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

A translational neuroscience perspective on loneliness: Narrative review focusing on social interaction, illness and oxytocin

This review addresses key findings on loneliness from the social, neurobiological and clinical fields. From a translational perspective, results from studies in humans and animals are included, with a focus on social interaction, mental and physical illness and the role of oxytocin in loneliness. In terms of social interactions, lonely individuals tend to exhibit a range of abnormal behaviors based on dysfunctional social cognitions that make it difficult for them to form meaningful relationships. Neurobiologically, a link has been established between loneliness and the hypothalamic peptide hormone oxytocin. Since social interactions and especially social touch regulate oxytocin signaling, lonely individuals may have an oxytocin imbalance, which in turn affects their health and well-being. Clinically, loneliness is a predictor of physical and mental illness and leads to increased morbidity and mortality. There is evidence that psychopathology is both a cause and a consequence of loneliness. The final section of this review summarizes the findings from social, neurobiological and clinical perspectives to present a new model of the complex construct of loneliness.

A Genetically Informed Study of the Association Between Perceived Stress and Loneliness

Although research shows a strong positive association between perceived stress and loneliness, the genetic and environmental etiology underlying their association remains unknown. People with a genetic predisposition to perceived stress, for example, may be more prone to feeling lonely and vice versa. Conversely, unique factors in people's lives may explain differences in perceived stress levels that, in turn, affect feelings of loneliness. We tested whether genetic factors, environmental factors, or both account for the association between perceived stress and loneliness. Participants were 3,066 individual twins (nFemale = 2,154, 70.3%) from the Washington State Twin Registry who completed a survey during April-May, 2020. Structural equation modeling was used to analyze the item-level perceived stress and loneliness measures. The correlation between latent perceived stress and latent loneliness was .68. Genetic and nonshared environmental variance components underlying perceived stress accounted for 3.71% and 23.26% of the total variance in loneliness, respectively. The genetic correlation between loneliness and perceived stress was .45 and did not differ significantly between men and women. The nonshared environmental correlation was .54 and also did not differ between men and women. Findings suggest that holding constant the strong genetic association between perceived stress and loneliness, unique life experiences underlying people's perceived stress account for individual differences in loneliness.

Neurogenetics and Epigenetics of Loneliness

Loneliness, an established risk factor for both, mental and physical morbidity, is a mounting public health concern. However, the neurobiological mechanisms underlying loneliness-related morbidity are not yet well defined. Here we examined the role of genes and associated DNA risk polymorphic variants that are implicated in loneliness via genetic and epigenetic mechanisms and may thus point to specific therapeutic targets. Searches were conducted on PubMed, Medline, and EMBASE databases using specific Medical Subject Headings terms such as loneliness and genes, neuro- and epigenetics, addiction, affective disorders, alcohol, anti-reward, anxiety, depression, dopamine, cancer, cardiovascular, cognitive, hypodopaminergia, medical, motivation, (neuro)psychopathology, social isolation, and reward deficiency. The narrative literature review yielded recursive collections of scientific and clinical evidence, which were subsequently condensed and summarized in the following key areas: (1) Genetic Antecedents: Exploration of multiple genes mediating reward, stress, immunity and other important vital functions; (2) Genes and Mental Health: Examination of genes linked to personality traits and mental illnesses providing insights into the intricate network of interaction converging on the experience of loneliness; (3) Epigenetic Effects: Inquiry into instances of loneliness and social isolation that are driven by epigenetic methylations associated with negative childhood experiences; and (4) Neural Correlates: Analysis of loneliness-related affective states and cognitions with a focus on hypodopaminergic reward deficiency arising in the context of early life stress, eg, maternal separation, underscoring the importance of parental support early in life. Identification of the individual contributions by various (epi)genetic factors presents opportunities for the creation of innovative preventive, diagnostic, and therapeutic approaches for individuals who cope with persistent feelings of loneliness. The clinical facets and therapeutic prospects associated with the current understanding of loneliness, are discussed emphasizing the relevance of genes and DNA risk polymorphic variants in the context of loneliness-related morbidity.

The CLoCk study: A retrospective exploration of loneliness in children and young people during the COVID-19 pandemic, in England

BACKGROUND

During the COVID-19 pandemic children and young people (CYP) were socially restricted during a stage of life crucial to development, potentially putting an already vulnerable population at higher risk of loneliness, social isolation, and poorer wellbeing. The objectives of this study are to conduct an exploratory analysis into loneliness before and during the pandemic, and determine which self-reported factors are associated with loneliness.

METHODS AND FINDINGS

Participants from The Children with Long COVID (CLoCk) national study were invited to take part via an online survey, with a total of 31,017 participants taking part, 31,016 of which reported on their experience of loneliness. Participants retrospectively answered questions on demographics, lifestyle, physical health and mental health and loneliness before the pandemic and at the time of answering the survey. Before the pandemic 6.5% (2,006/31,016) of participants reported experiencing loneliness "Often/Always" and at the time of survey completion 17.4% (5,395/31,016) reported feeling lonelier. There was an association between meeting the research definition of long COVID and loneliness [3.49 OR, 95%CI 3.28-3.72]. CYP who reported feeling lonelier at the time of the survey than before the pandemic were assigned female at birth, older CYP, those from Black/African/Caribbean/Black British or other ethnicity groups, those that had 3-4 siblings and lived in more deprived areas.

CONCLUSIONS

We demonstrate associations between multiple factors and experiences of loneliness during the pandemic. There is a need for a multi-faceted integrated approach when developing interventions targeted at loneliness. It is important to follow up the CYP involved at regular intervals to investigate the progression of their experience of loneliness over time.

Loneliness during the Covid-19 pandemic in Germany: Impact of social factors and polygenic risk scores on interpersonal differences in loneliness and mental health

OBJECTIVES

The outbreak of Covid-19 negatively affected mental health and increased loneliness. The subjective feeling of loneliness is influenced by genetic and social factors and has a negative impact on mental health.

METHODS

From March 2020 to June 2021 loneliness was investigated in N = 517 individuals using monthly acquired questionnaire data and Latent Growth Curve Analysis. Associations of social factors and polygenic risk scores (PRSs, n = 361) with class membership were investigated.

RESULTS

Three classes ("average", 40%; "not lonely", 38%; "elevated loneliness", 22%) were identified, that differ significantly regarding loneliness, mental dysfunction, and response to the lockdown phases. Individuals with a high PRS for neuroticism are more likely to belong to the "elevated loneliness" class, living with another person is a protective factor.

CONCLUSION

As the "elevated loneliness" class was at the highest risk of mental dysfunction, our findings underscore the importance of identifying those individuals to implement counteractive measures.

Loneliness and depression: bidirectional mendelian randomization analyses using data from three large genome-wide association studies

Major depression (MD) is a serious psychiatric illness afflicting nearly 5% of the world's population. A large correlational literature suggests that loneliness is a prospective risk factor for MD; correlational assocations of this nature may be confounded for a variety of reasons. This report uses Mendelian Randomization (MR) to examine potentially causal associations between loneliness and MD. We report on analyses using summary statistics from three large genome wide association studies (GWAS). MR analyses were conducted using three independent sources of GWAS summary statistics. In the first set of analyses, we used available summary statistics from an extant GWAS of loneliness to predict MD risk. We used two sources of outcome data: the Psychiatric Genomics Consortium (PGC) meta-analysis of MD (PGC-MD; N = 142,646) and the Million Veteran Program (MVP-MD; N = 250,215). Finally, we reversed analyses using data from the MVP and PGC samples to identify risk variants for MD and used loneliness outcome data from UK Biobank. We find robust evidence for a bidirectional causal relationship between loneliness and MD, including between loneliness, depression cases status, and a continuous measure of depressive symptoms. The estimates remained significant across several sensitivity analyses, including models that account for horizontal pleiotropy. This paper provides the first genetically-informed evidence that reducing loneliness may play a causal role in decreasing risk for depressive illness, and these findings support efforts to reduce loneliness in order to prevent or ameliorate MD. Discussion focuses on the public health significance of these findings, especially in light of the SARS-CoV-2 pandemic.

Perceived Parental Support and Psychological Control, DNA Methylation, and Loneliness: Longitudinal Associations Across Early Adolescence

A broad range of factors have been associated with the development of adolescent loneliness. In the family context, a lack of parental support and high levels of parental psychological control have systematically been linked to loneliness. On the biological level, DNA methylation (which is an epigenetic process that suppresses gene expression) is believed to play a role in the development of loneliness. Specifically, high levels of DNA methylation in genes that play an important role in the functioning of the human stress response system are believed to elevate the risk of loneliness. Moreover, DNA methylation levels in these stress-related genes can be influenced by stressful environmental factors, suggesting a potential mediating role of DNA methylation in the association between parenting behaviors and loneliness. The current 3-year longitudinal study is the first study to examine the potential bidirectional longitudinal associations between loneliness, DNA methylation in stress-related genes, and both perceived parental support and psychological control. Furthermore, we explored the potential mediating role of DNA methylation in stress-related genes in the associations between perceived parenting and loneliness. The sample comprised 622 early adolescents (55% girls, M_{age T1} = 10.77 years, SD_{age T1} = 0.48) who were followed from Grade 5 to 7. Parental support, psychological control, and loneliness were assessed annually by adolescent self-report questionnaires and DNA methylation was determined from saliva samples. Cross-Lagged Panel Models (CLPM) revealed that higher levels of loneliness predicted lower perceived parental support and higher perceived psychological control over time, as well as higher DNA methylation in some stress-related genes, that is, the glucocorticoid receptor gene (NR3C1) and the brain-derived neurotrophic factor (BDNF). In addition, higher NR3C1 methylation was predictive of lower perceived parental support and higher psychological control over time. No evidence was found for a mediating role of DNA methylation. Overall, our longitudinal findings challenge the current focus on DNA methylation and parenting behaviors as risk factors for adolescent loneliness. Instead, they suggest that the less considered direction of effects, which implies that loneliness predicts DNA methylation and aspects of parenting such as support and psychological control, should receive greater attention in future research.

[Loneliness in childhood and adolescence. On the prevalence of a mental health risk factor among 11- to 15-year-old German students]

BACKGROUND

Loneliness is an increasingly present topic in both public and scientific discourse. There is increasing research on the prevalence of loneliness and health-related correlates in adults. However, there are only a few findings on loneliness in childhood and adolescence in Germany. Therefore, this study aims to present findings on the prevalence of loneliness among 11- to 15-year-old German students and examines associations with socio-demographic characteristics.

METHODS

The Health Behaviour in School-aged Children (HBSC) study is a global research collaboration under the auspices of the World Health Organization (WHO). In the federal state of Brandenburg, Germany, the study collected data from 3819 children and adolescents in grades 5, 7, and 9 (M_age = 13.5, SD = 1.6) at schools for general education. Loneliness was assessed using the University of California, Los Angeles (UCLA) Loneliness Scale and a single item.

RESULTS

Of the respondents, 13.2% reported feeling lonely most of the time and 3.6% always. Loneliness was more prevalent among girls and non-binary students, as well as among older students and children and adolescents of lower socioeconomic status.

DISCUSSION

The findings indicate that loneliness is a widespread phenomenon among children and adolescents. The result that girls, older pupils and children, and adolescents with lower family wealth report feeling lonelier is consistent with findings from other countries and provides starting points for planning prevention measures. Further research is needed in Germany on possible health-related correlates of loneliness.

The developmental course of loneliness in adolescence: Implications for mental health, educational attainment, and psychosocial functioning

The present study examined patterns of stability and change in loneliness across adolescence. Data were drawn from the Environmental Risk (E-Risk) Longitudinal Twin Study, a UK population-representative cohort of 2,232 individuals born in 1994 and 1995. Loneliness was assessed when participants were aged 12 and 18. Loneliness showed modest stability across these ages (r = .25). Behavioral genetic modeling indicated that stability in loneliness was explained largely by genetic influences (66%), while change was explained by nonshared environmental effects (58%). Individuals who reported loneliness at both ages were broadly similar to individuals who only reported it at age 18, with both groups at elevated risk of mental health problems, physical health risk behaviors, and education and employment difficulties. Individuals who were lonely only at age 12 generally fared better; however, they were still more likely to finish school with lower qualifications. Positive family influences in childhood predicted reduced risk of loneliness at age 12, while negative peer experiences increased the risk. Together, the findings show that while early adolescent loneliness does not appear to exert a cumulative burden when it persists, it is nonetheless a risk for a range of concomitant impairments, some of which can endure.

[Loneliness in Adolescents Against the Background of the COVID-19 Pandemic: A Risk Factor]

Loneliness in Adolescents Against the Background of the COVID-19 Pandemic: A Risk Factor Abstract. On the one hand, loneliness is the subjective experience of inadequate social integration and support. It is a normal phenomenon that can be dealt with in an emotional and action-oriented way within the framework of individual development. On the other hand, chronic loneliness goes hand in hand with considerable suffering, reduced quality of life, and an increased risk of psychological and somatic diseases. In the context of current social developments associated with the fragmentation of social networks and programmatic individualism, we discuss an increase in the problem of loneliness, especially among young people. Interventions to reduce the experience of loneliness are effective if, in addition to making social offers, they focus on the dynamics common in chronic loneliness (low self-esteem, evaluation of neutral communication as devaluing, etc.). There is no evidence that the use of virtual social networks persistently reduces feelings of loneliness in young people. In addition, the question arises on what basis of shared values and goals social integration of young people who experience loneliness will be possible in the future. According to the results of the first systematic surveys on the psychosocial consequences of the COVID-19 pandemic, we must assume that the loneliness problem of young people will continue to gain relevance because of the associated massive restrictions on real social life.

Childhood loneliness as a specific risk factor for adult psychiatric disorders

BACKGROUND

Loneliness is a major risk factor for both psychological disturbance and poor health outcomes in adults. This study aimed to assess whether childhood loneliness is associated with a long-term disruption in mental health that extends into adulthood.

METHODS

This study is based on the longitudinal, community-representative Great Smoky Mountains Study of 1420 participants. Participants were assessed with the structured Child and Adolescent Psychiatric Assessment interview up to eight times in childhood (ages 9-16; 6674 observations; 1993-2000) for childhood loneliness, associated psychiatric comorbidities and childhood adversities. Participants were followed up four times in adulthood (ages 19, 21, 25, and 30; 4556 observations of 1334 participants; 1999-2015) with the structured Young Adult Psychiatric Assessment Interview for psychiatric anxiety, depression, and substance use outcomes.

RESULTS

Both self and parent-reported childhood loneliness were associated with adult self-reported anxiety and depressive outcomes. The associations remained significant when childhood adversities and psychiatric comorbidities were accounted for. There was no evidence for an association of childhood loneliness with adult substance use disorders. More associations were found between childhood loneliness and adult psychiatric symptoms than with adult diagnostic status.

CONCLUSION

Childhood loneliness is associated with anxiety and depressive disorders in young adults, suggesting that loneliness - even in childhood - might have long-term costs in terms of mental health. This study underscores the importance of intervening early to prevent loneliness and its sequelae over time.

Loneliness, Depression, and Genetics in the Elderly: Prognostic Factors of a Worse Health Condition?

Loneliness is considered a prognostic factor for poorer health status in the elderly. It is proposed to analyze the role of loneliness in health status in terms of various factors. A total of 1747 individuals from the pilot survey of the Aging in Spain Longitudinal Study (ELES-PS) were reviewed. ELES is a cross-sectional study for collecting health variables, food habits, socioeconomic data, and cognitive and functional capacities, which was carried out on a Spanish representative sample of noninstitutionalized persons of 50 years of age or older. Moreover, since telomere shortening is associated with cellular senescence, 35 telomere-related SNPs and cognitive impairments were analyzed. The results characterize the "solos" as males of 50-60 years, who were overweight and had lower levels of hemoglobin and neutrophils. There is also an association between five SNPs related to telomere length and BDNF. A group of people with loneliness and depression was identified with poorer health and cognitive status, poorer perception of their quality of life, poorer quality of sleep, and lower physical activity. Therefore, it follows that telomeres and BDNF play a role as intermediaries between loneliness and depression and their relationship with a worse state of health.

Social connectedness as a determinant of mental health: A scoping review

Public health and epidemiologic research have established that social connectedness promotes overall health. Yet there have been no recent reviews of findings from research examining social connectedness as a determinant of mental health. The goal of this review was to evaluate recent longitudinal research probing the effects of social connectedness on depression and anxiety symptoms and diagnoses in the general population. A scoping review was performed of PubMed and PsychInfo databases from January 2015 to December 2021 following PRISMA-ScR guidelines using a defined search strategy. The search yielded 66 unique studies. In research with other than pregnant women, 83% (19 of 23) studies reported that social support benefited symptoms of depression with the remaining 17% (5 of 23) reporting minimal or no evidence that lower levels of social support predict depression at follow-up. In research with pregnant women, 83% (24 of 29 studies) found that low social support increased postpartum depressive symptoms. Among 8 of 9 studies that focused on loneliness, feeling lonely at baseline was related to adverse outcomes at follow-up including higher risks of major depressive disorder, depressive symptom severity, generalized anxiety disorder, and lower levels of physical activity. In 5 of 8 reports, smaller social network size predicted depressive symptoms or disorder at follow-up. In summary, most recent relevant longitudinal studies have demonstrated that social connectedness protects adults in the general population from depressive symptoms and disorders. The results, which were largely consistent across settings, exposure measures, and populations, support efforts to improve clinical detection of high-risk patients, including adults with low social support and elevated loneliness.

The role of sense of coherence and loneliness in borderline personality disorder traits: a longitudinal twin study

BACKGROUND

Borderline personality disorder (BPD) implies having problems with identity and relations with other people. However, not much is known about whether these indications of BPD are present in adolescence, i.e., before personality disorders usually are diagnosed. In this study, we examined the prediction of an aspect of identity (i.e., sense of coherence [SOC]) and social relations (i.e., perceived loneliness) throughout adolescence on BPD traits in young adulthood. In addition, we examined to what degree the predictive ability could be attributed to genetic and environmental factors. We also examined whether life events in adolescence were related to BPD traits.

METHODS

Three thousand three hundred ninety-one twins, consisting of seven national birth cohorts from Norway, participated in the study. SOC, loneliness and life events were measured three times throughout adolescence with self-report questionnaires, with 2 years in between measurements. BPD traits were measured at the end of adolescence around the age of 19 with a structured interview. Regression analyses were performed to examine the prediction of SOC, loneliness and life events on BPD traits. Cholesky decomposition models were then used to determine to what degree the associations were due to genetic and environmental influences.

RESULTS

The prediction of SOC and loneliness on BPD traits increased from R = .25 (when measured 6 years prior to the assessment of BPD traits) to R = .45 (when measured shortly before the assessment of BPD traits). In addition, negative life events considered dependent on a person's behavior were related to BPD traits. Negative independent and positive dependent life events did not contribute to the prediction of BPD traits. Cholesky decomposition models showed that SOC and loneliness were associated with BPD traits mainly due to shared genetic influences (i.e., the proportion due to genetic influences ranged from 71 to 86%). Adding negative dependent life events to the prediction of BPD traits did not change these percentages.

CONCLUSIONS

These findings indicate that the weaker SOC, the stronger feelings of loneliness, and the negative life events associated with BPD traits are mainly consequences of the genetic aspects of BPD traits, rather than having direct effects on levels of BPD symptoms.

Loneliness at Older Ages in the United States: Lonely Life Expectancy and the Role of Loneliness in Health Disparities

We provide an empirical foundation for research on the demography of loneliness at older ages. First, we use published life tables and data from the U.S.-based Health and Retirement Study for the period 2008-2016 to calculate lonely life expectancy for Americans aged 55 or older. Using Sullivan's method, we demonstrate pronounced differences in lonely life expectancy by sex, race/ethnicity, and educational attainment that correspond to well-established patterns of stratification in other dimensions of well-being. Next, we estimate models that decompose observed sex, racial/ethnic, and educational differences in three key health outcomes into the part explained (in a statistical accounting sense) by loneliness and the part accounted for by other factors. We find little evidence of an important role for loneliness in understanding disparities in mortality and the onset of physical disability and cognitive impairment among Americans aged 55 or older, net of several established correlates of health disparities. These descriptive findings provide an empirical foundation for continued development of a demography of loneliness at older ages in response to the anticipated growth in scientific and policy emphasis on loneliness and the fundamental life changes that have accompanied the COVID-19 pandemic.

Associations of loneliness with risk of Alzheimer's disease dementia in the Framingham Heart Study

INTRODUCTION

The relationship between persistent loneliness and Alzheimer's disease (AD) is unclear. We examined the relationship between different types of mid-life loneliness and the development of dementia and AD.

METHODS

Loneliness was assessed in cognitively normal adults using one item from the Center for Epidemiologic Studies Depression Scale. We defined loneliness as no loneliness, transient loneliness, incident loneliness,or persistent loneliness, and applied Cox regression models and Kaplan-Meier plots with dementia and AD as outcomes (n = 2880).

RESULTS

After adjusting for demographics, social network, physical health, and apolipoprotein E ε4, persistent loneliness was associated with higher (hazard ratio [HR], 1.91; 95% confidence interval [CI] 1.25-2.90; P < .01), and transient loneliness with lower (HR, 0.34; 95% CI 0.14-0.84; P < .05), risk of dementia onset, compared to no loneliness. Results were similar for AD risk.

DISCUSSION

Persistent loneliness in mid-life is an independent risk factor for dementia and AD, whereas recovery from loneliness suggests resilience to dementia risk.

Solitary Living Brings a Decreased Weight and an Increased Agility to the Domestic Silkworm, Bombyx mori

Simple Summary

We identified and validated that solitary living brings a decreased weight and an increased agility to silkworms. Solitary silkworms have a faster movement in response to food or physical stress than group-living counterparts. These contradict previous thoughts that solitary or lonely life is always harmful to animals or humans. We identified differently expressed genes (DEGs) and microRNAs (DEmiRNAs) resulted from solitary living. These DEGs and DEmiRNAs are functionally associated with the phenotypic changes led by solitary living.

Abstract

The domestic silkworms, Bombyx mori, always live in groups and little is known of the outcomes of solitary living. We bred solitary silkworms and performed a comprehensive investigation of the difference between solitary and group-living silkworms. The results show that solitary silkworms had significantly lower weights than group-living counterparts. Moreover, solitary silkworms had faster movements under food luring or heat stress than the group-living ones, supported by extensive behavior experiments. These findings inferred that an increased agility resulted from solitary living. For an understanding of the molecular mechanism associated with solitary living, we performed integrated mRNA and miRNA (microRNA) sequencing of tissues for solitary and group-living silkworms. We identified 165 differently expressed genes (DEGs) and 6 differently expressed miRNAs between the solitary and group-living silkworms. Functional and pathway analyses indicated that these DEGs are associated with weight loss and agility increase. These findings compose a sketch depicting an association between the phenotypes and genes resulted from solitary living and refresh the understanding of solitary living and loneliness, which has an increased prevalence in our modern society.

A genetic screen for Drosophila social isolation mutants and analysis of sex pistol

Prolonged periods of forced social isolation is detrimental to well-being, yet we know little about which genes regulate susceptibility to its effects. In the fruit fly, Drosophila melanogaster, social isolation induces stark changes in behavior including increased aggression, locomotor activity, and resistance to ethanol sedation. To identify genes regulating sensitivity to isolation, I screened a collection of sixteen hundred P-element insertion lines for mutants with abnormal levels of all three isolation-induced behaviors. The screen identified three mutants whose affected genes are likely central to regulating the effects of isolation in flies. One mutant, sex pistol (sxp), became extremely aggressive and resistant to ethanol sedation when socially isolated. sxp also had a high level of male–male courtship. The mutation in sxp reduced the expression of two minor isoforms of the actin regulator hts (adducin), as well as mildly reducing expression of CalpA, a calcium-dependent protease. As a consequence, sxp also had increased expression of the insulin-like peptide, dILP5. Analysis of the social behavior of sxp suggests that these minor hts isoforms function to limit isolation-induced aggression, while chronically high levels of dILP5 increase male–male courtship.

Association of Loneliness and Wisdom With Gut Microbial Diversity and Composition: An Exploratory Study

Loneliness and wisdom have opposite effects on health and well-being. Loneliness is a serious public health problem associated with increased morbidity and mortality. Wisdom is associated with better health and well-being. We have consistently found a strong negative correlation between loneliness and wisdom. The present study aimed to investigate the association of loneliness and wisdom with the gut microbiome. One hundred eighty-four community-dwelling adults (28-97 years) completed validated self-report-based measures of loneliness, wisdom, compassion, social support, and social engagement. Fecal samples were collected and profiled using 16S rRNA sequencing. Linear regression analyses, controlling for age and body mass index, revealed that lower levels of loneliness and higher levels of wisdom, compassion, social support, and social engagement were associated with greater phylogenetic richness and diversity of the gut microbiome. Partial least squares (PLS) analysis to investigate multivariate relationships extracted two composite variables. Linear regression model predicting alpha-diversity with PLS components revealed that a linear combination of all psychosocial predictors (with negative loading for loneliness and positive loadings for all others, including wisdom, compassion, social support, and social engagement) was significantly associated with alpha-diversity. For beta-diversity, compassion and wisdom accounted for a significant proportion of variance in overall microbial community composition. Findings may have implications for interventions to reduce loneliness and possibly its health-related adverse consequences. Future research should explore whether increasing compassion and wisdom may improve loneliness and overall well-being as well as microbial diversity.

A longitudinal twin study of victimization and loneliness from childhood to young adulthood

The present study used a longitudinal and discordant twin design to explore in depth the developmental associations between victimization and loneliness from mid-childhood to young adulthood. The data were drawn from the Environmental Risk (E-Risk) Longitudinal Twin Study, a birth cohort of 2,232 individuals born in England and Wales during 1994-1995. Diverse forms of victimization were considered, differing across context, perpetrator, and timing of exposure. The results indicated that exposure to different forms of victimization was associated with loneliness in a dose-response manner. In childhood, bullying victimization was uniquely associated with loneliness, over and above concurrent psychopathology, social isolation, and genetic risk. Moreover, childhood bullying victimization continued to predict loneliness in young adulthood, even in the absence of ongoing victimization. Within-twin pair analyses further indicated that this longitudinal association was explained by genetic confounds. In adolescence, varied forms of victimization were correlated with young adult loneliness, with maltreatment, neglect, and cybervictimization remaining robust to controls for genetic confounds. These findings indicate that vulnerability to loneliness in victimized young people varies according to the specific form of victimization in question, and also to the developmental period in which it was experienced.

Understanding loneliness in the twenty-first century: an update on correlates, risk factors, and potential solutions

PURPOSE

Loneliness is increasingly recognised as the next critical public health issue. A plausible reason for this concern may be related to emerging societal trends affecting the way we relate, communicate, and function in our social environment. In 2006, a prominent review of the clinical significance of loneliness was published. However, there has not been a comprehensive update on known and emerging risk factors and correlates of loneliness since then. Furthermore, there is no conceptual model that has been developed to better account for the complexity of loneliness and to inform the development of evidence-based solutions as we challenge the issues of the twenty-first century.

METHODS

We reviewed the current literature to identify either known or emerging risk factors and correlates of loneliness since 2006. This includes new or known evidence on: (1) demography; (2) health, including physical health; mental health; cognitive health; brain, biology, and genetics; and (3) socio-environmental factors including digital communication and the workplace.

RESULTS

We synthesized the literature according to a new proposed conceptual model of loneliness which showed the interplay between known and emerging correlates and risk factors from demography, health, to socio-environmental factors. In the conceptual model of loneliness, we illustrated how solutions can be delivered and tailored to an individual based on their life circumstances and preferences.

CONCLUSION

We concluded by making specific recommendations in advancing our scientific understanding of loneliness. Our knowledge can only be deepened if we increase scientific rigour via accounting for confounding variables and using longitudinal, multi-disciplinary, and multiple methodologies in research. We also call for the rigorous evaluation of programs targeting loneliness.

A critical review and development of a conceptual model of exclusion from social relations for older people

Social exclusion is complex and dynamic, and it leads to the non-realization of social, economic, political or cultural rights or participation within a society. This critical review takes stock of the literature on exclusion of social relations. Social relations are defined as comprising social resources, social connections and social networks. An evidence review group undertook a critical review which integrates, interprets and synthesizes information across studies to develop a conceptual model of exclusion from social relations. The resulting model is a subjective interpretation of the literature and is intended to be the starting point for further evaluations. The conceptual model identifies individual risks for exclusion from social relations (personal attributes, biological and neurological risk, retirement, socio-economic status, exclusion from material resources and migration). It incorporates the evaluation of social relations, and the influence of psychosocial resources and socio-emotional processes, sociocultural, social-structural, environmental and policy contextual influences on exclusion from social relations. It includes distal outcomes of exclusion from social relations, that is, individual well-being, health and functioning, social opportunities and social cohesion. The dynamic relationships between elements of the model are also reported. We conclude that the model provides a subjective interpretation of the data and an excellent starting point for further phases of conceptual development and systematic evaluation(s). Future research needs to consider the use of sophisticated analytical tools and an interdisciplinary approach in order to understand the underlying biological and ecopsychosocial associations that contribute to individual and dynamic differences in the experience of exclusion from social relations.

The Stability and Change of Loneliness Across the Life Span: A Meta-Analysis of Longitudinal Studies

Individuals experience loneliness when they perceive a deficiency in the quality or quantity of their social relationships. In the present meta-analysis, we compiled data from 75 longitudinal studies conducted in Asia, Australia, Europe, and North America (N = 83, 679) to examine the rank-order and mean-level development of loneliness across the life span. Data were analyzed using two- and three-level meta-analyses and generalized additive mixed models. The results indicate that the rank order of loneliness is as stable as the rank order of personality traits and follows an inverted U-shaped trajectory across the life span. Regarding mean-level development, loneliness was found to decrease throughout childhood and to remain essentially stable from adolescence to oldest old age. Thus, in contrast to other personality characteristics, changes in loneliness are not generally related to age. Implications for theory are discussed.

A SNP, Gene, and Polygenic Risk Score Approach of Oxytocin-Vasopressin Genes in Adolescents' Loneliness

Not much is known regarding underlying biological pathways to adolescents' loneliness. Insight in underlying molecular mechanisms could inform intervention efforts aimed at reducing loneliness. Using latent growth curve modeling, baseline levels and development of loneliness were studied in two longitudinal adolescent samples. Genes (OXTR, OXT, AVPR1A, AVPR1B) were examined using SNP-based, gene-based, and polygenic risk score (PRS) approaches. In both samples, SNP- and gene-based tests showed involvement of the OXTR gene in development of loneliness, though, significance levels did not survive correction for multiple testing. The PRS approach provided no evidence for relations with loneliness. We recommend alternative phenotyping methods, including environmental factors, to consider epigenetic studies, and to examine possible endophenotypes in relation to adolescents' loneliness.

Candidate Biomarkers of Suicide Crisis Syndrome: What to Test Next? A Concept Paper

BACKGROUND

There has been increasing interest in both suicide-specific diagnoses within the psychiatric nomenclature and related biomarkers. Because the Suicide Crisis Syndrome-an emotional crescendo of several interrelated symptoms-seems to be promising for the identification of individuals at risk of suicide, the aim of the present paper is to review the putative biological underpinnings of the Suicide Crisis Syndrome symptoms (entrapment, affective disturbance, loss of cognitive control, hyperarousal, social withdrawal).

METHODS

A PubMed literature search was performed to identify studies reporting a link between each of the 5 Suicide Crisis Syndrome symptoms and biomarkers previously reported to be associated with suicidal outcomes.

RESULTS

Disturbances in the hypothalamic-pituitary-adrenal axis, with dysregulated corticotropin-releasing hormone and cortisol levels, may be linked to a sense of entrapment. Affective disturbance is likely mediated by alterations in dopaminergic circuits involved in reward and antireward systems as well as endogenous opioids. Loss of cognitive control is linked to altered neurocognitive function in the areas of executive function, attention, and decision-making. Hyperarousal is linked to autonomic dysregulation, which may be characterized by a reduction in both heart rate variability and electrodermal activity. Social withdrawal has been associated with oxytocin availability. There is also evidence that inflammatory processes may contribute to individual Suicide Crisis Syndrome symptoms.

CONCLUSION

The Suicide Crisis Syndrome is a complex syndrome that is likely the consequence of distinct changes in interconnected neural, neuroendocrine, and autonomic systems. Available clinical and research data allow for development of empirically testable hypotheses and experimental paradigms to scrutinize the biological substrates of the Suicide Crisis Syndrome.

Epigenetic Modifications in Stress Response Genes Associated With Childhood Trauma

Adverse childhood experiences (ACEs) may be referred to by other terms (e.g., early life adversity or stress and childhood trauma) and have a lifelong impact on mental and physical health. For example, childhood trauma has been associated with posttraumatic stress disorder (PTSD), anxiety, depression, bipolar disorder, diabetes, and cardiovascular disease. The heritability of ACE-related phenotypes such as PTSD, depression, and resilience is low to moderate, and, moreover, is very variable for a given phenotype, which implies that gene by environment interactions (such as through epigenetic modifications) may be involved in the onset of these phenotypes. Currently, there is increasing interest in the investigation of epigenetic contributions to ACE-induced differential health outcomes. Although there are a number of studies in this field, there are still research gaps. In this review, the basic concepts of epigenetic modifications (such as methylation) and the function of the hypothalamic-pituitary-adrenal (HPA) axis in the stress response are outlined. Examples of specific genes undergoing methylation in association with ACE-induced differential health outcomes are provided. Limitations in this field, e.g., uncertain clinical diagnosis, conceptual inconsistencies, and technical drawbacks, are reviewed, with suggestions for advances using new technologies and novel research directions. We thereby provide a platform on which the field of ACE-induced phenotypes in mental health may build.

Does loneliness 'get under the skin'? Associations of loneliness with subsequent change in inflammatory and metabolic markers

Objectives: To expand existing knowledge about the associations of late-life loneliness with subsequent change in inflammation as well as with metabolic dysregulation, using national representative longitudinal data. The current analysis also explores age, gender, and race differences in these pre-disease pathways. Method: The analysis is based on data from the 2005-06 and 2010-11 waves of the National Social Life, Health, and Aging Project (NSHAP)-a longitudinal survey conducted among a representative sample of community-dwelling Americans aged 57-85. Adjusted logistic regression models were used to examine associations of loneliness with changes in C-reactive protein (CRP), Glycated hemoglobin (HbA1c), Body Mass Index (BMI), Waist Circumference (WC), and metabolic burden. Results: Loneliness was found to be associated with a change for the worse in most of the metabolic bio-markers. Specifically, lonely older adults had 39%-71% higher odds of developing prospective risk levels in three out of the four metabolic bio-markers that were measured: HbA1c, BMI, and metabolic burden. Salient differences by race were found in this regard. Whereas loneliness was not significantly associated with HbA1c risk levels and BMI among the Whites, the prospective risk of high HbA1c was more than five times greater and the risk of high BMI scores was three times greater among Hispanics who experienced loneliness than among the not-lonely Hispanic group. Conclusions: The robust impact of loneliness on prospective changes for the worse in levels of various metabolic bio-markers that are closely associated with morbidity highlights the need for prevention, coping with, and reducing loneliness.

Differential impact of transient and chronic loneliness on health status. A longitudinal study

Objective: Loneliness is associated with worse health status outcomes. Yet, the present study is one of the first to identify how patterns of loneliness (transient and chronic) are associated with health over time. Design: A total of 2,390 individuals were interviewed in 2011-2012 and 2014-2015 in a follow-up study conducted over a nationally representative sample of Spain. After confirming a longitudinal relationship between loneliness and health status, a growth curve mixture modeling was used to examine health trajectories. Main outcome measures: The three-item UCLA Loneliness Scale was used to assess loneliness. Health status was measured with self-reported questions regarding ten domains (vision, mobility, and self-care, among others), and seven measured tests (including grip strength, walking speed and immediate and delayed verbal recall). Results: A quarter of participants were lonely at baseline. Both the group of transient and chronic loneliness showed a negative significant relationship with health status at follow-up, (β = -0.063 and β = -0.075 respectively, p < 0.001). Nevertheless, the health status did not change across time in any group. Conclusion: People experiencing chronic loneliness had the worst health status. Different patterns of loneliness could benefit from the appropriate interventions.

Genetic Contributions to Loneliness and Their Relevance to the Evolutionary Theory of Loneliness

Loneliness is a negative and distressing emotional state that arises from a discrepancy between one's desired and achieved levels of social connectedness. The evolutionary theory of loneliness (ETL) posits that experiencing loneliness is an inherited adaptation that signals that salutary social relations are endangered or damaged and prompts people to reconnect to significant others. The basic tenets of the ETL has led researchers to examine the genetic underpinnings of loneliness. The current review provides an updated overview of genetic studies on loneliness and discusses the importance of genetic research for the ETL. The most recent studies suggest that the many genes that contribute to a small degree to differences in loneliness partially overlap with genes that contribute to neuroticism, but not with depression. In addition, the genetic studies discussed in this review show that genes are unlikely to have a direct effect on loneliness. Instead, environmental factors determine in a dynamic fashion how genes that contribute to loneliness are expressed. Future research on epigenetic processes, such as DNA methylation, can further elucidate the dynamic interplay between genes and the environment and how this interplay contributes to loneliness.

Associations between loneliness and personality are mostly driven by a genetic association with Neuroticism

OBJECTIVE

Loneliness is an aversive response to a discrepancy between desired and actual social relationships and correlates with personality. We investigate the relationship of loneliness and personality in twin family and molecular genetic data.

METHOD

Phenotypic correlations between loneliness and the Big Five personality traits were estimated in 29,625 adults, and in a group with genome-wide genotype data (N = 4,222), genetic correlations were obtained. We explored whether genetic correlations may reflect causal relationships by investigating within monozygotic twin pair differences (N_pairs  = 2,662), by longitudinal within-subject changes in personality and loneliness (N = 4,260-9,238 longitudinal comparisons), and by longitudinal cross-lagged panel analyses (N = 15,628). Finally, we tested whether genetic correlations were due to cross-trait assortative mating (N_{spouse pairs}  = 4,436).

RESULTS

The strongest correlations with loneliness were observed for Neuroticism (r = .55) and Extraversion (r = -.33). Only Neuroticism showed a high correlation with loneliness independent of other personality traits (r = .50), so follow-up analyses focused on Neuroticism. The genetic correlation between loneliness and Neuroticism from genotyped variants was .71; a significant reciprocal causal relationship and nonsignificant cross-trait assortative mating imply that this is at least partly due to mediated pleiotropy.

CONCLUSIONS

We show that the relationship between loneliness and personality is largely explained by its relationship with Neuroticism, which is substantially genetic in nature.

Advancing social connection as a public health priority in the United States

A robust body of scientific evidence has indicated that being embedded in high-quality close relationships and feeling socially connected to the people in one's life is associated with decreased risk for all-cause mortality as well as a range of disease morbidities. Despite mounting evidence that the magnitude of these associations is comparable to that of many leading health determinants (that receive significant public health resources), government agencies, health care providers and associations, and public or private health care funders have been slow to recognize human social relationships as either a health determinant or health risk marker in a manner that is comparable to that of other public health priorities. This article evaluates current evidence (on social relationships and health) according to criteria commonly used in determining public health priorities. The article discusses challenges for reducing risk in this area and outlines an agenda for integrating social relationships into current public health priorities. (PsycINFO Database Record

Predicting loneliness with polygenic scores of social, psychological and psychiatric traits

Loneliness is a heritable trait that accompanies multiple disorders. The association between loneliness and mental health indices may partly be due to inherited biological factors. We constructed polygenic scores for 27 traits related to behavior, cognition and mental health and tested their prediction for self-reported loneliness in a population-based sample of 8798 Dutch individuals. Polygenic scores for major depressive disorder (MDD), schizophrenia and bipolar disorder were significantly associated with loneliness. Of the Big Five personality dimensions, polygenic scores for neuroticism and conscientiousness also significantly predicted loneliness, as did the polygenic scores for subjective well-being, tiredness and self-rated health. When including all polygenic scores simultaneously into one model, only 2 major depression polygenic scores remained as significant predictors of loneliness. When controlling only for these 2 MDD polygenic scores, only neuroticism and schizophrenia remain significant. The total variation explained by all polygenic scores collectively was 1.7%. The association between the propensity to feel lonely and the susceptibility to psychiatric disorders thus pointed to a shared genetic etiology. The predictive power of polygenic scores will increase as the power of the genome-wide association studies on which they are based increases and may lead to clinically useful polygenic scores that can inform on the genetic predisposition to loneliness and mental health.

BDNF Val66Met polymorphism modulates the effect of loneliness on white matter microstructure in young adults

Loneliness is a common experience. Susceptibility to loneliness is a stable trait and is heritable. Previous studies have suggested that loneliness may impact regional gray matter density and brain activation to social stimuli, but its relation to white matter structure and how it may interact with genetic factors remains unclear. In this study, we investigated whether and how a common polymorphism (Val66Met) in the brain-derived neurotrophic factor gene modulated the association between loneliness and white matter microstructure in 162 young adults. The tract-based spatial statistics analyses revealed that the relationships between loneliness and white matter microstructures were significantly different between Val/Met heterozygotes and Val/Val homozygotes. Specifically, loneliness was significantly correlated with reduced fractional anisotropy and increased radial diffusivity in widespread white matter fibers within Val/Met heterozygotes. It was also significantly correlated with increased radial diffusivity in Met/Met genotypes but showed no significant association with white matter measures in Val/Val genotypes. Furthermore, the associations between loneliness and fractional anisotropy (or radial diffusivity) in Val/Met heterozygotes turned out to be global effects. These results provide evidence that loneliness may interact with the BDNF Val66Met polymorphism to shape the microstructures of white matter, and the Val/Met heterozygotes may be more susceptible to social environment.

Altered brain activity and the effect of personality traits in excessive smartphone use during facial emotion processing

Excessive smartphone use is a phenomenon related to maladaptive smartphone use, leading to negative consequences. This study set out with the aim of assessing the effects of excessive smartphone use on behavioral and neural responses during facial emotional processing. We examined 25 excessive smartphone users and 27 normal control users using functional MRI during facial emotion processing and investigated Behavioral Inhibition System/Behavioral Activation System (BIS/BAS). The excessive smartphone use group (SP) showed neural deactivation in the dorsolateral prefrontal cortex (DLPFC) and dorsal anterior cingulate cortex (dACC) during the presentation of an angry face and emotional transition compared to that of the normal control group (NC). Additionally, the SP revealed neural deactivation of the superior temporal sulcus and temporo-parietal junction related to social interaction during emotional transition compared to the NC. We found that BAS-Reward Responsiveness level was correlated with behavioral responses during repeated happy faces related to emotional reward in SP compared to NC. It can thus be suggested that excessive smartphone use is likely to fail on cognitive control during emotional processing, and this impairment might be influenced on emotional processing related to social interaction.

Sleeping with one eye open: loneliness and sleep quality in young adults

BACKGROUND

Feelings of loneliness are common among young adults, and are hypothesized to impair the quality of sleep. In the present study, we tested associations between loneliness and sleep quality in a nationally representative sample of young adults. Further, based on the hypothesis that sleep problems in lonely individuals are driven by increased vigilance for threat, we tested whether past exposure to violence exacerbated this association.

METHOD

Data were drawn from the Environmental Risk (E-Risk) Longitudinal Twin Study, a birth cohort of 2232 twins born in England and Wales in 1994 and 1995. We measured loneliness using items from the UCLA Loneliness Scale, and sleep quality using the Pittsburgh Sleep Quality Index. We controlled for covariates including social isolation, psychopathology, employment status and being a parent of an infant. We examined twin differences to control for unmeasured genetic and family environment factors.

RESULTS

Feelings of loneliness were associated with worse overall sleep quality. Loneliness was associated specifically with subjective sleep quality and daytime dysfunction. These associations were robust to controls for covariates. Among monozygotic twins, within-twin pair differences in loneliness were significantly associated with within-pair differences in sleep quality, indicating an association independent of unmeasured familial influences. The association between loneliness and sleep quality was exacerbated among individuals exposed to violence victimization in adolescence or maltreatment in childhood.

CONCLUSIONS

Loneliness is robustly associated with poorer sleep quality in young people, underscoring the importance of early interventions to mitigate the long-term outcomes of loneliness. Special care should be directed towards individuals who have experienced victimization.

Reciprocal Influences Between Loneliness and Self-Centeredness: A Cross-Lagged Panel Analysis in a Population-Based Sample of African American, Hispanic, and Caucasian Adults

Loneliness has been posited to increase the motivation to repair or replace deficient social relationships and, seemingly paradoxically, to increase the implicit motivation for self-preservation. In the current research, we report a cross-lagged panel analysis of 10 waves of longitudinal data ( N = 229) on loneliness and self-centeredness (as gauged by Feeney and Collins's measure of chronic self-focus) in a representative sample of middle-aged and older adults. As predicted by the proposition that loneliness increases the implicit motivation for self-preservation, loneliness in the current year predicts self-centeredness in the subsequent year beyond what is explained by current-year demographic variables, self-centeredness, depressive symptomatology, and overall negative mood. Analyses also show that self-centeredness in the current year (net covariates) predicts loneliness in the subsequent year, a reciprocal relationship that could potentially contribute to the maintenance of loneliness. Theoretical and practical implications are discussed.

Invited Commentary: Evolution of Social Networks, Health, and the Role of Epidemiology

Almost 40 years ago, Berkman and Syme demonstrated that social networks were related to the risk of early mortality (Am J Epidemiol. 1979;109(2):186-204). Their study was highly innovative because they directly measured and quantified social networks in a large prospective population-based survey with mortality follow-up. The results of the study showed robust network gradients, whereby those with fewer networks and weaker social ties had significantly higher mortality rates. The important influence of social networks that Berkman and Syme noted many years ago is likely to heighten in the future, as demographic characteristics shift and individuals become more inclined to socialize through online platforms instead of real-world interactions. Berkman and Syme's research in 1979 continues to play a key role in shaping recent efforts to uncover the influence of social networks on health. Looking back on their findings may help epidemiologists better understand the importance of both online and offline networks for population health today.

Genome-Wide Association Study of Loneliness Demonstrates a Role for Common Variation

Loneliness is a complex biological trait that has been associated with numerous negative health outcomes. The measurement and environmental determinants of loneliness are well understood, but its genetic basis is not. Previous studies have estimated the heritability of loneliness between 37 and 55% using twins and family-based approaches, and have explored the role of specific candidate genes. We used genotypic and phenotypic data from 10 760 individuals aged ⩾50 years that were collected by the Health and Retirement Study (HRS) to perform the first genome-wide association study of loneliness. No associations reached genome-wide significance (p>5 × 10^-8). Furthermore, none of the previously published associations between variants within candidate genes (BDNF, OXTR, RORA, GRM8, CHRNA4, IL-1A, CRHR1, MTHFR, DRD2, APOE) and loneliness were replicated (p>0.05), despite our much larger sample size. We estimated the chip heritability of loneliness and examined coheritability between loneliness and several personality and psychiatric traits. Our estimates of chip heritability (14-27%) support a role for common genetic variation. We identified strong genetic correlations between loneliness, neuroticism, and a scale of 'depressive symptoms.' We also identified weaker evidence for coheritability with extraversion, schizophrenia, bipolar disorder, and major depressive disorder. We conclude that loneliness, as defined in this study, is a modestly heritable trait that has a highly polygenic genetic architecture. The coheritability between loneliness and neuroticism may reflect the role of negative affectivity that is common to both traits. Our results also reflect the value of studies that probe the common genetic basis of salutary social bonds and clinically defined psychiatric disorders.

A developmental social neuroscience model for understanding loneliness in adolescence

Loneliness is prevalent in adolescents. Although it can be a normative experience, children and adolescents who experience loneliness are often at risk for anxiety, depression, and suicide. Research efforts have been made to identify the neurobiological basis of such distressful feelings in our social brain. In adolescents, the social brain is still undergoing significant development, which may contribute to their increased and differential sensitivity to the social environment. Many behavioral studies have shown the significance of attachment security and social skills in adolescents' interactions with the social world. In this review, we propose a developmental social neuroscience model that extends from the social neuroscience model of loneliness. In particular, we argue that the social brain and social skills are both important for the development of adolescents' perceived loneliness and that adolescents' familial attachment sets the baseline for neurobiological development. By reviewing the related behavioral and neuroimaging literature, we propose a developmental social neuroscience model to explain the heightened perception of loneliness in adolescents using social skills and attachment style as neurobiological moderators. We encourage future researchers to investigate adolescents' perceived social connectedness from the developmental neuroscience perspective.

Social isolation, loneliness and depression in young adulthood: a behavioural genetic analysis

PURPOSE

To investigate the association between social isolation and loneliness, how they relate to depression, and whether these associations are explained by genetic influences.

METHODS

We used data from the age-18 wave of the Environmental Risk Longitudinal Twin Study, a birth cohort of 1116 same-sex twin pairs born in England and Wales in 1994 and 1995. Participants reported on their levels of social isolation, loneliness and depressive symptoms. We conducted regression analyses to test the differential associations of isolation and loneliness with depression. Using the twin study design, we estimated the proportion of variance in each construct and their covariance that was accounted for by genetic and environmental factors.

RESULTS

Social isolation and loneliness were moderately correlated (r = 0.39), reflecting the separateness of these constructs, and both were associated with depression. When entered simultaneously in a regression analysis, loneliness was more robustly associated with depression. We observed similar degrees of genetic influence on social isolation (40 %) and loneliness (38 %), and a smaller genetic influence on depressive symptoms (29 %), with the remaining variance accounted for by the non-shared environment. Genetic correlations of 0.65 between isolation and loneliness and 0.63 between loneliness and depression indicated a strong role of genetic influences in the co-occurrence of these phenotypes.

CONCLUSIONS

Socially isolated young adults do not necessarily experience loneliness. However, those who are lonely are often depressed, partly because the same genes influence loneliness and depression. Interventions should not only aim at increasing social connections but also focus on subjective feelings of loneliness.

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.

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.