Loneliness is common in adolescents and young adults, involving perceived (as opposed to objective) isolation from others (Rönkä et al., 2014; Qualter et al., 2015). Higher levels of loneliness are associated with a reduction in well-being and an increased risk for various forms of psychopathology, though much of the prior research has focused on the link with depression (Lasgaard et al., 2011; Vanhalst et al., 2012; Shevlin et al., 2014). Recently there has been a growing interest in the relationship between loneliness and psychosis (Stain et al., 2012; Sundermann et al., 2014) with 75–94% of people with psychotic disorders reporting feeling lonely some or more of the time (Badcock et al., 2015). However, the specific relationship between loneliness and psychosis proneness remains unclear (Lim and Gleeson, 2014).

From a clinical perspective, high trait schizotypy is clearly associated with an increased risk of schizophrenia-spectrum psychopathology (Barrantes-Vidal et al., 2013; Cicero et al., 2014). The term “schizotypy” (a contraction of schizophrenic phenotype) describes a cluster of personality traits that convey vulnerability to psychosis, including unusual perceptual experiences, magical thinking, and paranoid ideas (positive schizotypy) and constricted affect, anhedonia, and social anxiety (negative schizotypy). These characteristics broadly correspond to the positive (hallucinations, delusions) and negative (anhedonia, flattened affect) symptom dimensions of schizophrenia. The Wisconsin Schizotypy Scales (WSS) are widely used to assess schizotypal traits and confirmatory factor analysis indicates that a two-factor structure, composed of positive and negative schizotypy, underpins these scales (Gross et al., 2014, 2015). Furthermore, high schizotypy scores are correlated with psychotic-like, prodromal and schizophrenia-spectrum symptoms, as well as increased risk for developing psychosis in the future (e.g., Barrantes-Vidal et al., 2013; Cicero et al., 2014). However, the boundaries between loneliness and schizotypal traits may be difficult to discern (Nelson et al., 2013; Cohen et al., 2015). For example, it is unclear if loneliness is a consequence, or a core component, of positive schizotypy (e.g., unusual perceptual experiences, persecutory beliefs), given that paranoia and suspiciousness result in social distancing and/or withdrawal (Cohen et al., 2015). Alternatively, the conceptual overlap may be most evident on measures of negative schizotypy (e.g., social anhedonia, lack of pleasure), which often include items about feeling lonely and socially disconnected (e.g., WSS item 7: “I dont really feel very close to my friends”; Chapman et al., 1976; Gross et al., 2012). Therefore, loneliness and schizotypy may easily be conflated and treated as the same construct, i.e., as evidence of generalized psychopathology, reflecting a common liability to mental disorder (Caspi et al., 2014). However, if loneliness and schizotypy are similar but separable experiences, then the clinical consequences of assuming they are equivalent could be significant: for example, leading loneliness to be missed or mistaken for social anhedonia or a facet of paranoid thinking, rather than an independent problem with separate functional effects and treatment needs in people vulnerable to psychosis.

From a theoretical and empirical point of view, loneliness is generally recognized as being multi-dimensional in nature (Hawkley et al., 2005; Durak and Senol-Durak, 2010; Shevlin et al., 2015). These findings typically indicate that our mental map of the social world involves individual, interpersonal and collective aspects, each with different causes and consequences for social functioning (Hawkley et al., 2012). For example, Shevlin et al. (2015) investigated the multidimensional structure of the widely-used UCLA Loneliness Scale, version 3 (UCLA-3; Russell, 1996) using confirmatory factor analysis, and found that the best fitting model comprised three correlated factors. The first factor, termed Isolation, included items tapping feelings of being alone and lacking companionship. The second, Relational Connectedness, factor reflected more social aspects of loneliness, such as feeling there are other people one can turn to or talk to. Finally, the third factor, termed Collective Connectedness, consisted of items such as feeling in tune with others and part of a group of friends, and appears to relate to the sense of connection to a group or community. Importantly, poor social functioning has been associated with both negative and positive schizotypy, adversely impacting on quality of life (Cohen and Davis, 2009; Abbott et al., 2012). However, there have been no previous studies examining whether these individual, interpersonal, and collective factors of loneliness are differentially related to positive and negative schizotypy traits, which would bolster the utility of a multi-dimensional loneliness model.

Motivated by these clinical and theoretical issues, the goal of the current study was to determine if loneliness and schizotypal traits are distinct and separable constructs, or whether they are all aspects of a single, common dimension: general psychopathology (the “p” factor; Caspi et al., 2014).

The average for the UCLA-3 total score was 42.62 (SD = 10.69, range = 20–71). The mean PAb score was 1.02 (SD = 2.22, range = 0–14) and for the SA score was 2.63 (SD = 2.88, range = 0–15).

The aim of the study reported here was to investigate the relationship between subjective feelings of loneliness and positive and negative trait schizotypy. The mean loneliness score was higher in the current sample of predominantly young adults compared to that previously reported in adolescents, aged 16–18 years, however, the spread of scores was similar (Lasgaard, 2007; Shevlin et al., 2014). Mean scores and score distributions for the PAb and SA scales were comparable to those previously reported in college students (Winterstein et al., 2011; Gross et al., 2012). The first phase of our analysis supported the multidimensional nature of loneliness assessed with the UCLA-3 Loneliness scale, with two and three-factor models providing acceptable fit to the data (Wilson et al., 1992; Hawkley et al., 2005). Increasing evidence demonstrates that the latter provides the best fit from adolescence to adulthood and generalizes across gender and culture (Hawkley et al., 2005, 2012; Shevlin et al., 2015), and the present findings suggest that this model—consisting of correlated facets of Isolated, Relational and Collective loneliness—extends to a predominantly young adult Australian sample. Consequently, we utilized the three-factor model in the remainder of our analyses.

The second phase of our analyses confirmed that a two-factor structure, of positive and negative schizotypy, provided a good fit to the current data, consistent with previous evidence (Gross et al., 2015). The final phases of our analyses suggest that loneliness and psychosis proneness, or schizotypy, are distinct and separable constructs, when measured in terms of positive (Perceptual Aberration) and negative (Social Anhedonia) schizotypy. Finally, our confirmatory bi-factor analysis provided strong support for a latent general factor of psychopathology, with high loadings on all (but one) of the individual items assessing loneliness and schizotypal traits. This finding is consistent with previous proposals that the structure of mental disorders is best characterized by a model which includes a common or general psychopathology factor “p,” in addition to more specific factors or dimensions (Caspi et al., 2014; Laceulle et al., 2015). Of particular importance here, when taking into account the existence of this latent general factor, the (three) loneliness and (two) schizotypy sub-factors remained, supporting the hypothesis that the three facets of loneliness and two facets of positive and negative schizotypal traits are separate constructs that explain a significant proportion of variance not accounted for by general psychopathology. Furthermore, the significant, positive correlations initially observed between isolation, related and collective connectedness and positive and negative schizotypy became non-significant once the shared variance with p was removed. Thus, while loneliness and schizotypy commonly co-occur and have previously been considered as reflecting the same construct (Gruzelier, 1996; Gross et al., 2012), the current findings suggest that they are distinct manifestations of psychological functioning. These results may be particularly important in view of recent evidence that schizotypal items assessing a lack of close interpersonal relations significantly predict transition to psychosis (Salokangas et al., 2013). The present study suggests that a sense of disconnection from others, i.e., loneliness, may in fact be a separate feature from trait schizotypy, with a different mechanistic influence on psychosis proneness. For example, Murphy and colleagues have argued that loneliness is a significant contextual vulnerability factor that can strengthen the relationship between negative childhood experiences from others and emergence of psychotic symptoms (Murphy et al., 2015).

Although loneliness is conceptually independent from schizotypy, the present findings suggest that there is overlap between the two constructs as part of a general psychopathology domain. This is not surprising, as previous research has shown that loneliness is associated with a range of personality “health” factors (i.e., emotional stability, surgency, agreeableness, conscientiousness, shyness, and sociability (Cacioppo et al., 2006), as well as depression, anxiety, and other psychopathological factors (Leary, 1990; Cacioppo and Cacioppo, 2014). Importantly, general psychopathology is an important construct for understanding schizophrenia-spectrum pathology and schizotypy, as negative affect/neuroticism, depression, and anxiety, are common in schizotypy (Horan et al., 2008; Hartley et al., 2013), and may serve as non-specific vulnerability markers or “state” markers of risk; potentially signaling onset of psychosis, relapse or decompensation more generally. Importantly, loneliness is commonly—but not universally—reported in people with, or vulnerable to, psychosis and is hypothesized to increase the risk of a range of mental illnesses, especially in adolescence (Lim and Gleeson, 2014; Shevlin et al., 2014; Badcock et al., 2015). Thus, feeling lonely may need to be understood both as a separate problem from schizotypy and as a marker of general psychopathology, rather than as an inherent feature of high schizotypy, requiring routine assessment and targeted intervention.

One limitation of the current study is that it focussed on investigating links between loneliness and perceptual aberration and social anhedonia, whilst schizotypy encompasses a broader range of traits. Factor analysis of schizotypy measures typically reveals 2–4 separate schizotypy factors (Badcock and Dragović, 2006; Fonseca-Pedrero et al., 2013; Gross et al., 2014). It is possible, therefore, that loneliness is specifically related to other facets of positive and negative schizotypy not assessed here, or to odd speech and behavior, associated with disorganized schizotypy. A second limitation is that the study samples were comprised of high functioning young adults enrolled at university, and therefore may not be representative of the general population. Consequently, replicating these results in a more diverse community sample would be useful. Similarly, the gender ratio in the current sample was uneven, the number of male participants was relatively small and separate analysis by sex was not undertaken. However, in future research it may be interesting to investigate whether the structure of the relationship between loneliness and schizotypy is consistent across sex. Finally, the measures employed in this study were exclusively self-report in nature and the methods of assessing random/careless responding differed at each study site, with unknown influence on the results. Although loneliness is an inherently subjective phenomenon, understanding its behavioral and neurobiological concomitants and how they relate to interviewer-based, or endophenotype-based measures of schizotypy is an important direction for future research.

JB, EB, AC, and DB developed the idea for the study. RB conducted the confirmatory and bi-factor analyses and provided overall statistical advice. JB wrote the first draft of the manuscript. All authors helped with revisions to the manuscript and approved the final version of the article for submission.