Retirement Study (56% female, mean age 73). Mixed effect linear model analyses quantified the strength of association between CTRA gene expression and measures of loneliness and eudaimonic well-being in separate and joint analyses.

RESULTS

As in previous studies, separate analyses found CTRA gene expression to be up-regulated in association with loneliness and down-regulated in association with eudaimonic well-being. In joint analyses, effects of loneliness were completely abrogated whereas eudaimonic well-being continued to associate with CTRA down-regulation. Similar eudaimonia-dominant effects were observed for positive and negative affect, optimism and pessimism, and anxiety symptoms. All results were independent of demographic and behavioral health risk factors.

CONCLUSIONS

Eudaimonic well-being may have the potential to compensate for the adverse impact of loneliness on CTRA gene expression. Findings suggest a novel approach to targeting the health risks associated with social isolation by promoting purpose and meaning in life.

Inter-individual differences in the experience of negative emotion predict variations in functional brain architecture

Current evidence suggests that two spatially distinct neuroanatomical networks, the dorsal attention network (DAN) and the default mode network (DMN), support externally and internally oriented cognition, respectively, and are functionally regulated by a third, frontoparietal control network (FPC). Interactions among these networks contribute to normal variations in cognitive functioning and to the aberrant affective profiles present in certain clinical conditions, such as major depression. Nevertheless, their links to non-clinical variations in affective functioning are still poorly understood. To address this issue, we used fMRI to measure the intrinsic functional interactions among these networks in a sample of predominantly younger women (N=162) from the Human Connectome Project. Consistent with the previously documented dichotomous motivational orientations (i.e., withdrawal versus approach) associated with sadness versus anger, we hypothesized that greater sadness would predict greater DMN (rather than DAN) functional dominance, whereas greater anger would predict the opposite. Overall, there was evidence of greater DAN (rather than DMN) functional dominance, but this pattern was modulated by current experience of specific negative emotions, as well as subclinical depressive and anxiety symptoms. Thus, greater levels of currently experienced sadness and subclinical depression independently predicted weaker DAN functional dominance (i.e., weaker DAN-FPC functional connectivity), likely reflecting reduced goal-directed attention towards the external perceptual environment. Complementarily, greater levels of currently experienced anger and subclinical anxiety predicted greater DAN functional dominance (i.e., greater DAN-FPC functional connectivity and, for anxiety only, also weaker DMN-FPC coupling). Our findings suggest that distinct affective states and subclinical mood symptoms have dissociable neural signatures, reflective of the symbiotic relationship between cognitive processes and emotional states.

Myeloid differentiation architecture of leukocyte transcriptome dynamics in perceived social isolation

To define the cellular mechanisms of up-regulated inflammatory gene expression and down-regulated antiviral response in people experiencing perceived social isolation (loneliness), we conducted integrative analyses of leukocyte gene regulation in humans and rhesus macaques. Five longitudinal leukocyte transcriptome surveys in 141 older adults showed up-regulation of the sympathetic nervous system (SNS), monocyte population expansion, and up-regulation of the leukocyte conserved transcriptional response to adversity (CTRA). Mechanistic analyses in a macaque model of perceived social isolation confirmed CTRA activation and identified selective up-regulation of the CD14(++)/CD16(-) classical monocyte transcriptome, functional glucocorticoid desensitization, down-regulation of Type I and II interferons, and impaired response to infection by simian immunodeficiency virus (SIV). These analyses identify neuroendocrine-related alterations in myeloid cell population dynamics as a key mediator of CTRA transcriptome skewing, which may both propagate perceived social isolation and contribute to its associated health risks.

Loneliness and Health in Older Adults: A Mini-Review and Synthesis

Increasing evidence suggests that perceived social isolation or loneliness is a major risk factor for physical and mental illness in later life. This review assesses the status of research on loneliness and health in older adults. Key concepts and definitions of loneliness are identified, and the prevalence, correlates, and health effects of loneliness in older individuals are reviewed. Theoretical mechanisms that underlie the association between loneliness and health are also described, and illustrative studies examining these mechanisms are summarized. Intervention approaches to reduce loneliness in old age are highlighted, and priority recommendations for future research are presented.

Loneliness across the life span

Most people have experienced loneliness and have been able to overcome it to reconnect with other people. In the current review, we provide a life-span perspective on one component of the evolutionary theory of loneliness-a component we refer to as the reaffiliation motive (RAM). The RAM represents the motivation to reconnect with others that is triggered by perceived social isolation. Loneliness is often a transient experience because the RAM leads to reconnection, but sometimes this motivation can fail, leading to prolonged loneliness. We review evidence of how aspects of the RAM change across development and how these aspects can fail for different reasons across the life span. We conclude with a discussion of age-appropriate interventions that may help to alleviate prolonged loneliness.

The genetics of loneliness: linking evolutionary theory to genome-wide genetics, epigenetics, and social science

As a complex trait, loneliness is likely to be influenced by the interplay of numerous genetic and environmental factors. Studies in behavioral genetics indicate that loneliness has a sizable degree of heritability. Candidate-gene and gene-expression studies have pointed to several genes related to neurotransmitters and the immune system. The notion that these genes are related to loneliness is compatible with the basic tenets of the evolutionary theory of loneliness. Research on gene-environment interactions indicates that social-environmental factors (e.g., low social support) may have a more pronounced effect and lead to higher levels of loneliness if individuals carry the sensitive variant of these candidate genes. Currently, there is no extant research on loneliness based on genome-wide association studies, gene-environment-interaction studies, or studies in epigenetics. Such studies would allow researchers to identify networks of genes that contribute to loneliness. The contribution of genetics to loneliness research will become stronger when genome-wide genetics and epigenetics are integrated and used along with well-established methods in psychology to analyze the complex process of gene-environment interplay.

Loneliness and social isolation as risk factors for mortality: a meta-analytic review

Actual and perceived social isolation are both associated with increased risk for early mortality. In this meta-analytic review, our objective is to establish the overall and relative magnitude of social isolation and loneliness and to examine possible moderators. We conducted a literature search of studies (January 1980 to February 2014) using MEDLINE, CINAHL, PsycINFO, Social Work Abstracts, and Google Scholar. The included studies provided quantitative data on mortality as affected by loneliness, social isolation, or living alone. Across studies in which several possible confounds were statistically controlled for, the weighted average effect sizes were as follows: social isolation odds ratio (OR) = 1.29, loneliness OR = 1.26, and living alone OR = 1.32, corresponding to an average of 29%, 26%, and 32% increased likelihood of mortality, respectively. We found no differences between measures of objective and subjective social isolation. Results remain consistent across gender, length of follow-up, and world region, but initial health status has an influence on the findings. Results also differ across participant age, with social deficits being more predictive of death in samples with an average age younger than 65 years. Overall, the influence of both objective and subjective social isolation on risk for mortality is comparable with well-established risk factors for mortality.

The ice-breaker effect: singing mediates fast social bonding

It has been proposed that singing evolved to facilitate social cohesion. However, it remains unclear whether bonding arises out of properties intrinsic to singing or whether any social engagement can have a similar effect. Furthermore, previous research has used one-off singing sessions without exploring the emergence of social bonding over time. In this semi-naturalistic study, we followed newly formed singing and non-singing (crafts or creative writing) adult education classes over seven months. Participants rated their closeness to their group and their affect, and were given a proxy measure of endorphin release, before and after their class, at three timepoints (months 1, 3 and 7). We show that although singers and non-singers felt equally connected by timepoint 3, singers experienced much faster bonding: singers demonstrated a significantly greater increase in closeness at timepoint 1, but the more gradual increase shown by non-singers caught up over time. This represents the first evidence for an 'ice-breaker effect' of singing in promoting fast cohesion between unfamiliar individuals, which bypasses the need for personal knowledge of group members gained through prolonged interaction. We argue that singing may have evolved to quickly bond large human groups of relative strangers, potentially through encouraging willingness to coordinate by enhancing positive affect.

Why may allopregnanolone help alleviate loneliness?

Impaired biosynthesis of Allopregnanolone (ALLO), a brain endogenous neurosteroid, has been associated with numerous behavioral dysfunctions, which range from anxiety- and depressive-like behaviors to aggressive behavior and changes in responses to contextual fear conditioning in rodent models of emotional dysfunction. Recent animal research also demonstrates a critical role of ALLO in social isolation. Although there are likely aspects of perceived social isolation that are uniquely human, there is also continuity across species. Both human and animal research show that perceived social isolation (which can be defined behaviorally in animals and humans) has detrimental effects on physical health, such as increased hypothalamic pituitary adrenal (HPA) activity, decreased brain-derived neurotrophic factor (BDNF) expression, and increased depressive behavior. The similarities between animal and human research suggest that perceived social isolation (loneliness) may also be associated with a reduction in the synthesis of ALLO, potentially by reducing BDNF regulation and increasing HPA activity through the hippocampus, amygdala, and bed nucleus of the stria terminalis (BNST), especially during social threat processing. Accordingly, exogenous administration of ALLO (or ALLO precursor, such as pregnenolone), in humans may help alleviate loneliness. Congruent with our hypothesis, exogenous administration of ALLO (or ALLO precursors) in humans has been shown to improve various stress-related disorders that show similarities between animals and humans i.e., post-traumatic stress disorders, traumatic brain injuries. Because a growing body of evidence demonstrates the benefits of ALLO in socially isolated animals, we believe our ALLO hypothesis can be applied to loneliness in humans, as well.

Loneliness: clinical import and interventions

In 1978, when the Task Panel report to the US President's Commission on Mental Health emphasized the importance of improving health care and easing the pain of those suffering from emotional distress syndromes including loneliness, few anticipated that this issue would still need to be addressed 40 years later. A meta-analysis (Masi et al., 2011) on the efficacy of treatments to reduce loneliness identified a need for well-controlled randomized clinical trials focusing on the rehabilitation of maladaptive social cognition. We review assessments of loneliness and build on this meta-analysis to discuss the efficacy of various treatments for loneliness. With the advances made over the past 5 years in the identification of the psychobiological and pharmaceutical mechanisms associated with loneliness and maladaptive social cognition, there is increasing evidence for the potential efficacy of integrated interventions that combine (social) cognitive behavioral therapy with short-term adjunctive pharmacological treatments.

Toward a neurology of loneliness

Social isolation has been recognized as a major risk factor for morbidity and mortality in humans for more than a quarter century. The brain is the key organ of social connections and processes, however, and the same objective social relationship can be experienced as caring and protective or as exploitive and isolating. We review evidence that the perception of social isolation (i.e., loneliness) impacts brain and behavior and is a risk factor for broad-based morbidity and mortality. However, the causal role of loneliness on neural mechanisms and mortality is difficult to test conclusively in humans. Mechanistic animal studies provide a lens through which to evaluate the neurological effects of a member of a social species living chronically on the social perimeter. Experimental studies show that social isolation produces significant changes in brain structures and processes in adult social animals. These effects are not uniform across the brain or across species but instead are most evident in brain regions that reflect differences in the functional demands of solitary versus social living for a particular species. The human and animal literatures have developed independently, however, and significant gaps also exist. The current review underscores the importance of integrating human and animal research to delineate the mechanisms through which social relationships impact the brain, health, and well-being.