Loneliness in Young Adulthood: a Protocol for a Scoping Review of the Quantitative and Qualitative Literature.

Loneliness refers to the distressing feeling that accompanies the experience of perceiving the quantity or quality of one’s social relationships as inadequate (1). There is increasing recognition of the prevalence of loneliness in young adults. Despite this, there is no existing scoping review on loneliness in young adulthood. Young adults (18-25 years) are in a critical life stage involving diverse social, demographic, biological and cognitive transitions which may affect the development of loneliness. Evidence that loneliness is a risk factor for poorer mental and physical health further emphasises the need to understand the experience in this age group. Therefore, the aim of our scoping review is to provide a summary of the quantitative and qualitative literature on loneliness in young adulthood. and and dissertations, and book will be excluded. Articles will following Results will in in


Abstract
Background Loneliness refers to the distressing feeling that accompanies the experience of perceiving the quantity or quality of one's social relationships as inadequate (1). There is increasing recognition of the prevalence of loneliness in young adults. Despite this, there is no existing scoping review on loneliness in young adulthood. Young adults (18-25 years) are in a critical life stage involving diverse social, demographic, biological and cognitive transitions which may affect the development of loneliness. Evidence that loneliness is a risk factor for poorer mental and physical health further emphasises the need to understand the experience in this age group. Therefore, the aim of our scoping review is to provide a summary of the quantitative and qualitative literature on loneliness in young adulthood.

Methods
The proposed scoping review will follow the Joanna Briggs Institute methodology and Arksey and O'Malley's (2005) framework for scoping reviews. Peer-reviewed journal articles and grey literature in the form of reports or di cult-to-locate studies will be identi ed by: (a) electronic database searching, (b) contacting national and international researchers in the eld, and (c) by posting general requests for relevant information on Twitter. We will include all study designs published in English from 2000 to 2021 where loneliness (de ned as subjective) is a key focus of the work and the mean age of participants is ≥ 18 and ≤ 25 years. Editorials, commentaries, opinion pieces, dissertations, and book chapters will be excluded. Articles will be selected for inclusion following screening of titles/abstracts succeeded by fulltext screening. Results will be presented in a narrative form to provide a descriptive summary of the literature on loneliness in young adults.

Discussion
The results of this scoping review will provide an up-to-date overview of available research related to loneliness in young adults and will inform our future research in the area. Results will be shared through a peer-reviewed journal publication and conference presentations.

Registration
This protocol is registered with the Open Science Framework (OSF) available at: https://osf.io/jfcmp.

Background
Loneliness has been de ned as the distressing feeling that accompanies the experience of perceiving one's social relationships as inadequate, either quantitatively or qualitatively (1). Although loneliness is often viewed as a problem of older adulthood, the need to belong and maintain social connections with others is observed across the lifespan and all age groups are susceptible to loneliness when these needs are not satis ed (2, 3). In fact, loneliness is particularly prevalent in young adults; about 40% of young adults (16-24 years) report feeling lonely often or very often (4). Despite its prevalence, loneliness has only recently been recognised as an important issue in young adult groups and to our knowledge, there is no existing scoping review on the topic.
Young adults are of particular interest in loneliness research considering that this life stage which marks the transition from adolescence to adulthood is characterised by important developmental milestones, cognitive and physical maturation, identity exploration, and increased autonomy which may place this age group at increased risk for loneliness (5)(6)(7). While short-term, or transient, loneliness is unlikely to have long-lasting consequences and might even function to motivate connection with others (5,8,9), almost one in ten (9%) young adults will experience persistent loneliness (10). Individuals who suffer from persistent, or chronic, loneliness tend to experience more long-term psychosocial problems and negative health outcomes, relative to those with short term loneliness (9).
The prevalence of loneliness in young adults is concerning considering that the experience is a wellestablished risk factor for poorer physical and mental health (8,11). Lonely young adults are more likely to experience depressive symptoms, to have poorer sleep quality, poorer self-rated general health, and altered cortisol activity (12)(13)(14)(15). Although the long-term health consequences of loneliness may not become apparent until later life, the negative experience and behaviours associated with loneliness may increase an individual's risk early in life. Young adults who experience loneliness are more likely to engage in health risk behaviours, to use negative stress coping strategies, and to have increased risk of disability and lower income in midlife (16,17).
Recognising that loneliness may have negative implications for young adults' health emphasises the need to consider the factors associated with loneliness in this group. Young adults have a similar, if not higher, prevalence of loneliness compared to older adults (10,18); however, research has tended to focus on the latter. Although loneliness is a universal experience at the individual level, the risk factors associated with loneliness and the in uence of these factors can vary across the lifespan. For example, the association between loneliness and less contact with friends is strongest for young adults (19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34) years) compared to early (35-49 years) and late (50-65 years) middle-aged adults (19). Therefore, there is a need for a greater understanding of the speci c risk factors for loneliness in young adulthood and the target groups for intervention, beyond those already identi ed (20).
Given its complex nature, it is unsurprising that research on loneliness spans many disciplines using different theoretical perspectives; some approaches are speci c to loneliness, but broader theories have also been applied in this area. A leading approach is the cognitive discrepancy model (2), which emphasises the role of individual traits and cognitive processes in the subjective experience of loneliness.
In contrast, the social needs approach (21) views loneliness as a multidimensional construct capturing an unmet need for intimate attachment (emotional loneliness) or meaningful friendships (social loneliness). Loneliness has attracted interest from sociologists, who have described loneliness as a product of wider society (22). Other approaches relevant in loneliness research relate to social connectedness, such as Bowlby's (23) attachment theory which highlights the in uence of early attachments on future psychological well-being. Finally, broader developmental approaches, such as Bronfenbrenner's (24) model of ecological systems, have demonstrated usefulness in loneliness research (25). Theoretical approaches applied in loneliness research are not mutually exclusive and may overlap. Therefore, the research on loneliness in young adults is likely to be somewhat disparate; yet no review of loneliness in young adults has speci cally considered the theoretical approaches employed in this area of research.
De nition of loneliness and young adults Throughout this review, we will use the term loneliness to refer to the distressing feeling that accompanies the experience of perceiving one's social relationships as inadequate, either quantitatively or qualitatively (1). Across age groups, loneliness is subjective and is only weakly associated with objective measures of contact with friends and family (18,26). Loneliness is distinct from social isolation, solitude, and living alone, such that these situations may not be unpleasant; adolescents' perception of loneliness in comparison to aloneness suggests that loneliness is seen as aversive and related to negative affect like sadness, whereas aloneness is not perceived negatively (27).
There is no de ned threshold for transition from adolescence to adulthood. Due to social and economic changes in contemporary societies, the process of achieving adulthood may continue for several years until the mid-to-late twenties (28). Many of the traditional markers of adulthood, such as the age of rst marriage, are now being achieved later in life than in previous years (29). This prolonged development from adolescence to adulthood has been termed "extended adolescence" by some developmental researchers (30). Others have described young, or "emerging", adulthood as a distinct period of the life course from age 18 to 25 years (6). Emerging adulthood is characterised by instability, possibilities, identity exploration, self-focus, and feeling in-between (7), and represents a separate life stage marking the transition from adolescence to adulthood involving a unique constellation of demographic, social, and psychological correlates (31), which may have bearing on the development of loneliness. Because the changes associated with the transition from adolescence to adulthood may be particularly relevant to the experience of loneliness, our review aims to focus on young or "emerging" adulthood.

Review aims and objectives
Despite increasing research interest in loneliness in young adults (16,32,33), our preliminary search of the literature (PsycInfo and Scopus on 9 th February 2021) revealed no existing scoping reviews on the topic. Given the broad nature of this topic, a scoping review, rather than a systematic review, will summarise the existing literature on loneliness in young adulthood. Scoping reviews are particularly useful for disparate literature or topics with emerging evidence (34). Like systematic reviews, scoping reviews use a methodologically rigorous process to nd and synthesise literature (35). Rather than follow a highly focused research question, scoping reviews aim to identify all relevant literature in an area (36). Therefore, conducting a scoping review is an iterative process which enables us to make (and document) necessary changes to the search strategy to identify relevant studies. We hope to provide a foundation to inform subsequent loneliness research in young adults ensuring that studies are based on the most up-todate knowledge in the area.
The aim of our scoping review is to provide an up-to-date summary of the qualitative and quantitative literature on loneliness in young or "emerging" adulthood. Our main objectives are (a) to identify what theoretical approaches are used in research on loneliness in young adulthood, (b) to summarise the evidence examining the correlates of and risk factors for loneliness in young adulthood, (c) to summarise how loneliness is conceptualised and measured in research in young adults, (d) to summarise the evidence on sex-gender differences in loneliness in young adults, (e) to identify and summarise the young adult groups that have been included in previous loneliness literature; in doing so, we may identify groups of young adults that are currently underrepresented in this area of research.

Study design
We will conduct a scoping review to provide a descriptive summary of the available research on loneliness in young adults. Our review will aggregate and articulate the ndings without formal analysis of the quality of the available studies. The following electronic databases will be searched: Scopus, PubMed, PsycArticles, PsycInfo, Medline, ScienceDirect, and Applied Social Sciences Index and Abstracts (ASSIA). The search will be limited to papers published from 2000 to 2021. This year limit is proposed for two reasons. First, Arnett's (6) seminal paper on emerging adulthood was published in the year 2000; considering that this is a highly cited paper, we expect to nd more peer-reviewed research on our target population after this year. Second, this year limit is in line with our aim to provide an up-to-date summary of the research on loneliness in young adults.
The search strategy includes combinations of keywords and terms related to our population group of interest (young adults) and loneliness ( Table 1). The search strategy will be adapted for each database listed above (example attached in Additional File 1). Following JBI guidance on scoping review methodology, a preliminary search of two databases (PsycInfo and Medline) will be carried out. The search terms may be modi ed to ensure that the search strategy is comprehensive. The term 'grey literature' is commonly understood to refer to electronic and print sources (e.g., reports, working papers, dissertations) that are not controlled by commercial publishing organisations (38). Prior to devising a search strategy for grey literature, we considered the bene ts and challenges of its inclusion in scoping reviews.
Overall, including grey literature provides the opportunity to produce a more comprehensive and timeliness picture of available evidence in an area. The emphasis on knowledge transfer in areas related to mental health, such as loneliness research, means that the publication of research in a scienti c journal may not be the best way to ensure that ndings are communicated to those most interested in the topic (e.g., youth mental health organisations, policy makers). It is likely that information relevant to loneliness in young adulthood will be found in reports by organisations or government bodies interested in youth mental health (e.g., Jigsaw, SpunOut.ie, National Youth Council of Ireland) and these sources may not be retrieved in database searching.
Of course, there are limitations to grey literature searching. First, the time consuming and heterogeneous nature of grey literature must be considered (39). Tricco (40) recounted the challenge in screening approximately 5000 titles and abstracts to include grey literature in a scoping review. Given that abstracts are not available for many grey literature documents, the full text of individual documents may need to be reviewed in order to determine eligibility. Second, there is no gold-standard method to systematically search for grey literature. Grey literature search strategies are better described as systematic rather than replicable; even if our search is replicable, other researchers may not retrieve the same results on replication. Previous review papers lack adequate descriptions of the search for and synthesis of results from grey literature sources (39). As such, we aim to be explicit in our methodology by outlining our considerations and including a level of detail in the nal scoping review paper that maximises the transparency of our grey literature search.
A key aspect of our grey literature search is maximising search sensitivity while retrieving search results that are feasible for screening. In line with this, we considered the use of search engines such as Google to source grey literature. Although search engines are easily accessible, the low speci city and sensitivity of this method, as well as the likelihood of nding documents already found elsewhere, reduces its effectiveness. Google results are in uenced by geographical location, previous search history, and popularity, meaning that the replicability and consistency of the search results may be compromised (41).
Having considered the importance of including grey literature, the advantages and disadvantages of different strategies, and the current variability in approaches to grey literature, we have devised a complementary search strategy which speci cally relates to sourcing grey literature documents in the form of reports or di cult to locate studies relating to loneliness in young adults. We have chosen two methods to source grey literature in our scoping review: (1) by contacting national and international researchers in the eld via I-LINK (International Loneliness and Isolation NetworK), and (2) by posting general requests for relevant information on Twitter and by mentioning relevant organisations ('@organisation') in such tweets, as described in Adams et al. (38).
We will identify key experts in the area via I-LINK; these researchers will be contacted and asked to nominate documents for inclusion in the review. By contacting other researchers in the area we may also identify studies in progress or recently published studies that are not retrieved in database searches. We will also post general requests targeting national and international organisations on the social media site Twitter and ask that other users repost these tweets, so as to increase the potential viewers. A previous review found this to be a more e cient process than email or professional press requests (38). We will receive and consider documents for inclusion in the review up until the point of submission to a journal. The reference lists of previous reviews and articles eligible for inclusion (from either the core search or the complementary grey literature search) will be examined to locate additional relevant studies.

Identifying relevant studies
Conducting a scoping review is an iterative process (34,37), meaning that some changes to the inclusion and exclusion criteria may be necessary following trial study selection. Any deviations from this protocol will be clearly detailed in the 'Methods' section of the nal scoping review paper.
Inclusion criteria a. Research where loneliness, de ned as 'subjective' or 'perceived social isolation', is a key focus of the work (determined by the inclusion of a relevant aim, objective, or research question related to understanding loneliness) or if the study measures and reports ndings related to loneliness under a broader concept, such as 'psychological well-being', 'mental health', or similar.
c. Mean age of participants is ≥ 18 and ≤ 25 years. Emerging adults share some characteristics with the life stages of adolescence and adulthood; samples that overlap our operationalization of emerging adulthood (e., ≥ 18 and ≤ 25 years) with a wider age range, but that report a mean age ≥ 18 and ≤ 25 years will be included.
d. Studies which focus on loneliness in clinical or speci c subpopulations, such as young adults with Type 1 diabetes (42), will be eligible for inclusion but may be summarised separately to studies of general or community samples in the results section of the review.
e. Research articles are not limited by setting or geographical location.
f. Articles published in English (that is the only language of the researchers).
g. Publications in the form of peer-reviewed articles or grey literature to include reports from relevant NGOs (e.g., SpunOut.ie, Jigsaw), loneliness organisations (e.g., Campaign to End Loneliness) and government bodies (e.g., National Youth Council of Ireland) or di cult to locate studies (i.e., empirical research which has only recently or not yet been published in a journal).
Exclusion criteria a. Articles not related to loneliness (such as those only focused on social isolation or living alone) or articles where loneliness is not considered a key aspect (e.g., loneliness is only included as a control variable).
b. Articles where the mean age of participants is < 18 or > 25 years, where the age of the population is not clearly described, or studies with a wider age range where data for emerging adults (i.e., ≥ 18 and ≤ 25 years) cannot be extrapolated.
d. Articles published in a language other than English.

Study selection
Articles retrieved from the database searches will be added to EndNoteX9 (Clarivate Analytics) to manage the references. The rst author (EK) will conduct database searching and removal of duplicates. Articles will be screened in two phases. First, the titles and abstracts of all articles retrieved from the database search, as well as reports and empirical studies located from additional sources, will be screened for eligibility by EK. Fifty percent of the titles and abstracts will be independently screened by a second reviewer (SS). Studies and reports that do not meet our initial inclusion criteria will be excluded. Second, the full-text of the remaining articles that appear to ful l the inclusion criteria will be obtained and examined for eligibility by EK. Fifty percent of the full-texts will be independently reviewed by SS. Studies that do not meet the inclusion criteria at this stage will be excluded and the reason for exclusion noted. Any disagreement between reviewers in decisions regarding the eligibility of studies will be resolved by a third reviewer. Rayyan QCRI software will be used to manage study selection. The study selection process, including reasons for exclusion, will be presented in a PRISMA ow diagram in the nal paper.

Data charting
Data charting will be carried out for all papers by one reviewer (EK). A proportion of the eligible papers (minimum of 10%) will also be assessed by SS for accuracy. The data charting form will be pre-piloted on a random selection of articles to ensure all relevant information is extracted. Uncertainty regarding the charting of results will be resolved through discussion, including a third author if necessary. The charting table will include the following details: Bibliographic information.
e. Assessment or measurement of loneliness: which psychometric measures of loneliness were used?
Were indirect or direct measures used, or both? Were psychometric measures adapted (i.e., was the wording of the psychometric measure adapted, was the assessment adapted to make it suitable for an online survey? j. Main ndings and conclusions.

Summarising of results
The reporting of results will be guided by the Preferred Reporting Items for Systematic reviews and Meta-Analysis extension for Scoping Reviews (PRISMA-ScR) (43). A completed PRISMA-P checklist is included as an additional le to this protocol (Additional File 2). The aim of our scoping review is to provide a descriptive summary of the available evidence; the quality of the studies included will not be assessed. All ndings will be included in a narrative review. Studies which report on clinical or speci c subpopulations, such as individuals with in ammatory bowel disease (44) or young adults with Type 1 diabetes (42), may be summarised separately to studies which focus on community samples. The reporting of our results will be guided by our research questions. The theoretical approaches used in research on loneliness in young adults (RQ1), correlates of and risk factors for loneliness (RQ2), and the means of conceptualising and measuring loneliness (RQ3) will be recorded in tables with accompanying narrative summaries. We will identify commonalities across studies and collate the extracted data. Observed sex differences (RQ4) and the young adult groups previously researched (RQ5) will be reported using a narrative summary. Included grey literature reports will also be summarised in the narrative review. The ndings from both kinds of literature will be combined to address our primary aim in providing a summary of the quantitative and qualitative literature on loneliness in young adults.

Discussion
This review aims to be the rst scoping review to provide a descriptive summary of the available literature on loneliness in young, or "emerging", adults. Speci cally, we aim to summarise: (a) the theoretical approaches used in this area, (b) the risk factors associated with loneliness in this group, (c) the conceptualisation and means of measuring loneliness in young adults, (d) the observed sex differences, if any, in loneliness in young adults, and (e) the young adult groups that have been included in previous loneliness research.
Following the completion of our scoping review, results will be shared through a peer-reviewed publication and conference presentations. This review forms part of a larger project which aims to identify the risk factors for loneliness in young adulthood. The results of this scoping review will provide an up-to-date overview of available research related to loneliness in young adults and will inform our future research in the area.

Limitations
The potential limitations to our scoping review include that older studies (published before the year 2000) and documents that are not available in electronic databases or online may be missed. However, we anticipate that the majority of relevant literature will be captured by our search strategies, including grey literature.

Declarations
Ethics approval and consent to participate Not applicable.

Consent for publication
Not applicable.
Availability of data and materials Not applicable.