Table 1 presents the descriptive characteristics of the study sample. Seven per cent of the participants were identified as severe emotional/social loners while 40% were serious emotional loners and 53% were slight emotional loners. Over the subjects’ life course, about 46% of the adolescents reported feeling lonely in middle school, 49% in high school, and about 64% during college. On the other hand, the mean score for AAE was less than 1 (M = 0.81; SD = 0.95); and 46% of the participants had not experienced any of the assessed AAEs. The ages of the subjects ranged 30–35 years, and more than three-quarters (83%) had attended a 4-year college.

The multivariate multinomial logistic regression models with progressive adjustments were shown in Table 2. Model 1 demonstrated that adults who reported experiencing adversity during adolescence were more likely to be severe emotional/social loners rather than emotional loners (the reference group) (RRR = 1.42, 95% CI: 1.23–1.63). Feelings of loneliness during middle school were found to be important even 15 years later because this feeling lonely increased the odds of being either a serious emotional loner (RRR = 1.66, 95% CI: 1.39–1.98) or a severe emotional/social loner during young adulthood (RRR = 2.27, 95% CI: 1.72–3.00) compared to the reference group. In contrast to the AAE impact on adulthood loneliness, the fact that the subject had attended a 4-year college reduced the odds of being a serious emotional loner (RRR = 0.69, 95% CI: 0.54–0.87) or a severe emotional/social loner (RRR = 0.64, 95% CI: 0.41–1).

Model 2 added feeling lonely during high school to Model 1. The association of AAEs and feeling lonely during middle school with adulthood loneliness showed no significant change, although the magnitude slightly decreased. Like feeling loneliness during middle school, feeling lonely during high school was also more likely to result in the subject becoming either a serious emotional loner (RRR = 1.91, 95% CI: 1.61–2.27) or a severe emotional/social loner (RRR = 1.98, 95% CI: 1.40–2.81), as compared to the reference group. Model 3 showed that feeling lonely during college also increased the odds of being either a serious emotional loner (RRR = 1.67, 95% CI: 1.37–2.02) or a severe emotional/social loner (RRR = 2.45, 95% CI: 1.63–3.67), even after adjusting for individual and demographic characteristics. Finally, as compared to the reference group, the odds of being a serious emotional loner or a severe emotional/social loner significantly decreased if the adult attended a 4-year college.

In order to further validate the application of the single item of loneliness, we used this single item loneliness as an outcome variable together with the same explanatory variables that examined the AAE effect with progressive adjustments as shown in Table 3. Basic Model demonstrated that adults who reported experiencing adversity during adolescence were more likely to report feeling of loneliness in emerging adulthood (AOR = 1.17, 95% CI: 1.07–1.26). Results were similar to those in Table 2: people who reported adversities during adolescence were also more likely to experience emotional loneliness (AOR = 1.11, 95% CI 1.03-1.19) (Model 1). This significant result was observed even after controlling for various covariates, as well as feelings of being lonely during middle school and high school (AOR = 1.08, 95% CI 1.00-1.16) (Model 2) but not college (AOR = 1.07, 95% CI 0.99-1.15) (Model 3). Feelings of being lonely when attending middle school, high school and college reduced the association between AAE and feeling of loneliness in emerging adulthood by 8.50%. That is, if those with AAEs experienced the same level of emotional loneliness as others, the difference in feeling of loneliness between these two groups would be about 8.50% smaller than the observed difference, net of the individual’s characteristics. The measures of feeling lonely across life stages consistently produced significant effects and it would seem that a more recent feeling of being lonely had a stronger effect than an earlier feeling of being lonely.

The main purpose of this study was to explore AAEs and their longitudinal association with loneliness during young adulthood using a large community cohort sample from a non-Western society. Consistent with the Belsky developmental model, our analysis found that AAEs had a profound impact on severe emotional/social loneliness during emerging adulthood, and this adversity effect was only reduced slightly after adjusting for other important individual risk factors and feeling lonely at various life stages. In addition, feelings of loneliness across important life stages resulted in a greater risk of becoming a serious emotional loners or severe emotional/social loners. The risk factors during early life consistently showed a significant longitudinal influence across each life stage. We also found that AAEs and feelings of loneliness across life stages “work” together additively, which may in turn shape the vulnerabilities of individuals, resulting in severe mental health problems later, such as loneliness during emerging adulthood.

Our results further extend the available current literature on the negative effects of adverse experiences during early life on an individual's health later in life by showing how these affect loneliness during adulthood and its relationship with adolescence adversities (Merrick et al., 2017; Sheikh, 2017). Given that loneliness has negative effects on both physical (Leigh-Hunt et al., 2017) and mental health (Cacioppo & Cacioppo, 2018), preventing children and adolescents from having such adverse experiences is warranted. It can be seen from the many and varied adverse experiences described in the original CDC-Kaiser ACE study (Felitti et al., 1998) and by WHO (World Health Organization, 2012) that a significant number of such experiences are family related. Thus reducing family adversity is crucial to promoting the mental health of children, adolescents and adults. Parenting skill training, for example, should help to reduce conflict and abuse in the home, and when this is employed it potentially ought to prevent the occurrence of such adversities in a family context (Center for Disease Control and Preventing, 2019a, 2019b).

Furthermore, our findings indicate that the effects of early negative experience can be profound and complex. External negative experiences, such as AAEs, appear to work in an additive manner with internal negative experiences such as feeling lonely, increasing the likelihood of the individual developing severe emotional and social loneliness. On the other hand, there may be a cascade effect, whereby external AAEs trigger internal negative emotions that together might lead to serious mental health problems during adulthood. We conducted an additional analysis showing that AAEs were significantly associated with both feelings of loneliness in middle and high school (results not shown), which in turn were related to serious loneliness during emerging adulthood.

In addition, this study also showed that the one-item measure of feelings of loneliness is a significant predictor of the loneliness construct. The results of the EFA suggest that the 1-item loneliness feeling loads on the latent structure of “emotional loneliness” along with the original three items of the emotional domain from the DJGS (Appendix 1), which CFA corroborated. These findings, along with our findings from the regression models, suggest that this single item of feeling lonely may be a valid measure for emotional loneliness. To further support this conclusion, we conducted additional series of analyses to valid this possibility. Suggested by prior studies (Davey, Barratt, Butow, & Deeks, 2007), our analysis showed that this single item of feeling lonely was positively associated with alcohol use and low self-esteem (results not shown).Therefore, our results not only suggest that this single item may be a valid indicator of emotional loneliness, but that it also echoes the suspicions of Cacioppo, Hughes, Waite, Hawkley, & Thisted, 2006 that loneliness and depressive symptomatology are often conflated together because this single item is derived from the depressive symptoms scale (Derogatis, 2000).

Our results further confirm previous research showing the benefit of education by the decreased risk of Taiwanese adults becoming severe emotional/social loners when they had a 4-year college degree. Prior research largely explored the effect of educational disparities on mental health, mostly depression (Shen, 2020; Todd & Teitler, 2019), and our study extends these results to loneliness among young people. In addition to the effect of attending a 4-year college, our analysis showed that parental education also had a marginal effect. Parents with higher levels of education had reduced risks of their children becoming emotional/social loner. Furthermore, as suggested by earlier studies, parental control has a negative effect on loneliness, which may act as the mechanism whereby the level of parental education affects long-term mental health in this non-Western society (Chiao et al., 2019).

The present study provides significant new insights on AAEs and their longitudinal association with loneliness during adulthood. However, the results need to be interpreted with care because of the limitations of this study. First, the TYP data is based on self-reported recall of AAEs and loneliness, raising the issue of recall bias. In addition, self-report measures of AAEs and loneliness may also suffer from shared method variance rather than the constructs the measures are hypothesized to represent. The measure of AAEs is not comprehensive; however, given that the significant effects of the AAEs we identified are consistent with prior studies of adverse childhood experiences (ACEs) (Kim et al., 2019; Suglia et al., 2018), we are confident more comprehensive measures of AAEs would not yield appreciably different results. Moreover, several AAE items were adopted from the reports of parents rather than adolescent children. The bias of social desirability is possibly limited. When the 6-item DJGS was measured by asking young adults to identify with their loneliness status, it may have had a stigma attached. However, the 6-item DJGS was validated in both Western and non-Western participants (Chiao et al., 2019; de Jong Gierveld, & Van Tiburg, 2010; Leung et al., 2008). Second, to present our results more concisely, we limited our analysis to feelings of loneliness across various life stages and its effects on adult loneliness by adjusting for individual and demographic characteristics as covariates. However, the Belsky developmental model proposes that it should be possible to distinguish the cumulative effects of feeling lonely across all life stages. Future research is needed to specifically identify the complex effects of cumulative negative emotions and how adverse experiences during early life may influence the development of loneliness during later life. Third, while prior studies have suggested that the influence of family dynamics and friendship are important in relation to loneliness (Rico-Uribe, Caballero, Olaya, Tobiasz-Adamczyk, & Koskinen, 2016), such information was not available in the present investigation. Fourth, attrition is always a matter of great concern when using a panel sample. In the present context, the participants with missing data are more likely to be suffering from mental health problems.

Nevertheless, to our knowledge this is the first report exploring AAEs and their longitudinal association with the latent structure of loneliness during emerging adulthood in a non-Western society. This study extends the existing literature by showing that AAE is associated with loneliness during adulthood. In addition, we employed a life-course perspective to examine feeling lonely across various life stage (middle school, high school, and college) and found them to be related additively to the development of both emotional and social loneliness during emerging adulthood. Furthermore, the validation analysis of the use of a single item as an outcome afforded us a better insight into the conceptual separation when using the single item for loneliness versus the 6-item DJGS.

This study was funded by Ministry of Science and Technology in Taiwan under grants 108-2410-H-010-014, 107-2511-H-010-001-MY2 and 109-2410-H-010-006-MY2. The conclusions herein are those of the authors and do not represent the viewpoint of the Ministry.