hyperactivity disorder symptoms and loneliness among adults in the general population

Research in Developmental Disabilities 62 (2017) 115–123

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Research in Developmental Disabilities

Attention-deficit/hyperactivity disorder symptoms and loneliness among adults in the general population Andrew Stickley a,b,c,∗ , Ai Koyanagi d,e , Hidetoshi Takahashi a , Vladislav Ruchkin f,g , Yoko Kamio a a Department of Child and Adolescent Mental Health, National Institute of Mental Health, National Center of Neurology and Psychiatry (NCNP), 4-1-1 Ogawahigashi, Kodaira, Tokyo 187-8553, Japan b The Stockholm Center for Health and Social Change (SCOHOST), Södertörn University, Huddinge, 141 89, Sweden c Department of Human Ecology, Graduate School of Medicine, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan d Parc Sanitari Sant Joan de Déu, Universitat de Barcelona, Fundació Sant Joan de Déu, Dr Antoni Pujadas, 42, Sant Boi de Llobregat, Barcelona 08830, Spain e Instituto de Salud Carlos III, Centro de Investigación Biomédica en Red de Salud Mental, CIBERSAM, Monforte de Lemos 3-5 Pabellón 11, Madrid 28029, Spain f Child Study Center, Yale University Medical School, New Haven, CT 06520, USA g Department of Child and Adolescent Psychiatry, Division of Neuroscience, Uppsala University, Uppsala, S-751 85, Sweden

a r t i c l e

i n f o

Article history: Received 27 April 2016 Received in revised form 7 September 2016 Accepted 4 January 2017 Number of reviews completed is 2 Keywords: ADHD Lonely Common mental disorders

a b s t r a c t Background: Research on the association between adult attention-deficit/hyperactivity disorder (ADHD) and loneliness is scarce even though factors which have been previously linked to loneliness, such as divorce and poorer mental health may be more prevalent among adults with ADHD. This study investigated the relation between ADHD symptoms/symptom severity and loneliness in the general adult population. Methods: Data from the Adult Psychiatric Morbidity Survey 2007 (N = 7403, aged ≥16 years) were analyzed. ADHD symptoms and common mental disorders (CMDs) were assessed with the Adult ADHD Self-Report Scale (ASRS) Screener and the Clinical Interview Schedule Revised, respectively. Loneliness was measured with a question from the Social Functioning Questionnaire. Multivariable logistic regression analysis was used to examine the associations. Results: In the fully adjusted model, an ASRS score ≥14 was strongly associated with loneliness (OR = 2.48 95%CI = 1.83–3.36). ADHD symptom severity was related to loneliness in a dose-response fashion. Over one-third of the association between ADHD symptoms and loneliness was explained by CMDs. Conclusions: Adults with more ADHD symptoms are at an increased risk of feeling lonely. Future research should determine how ADHD symptoms are linked to loneliness and if loneliness is affecting well-being. © 2017 Elsevier Ltd. All rights reserved.

What this paper adds? Research among adults in the general population has shown that loneliness is common and that for some people is associated with worse health and well-being. However, until now, there has been an absence of research focused specifically

∗ Corresponding author at: Department of Child and Adolescent Mental Health, National Institute of Mental Health, National Center of Neurology and Psychiatry (NCNP), 4-1-1 Ogawahigashi, Kodaira, Tokyo 187-8553, Japan. E-mail address: [email protected] (A. Stickley). http://dx.doi.org/10.1016/j.ridd.2017.01.007 0891-4222/© 2017 Elsevier Ltd. All rights reserved.

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on loneliness among adults with attention-deficit/hyperactivity disorder (ADHD), even though factors which have been previously linked to loneliness, such as divorce and poorer mental health may be more prevalent among adults with ADHD. To address this research gap the current study examined the association between ADHD symptoms and loneliness among adults in a large, nationally representative general population sample. Results showed that adults with a higher number of ADHD symptoms had increased odds for feeling lonely and that ADHD symptoms were linked to loneliness in a doseresponse fashion. Moreover, a meditational analysis further revealed that common mental disorders (CMDs) explained over one-third of the association while social support explained almost 6%. This research thus suggests that loneliness may be elevated in adults with ADHD and that in addition to poorer mental health, other factors may also be important for this association. Further research is now needed to determine what factors underlie this association and whether loneliness is affecting well-being among adults with ADHD.

1. Introduction Following earlier research which showed that for many children, attention-deficit/hyperactivity disorder (ADHD) symptoms persist into adulthood in varying degrees (Matte, Rohde, & Grevet, 2012), in the last 20 years there has been an increasing focus on ADHD among adults (Castellanos, 2015). A meta-analysis of community-based studies using DSM-IV diagnostic criteria found that the pooled prevalence of adult ADHD was 2.5% (95% confidence interval [CI]: 2.1–3.1%) and that this figure decreased with age (Simon, Czobor, Bálint, Mészáros, & Bitter, 2009), although a recent study from the Netherlands showed that symptoms are present even among elderly adults (aged 71–94) (Michielsen et al., 2012). ADHD has been linked to a number of functional impairments that impact on well-being and quality of life throughout adulthood in terms of family functioning, work and health (Barkley Murphy, & Fischer 2008; Das, Cherbuin, Butterworth, Anstey, & Easteal, 2012; Eakin et al., 2004; Murphy & Barkley, 1996; Nigg, 2013). Despite this, there are many aspects of adult ADHD that still need to be illuminated. For example, there has been little focus on the association between ADHD and loneliness even though there are several reasons to expect that loneliness may be linked to adult ADHD. Earlier research among children found that ADHD was associated with a deficit in adaptive social functioning (Greene et al., 1996; Stein, Szumowski, Blondis, & Roizen, 1995). This deficit might help explain why subsequent studies among adults with ADHD/ADHD symptoms have shown that they often experience more relationship difficulties, including poorer relationship quality (Das et al., 2012), worse marital functioning (Eakin et al., 2004), and reduced social support (Brod, Schmitt, Goodwin, Hodgkins, & Niebler, 2012). Research suggests they are also more likely to be single (Lensing, Zeiner, Sandvik, & Opjordsmoen, 2015), divorced, and have multiple marriages (Biederman et al., 2006; Murphy & Barkley, 1996). In turn, several of these factors have also been linked to loneliness in the general population (Fokkema, De Jong Gierveld, & Dykstra, 2012; Stickley et al., 2013; Yang & Victor, 2008). Furthermore, given the strong association between adult ADHD and a range of mental health conditions including mood and anxiety disorders (Kessler et al., 2006), it is also possible that comorbidity might be playing a role in the link between ADHD and loneliness. Many mental disorders have been linked to loneliness (Meltzer et al., 2013), while longitudinal research has suggested that among adults, depressive symptoms and loneliness may have a reciprocal influence on each other across time (Cacioppo, Hughes, Waite, Hawkley, & Thisted, 2006). Indeed, a recent study has indicated that depressive symptoms might be important for the association between adult ADHD symptoms and some forms of loneliness (Michielsen et al., 2015). The few studies that have examined the association between ADHD and loneliness have produced conflicting findings. Two studies among children and adolescents in Australia and Sweden respectively found that neither an ADHD diagnosis (Houghton, Roost, Carroll, & Brandtman, 2015) nor ADHD symptoms (Diamantopoulou, Henricsson, & Rydell, 2005) were linked to loneliness. In contrast, an earlier longitudinal study from Canada showed that some children who were diagnosed as being hyperactive at age 6–12 years old tended to report feelings of loneliness in early adulthood (average age 25.1) (Weiss & Hechtman, 1993). A more recent study among older adults in the Netherlands (aged 60–94, average age 71) also found that an ADHD diagnosis and symptoms were both linked to emotional loneliness (Michielsen et al., 2015). Comparing the results from these studies is difficult however, because of important methodological differences. For example, while two of the studies obtained information on child (internalizing problems) (Diamantopoulou et al., 2005) and adult (depressive symptoms) psychopathology (Michielsen et al., 2015), another study found that no adolescent participants had depression/depressive symptoms while specifically noting the link seen between depression and loneliness in earlier studies (Houghton et al., 2015). To the best of our knowledge, until now there has been an absence of research that has specifically focused on the association between ADHD/ADHD symptoms and loneliness among adults of all ages in the general population. As ADHD often remains undiagnosed in adults (Asherson et al., 2012; Lamberg, 2003), understanding this association in the general population may be particularly important, especially as loneliness has itself been associated with poorer health and wellbeing among adults (Hawkley & Cacioppo, 2010; Holt-Lunstad, Smith, Baker, Harris, & Stephenson, 2015). To address this deficit, the current study used data from a large community-based survey carried out in England. Given the scarcity of studies on ADHD and loneliness, the conflicting results between the few previous studies on this topic, and uncertainty concerning the role of other factors such as comorbid mental disorders in this association, the aims of the current study were: (1) to

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determine if loneliness is associated with ADHD symptoms/ADHD symptom severity among adults in the general population; and (2) to ascertain the extent to which this association is explained by social support and common mental disorders (CMDs). 2. Methods 2.1. Study sample The data used in this study came from the Adult Psychiatric Morbidity Survey (APMS) 2007, a nationally representative sample of the English adult population (aged ≥16 years) residing in private households (N = 7403). A full description of the survey and its methodology has been published previously (McManus, Meltzer, Brugha, Bebbington, & Jenkins, 2009). In brief, the survey was conducted by the National Center for Social Research (NatCen) in conjunction with the University of Leicester with fieldwork being undertaken in the period from October 2006 to December 2007. The small user Postcode Address File (PAF) was used as the sampling frame with stratified multistage random probability sampling being used to select respondents. In the first stage of a two-stage process, 519 postal sectors were selected as the primary sampling units (PSUs) after being stratified by Strategic Health Authority (SHA) region and socio-economic status. In the second stage, within each postal sector, 28 delivery points were randomly chosen. This resulted in 14,532 delivery points, which after a subsequent eligibility check of addresses, was reduced to 13,171 points. One person was randomly selected from within each household if two or more adults aged ≥16 were present. Information was obtained from respondents during face-to-face interviews using computer-assisted personal interviewing (CAPI), while interviewees self-completed parts of the interview that enquired about more sensitive topics (e.g., drug use, domestic violence) using computer-assisted self-interviewing (CASI). Responses were obtained from 7461 respondents to give a survey response rate of 57%. To ensure that the sample was representative of its intended target population, sampling weights were generated to account for non-response. The Royal Free Hospital and Medical School Research Ethics Committee provided ethical approval for the survey with all participants providing informed consent. 2.2. Measures 2.2.1. Loneliness One item from the Social Functioning Questionnaire (SFQ) was used to obtain information on loneliness (Tyrer et al., 2005). Using the past two weeks as the reference period, respondents were asked to assess to what extent they had felt ‘lonely and isolated from other people’. The response options were very much, sometimes, not often, and not at all. This variable was used as a four-point ordinal variable (only for the descriptive analysis) and also as a dichotomous variable with those who responded, sometimes and very much being categorized as lonely (Meltzer et al., 2013). 2.2.2. ADHD symptoms The Adult ADHD Self-Report Scale (ASRS) Screener was used to assess ADHD symptoms. This is the 6-item screening version of a longer 18-item scale that is used to assess DSM-IV ADHD symptoms among adults (Kessler et al., 2005; Kessler et al., 2007). Statements with a 5-point response scale (never-scored 0, rarely-1, sometimes-2, often-3, and very often-4) that enquire about inattention (4 items) and hyperactivity (2 items) in the previous six months were used to assess symptoms. The responses to individual items were added to give a total score that could range from 0 to 24. We categorized respondents’ scores in two different ways: (1) by using a dichotomized measure: 0–13 vs. 14–24; and (2) by using four strata: Stratum I: 0–9; II: 10–13; III: 14–17; and IV: 18–24 (Kessler et al., 2007). 2.2.3. Sociodemographic variables Information was obtained on respondents’ sex, age (16–34, 35–59, ≥60 years), ethnicity (white British or other), equivalized income tertiles (high ≥£29,826, middle £14,057 to < £29,826, low <£14,057), educational qualifications [(degree, non-degree, A-level, GCSE, other): yes or no)], marital status (married/cohabiting, single, widowed/divorced/separated), and employment status (employed, unemployed, economically inactive). 2.2.4. Self-rated health Information on respondents’ health was obtained by asking, “How is your health in general?” The answers were dichotomized as: excellent, very good, or good (scored 0); and fair or poor (scored 1). 2.2.5. Difficulty with activities of daily living Difficulty with seven types of activities of daily living (ADL) was assessed: personal care, getting out and about or using transport, medical care, household activities, practical activities, dealing with paperwork, and managing money. The response options were: (1) “No, no difficulty at all”; (2) “Yes, some difficulty”; and (3) “Yes, a lot of difficulty”. Those who answered (2) or (3) to any of the seven types of ADL were considered to have ADL difficulty.

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2.2.6. Social support To determine the level of social support that respondents felt they received from their family and friends, they were presented with a list of seven statements concerning whether: family and friends did things to make them happy, made them feel loved, could be relied on no matter what, would see that they were taken care of no matter what, accepted them just the way they are, made them feel an important part of their lives, and gave them support and encouragement. There were three response options, ‘not true’ (coded 0), ‘partly true’ (coded 1), or ‘certainly true’ (coded 2). The answers to these items were summed to create a scale score that ranged from 0 to 14 with higher scores indicating greater social support. The scale had a good degree of internal consistency (Cronbach’s ␣ = 0.88). 2.2.7. Common mental disorders (CMDs) Six types of CMDs were assessed: depressive episode, mixed anxiety and depression, generalized anxiety disorder, panic disorder, phobia, and obsessive-compulsive disorder. This was done using the Clinical Interview Schedule Revised (CIS-R) which identifies past-week non-psychotic symptoms to generate ICD-10 diagnoses. In the current study, any CMD referred to having one or more of the six CMDs. 2.3. Statistical analysis The analysis was performed with Stata version 14.1 (Stata Corp LP, College Station, Texas). A descriptive analysis was conducted to characterize the study sample by different levels of loneliness and the presence of ADHD symptoms. The difference in participant characteristics by different levels of loneliness was tested with Chi-square tests and a one-way Analysis of Variance (ANOVA) for categorical and continuous variables (social support), respectively. A multivariable logistic regression analysis was undertaken to assess the association between ADHD symptoms/ADHD symptom severity (independent variable) and loneliness (dependent variable). To determine how the inclusion of different control variables affected the association between ADHD and loneliness, a hierarchical analysis was undertaken with different blocks of variables being entered sequentially in different models. Four different models were constructed. A base model adjusted for sex, age, ethnicity, income, educational qualifications, marital status, employment status, self-rated health, and ADL difficulties (Model 1). To determine the effects of social support and CMDs, social support (Model 2) and CMDs (Model 3) were added individually to the base model. Both variables were then included together in the fully adjusted model (Model 4). For the analysis using the four category ADHD variable, tests for trend were conducted by including ADHD as a continuous variable in the models. We also examined the mediating role of CMDs and social support in the association between ADHD symptoms (0–13 vs. 14–24) and loneliness by performing a meditational analysis using the khb (Karlson Holm Breen) command in Stata (Breen, Karlson, & Holm, 2013). This analytical technique, which can be used with logistic regression analysis, decomposes the total effect of a variable into its direct and indirect (i.e., mediational) components. Using this method, the mediated percentage (i.e., the percentage of the total effect that is explained by the mediator) can be calculated. In these analyses, we controlled for sex, age, ethnicity, income, educational qualifications, marital status, employment status, self-rated health, and ADL difficulties. In addition, the analysis assessing the mediating effect of CMDs was adjusted for social support, while the analysis for social support was adjusted for CMDs. In all regression analyses, covariates were included as categorical variables with the exception of social support, and the dichotomous loneliness variable was used. In order to keep as many participants in the analysis as possible, we created a missing category for the income variable as 20.7% of the respondents did not provide this information. In all analyses, the sample weighting and the complex study design were taken into account. The results are presented as odds ratios (OR) and 95% confidence intervals (CIs). The level of statistical significance was set at P < 0.05. 3. Results The mean (SD) age of the sample was 46.4 (18.6) years, and 51.4% were female. The proportion of individuals in each ADHD symptom stratum was: 76.2% (stratum I); 18.1% (II); 4.6% (III); and 1.2% (IV). The prevalence of loneliness was: 16.5% (not often); 17.4% (sometimes); and 3.1% (very much). Increasing ADHD scores were associated with a higher prevalence of loneliness (sometimes or very much): 14.6% (stratum I); 33.6% (II); 53.5% (III); and 67.0% (IV). The sample characteristics by different levels of loneliness according to the presence of ADHD symptoms are presented in Table 1. For the non-ADHD group, there were significant differences in reported levels of loneliness across all sample characteristics except ethnicity. Among those with ADHD symptoms, there were statistically significant differences for only five sample characteristics: income, educational qualifications, self-rated health, social support, and CMDs. The results from the analysis examining the association between ADHD symptoms and loneliness are presented in Table 2. When ASRS ≥ 14 was the cut-off point, in the analysis that controlled for sociodemographic characteristics and health variables (Model 1), there was a strong association between ADHD symptoms and loneliness (OR = 4.11, 95%CI = 3.15–5.35). Further adjustment for social support or CMDs (Model 2 and 3) attenuated the OR with a greater reduction occurring when CMDs were included in the analysis. Even after full adjustment ADHD symptoms continued to be strongly associated with loneliness (OR = 2.48, 95%CI = 1.83–3.36) (Model 4). Similar results were obtained when the ADHD symptom strata were used to examine the association with loneliness. There was a significant dose-response effect across all models with

Table 1 Sample characteristics by different levels of loneliness and the presence of ADHD symptoms.a No ADHD

ADHD

Loneliness

Loneliness Not at all

Not often

Sometimes

Very much

P-valueb

2.0 2.4

<0.001

26.5 21.3

21.4 18.0

40.7 38.5

11.3 22.3

0.054

17.5 16.6 13.5

2.2 2.3 2.1

<0.001

24.3 19.5 42.6

20.2 20.6 14.0

40.7 40.5 31.7

14.8 19.4 11.7

0.131

16.2 17.2

15.8 17.2

2.1 3.0

0.250

22.8 31.0

19.4 22.1

41.0 31.2

16.8 15.8

0.535

66.4 66.2 65.4

18.4 16.4 13.4

14.2 15.0 17.5

0.9 2.3 3.7

<0.001

26.3 27.1 21.9

29.3 13.5 15.1

37.4 38.3 43.1

7.1 21.1 19.9

0.046

No Yes

68.8 64.3

11.3 18.0

16.3 15.9

3.6 1.8

<0.001

26.0 23.5

10.7 22.6

37.3 40.2

26.0 13.8

0.017

Marital status

Married/cohabiting Single Widowed/divorced/separated

72.3 53.7 52.6

14.6 21.1 17.0

11.8 22.1 25.4

1.3 3.1 5.1

<0.001

26.7 25.2 10.9

20.9 18.8 18.2

38.7 37.6 48.9

13.7 18.4 22.1

0.212

Employment status

Employed Unemployed Economically inactive

64.8 61.1 66.8

18.1 15.3 13.5

15.5 16.6 16.9

1.6 7.0 2.8

<0.001

23.6 26.5 24.2

24.7 11.2 14.7

40.1 38.5 39.3

11.6 23.8 21.9

0.182

Self-rated health

Excellent/very good/good Fair/poor

67.6 55.4

16.8 14.3

13.9 25.5

1.6 4.8

<0.001

29.1 14.9

22.1 15.5

36.9 44.6

11.9 25.0

0.001

ADL difficultye

No Yes

68.9 57.7

16.8 15.4

13.2 22.3

1.2 4.6

<0.001

25.7 22.7

25.1 15.8

37.0 41.7

12.2 19.8

0.093

Social supportf Common mental disordersg

Mean (SD) No Yes

13.4 (1.5) 71.2 29.0

13.1 (1.9) 16.1 18.2

12.6 (2.5) 11.8 42.7

<0.001 <0.001

12.9 (1.8) 37.5 14.1

13.2 (1.3) 24.9 16.0

12.5 (2.1) 31.5 45.6

10.8 (3.6) 6.1 24.2

<0.001 <0.001

Category

Not at all

Not often

Sometimes

Sex

Male Female

68.2 62.8

16.1 16.6

13.8 18.1

Age (year)

16–34 35–59 ≥60

59.9 64.6 72.7

20.4 16.5 11.7

British white

Yes No

65.9 62.6

Incomec

High Middle Low

Qualificationsd

Very much

11.3 (4.0) 1.0 10.1

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P-value

Characteristic

b

Abbreviations: ADHD Attention-deficit/hyperactivity disorder; SD Standard deviation; ADL Activities of daily living. a ADHD was defined as a score of 14 or higher on the 6-item Adult ADHD Self-Report Scale (ASRS) Screener. b P-values were calculated by chi-square tests and one-way ANOVA for categorical and continuous variables respectively. c Income categories were based on equivalized income tertiles (high ≥£29,826, middle £14,057 to <£29,826, low <£14,057). d Qualifications referred to degree, non-degree, A-level, GCSE, or other educational qualifications. e ADL difficulty referred to having difficulty with at least one of: personal care, getting out and about or using transport, medical care, household activities, practical activities, dealing with paperwork, and managing money. f Seven items were used to identify the level of social support with each item having scores of 0, 1, or 2. The scores for the 7 items were added to create a scale ranging from 0 to 14 with higher scores corresponding to higher levels of social support. g Common mental disorders referred to having at least one of: depressive episode, mixed anxiety and depression, generalized anxiety disorder, panic disorder, phobia, and obsessive-compulsive disorder. 119

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Table 2 The association between ADHD symptoms/symptom severity and loneliness estimated by multivariable logistic regression analysis. Model 1 OR (95%CI)

Model 2 OR (95%CI)

P-value

Model 3 OR (95%CI)

P-value

Model 4 OR (95%CI)

P-value

ADHD symptoms Adult ADHD Self-Report Scale Screener 1.00 0–13 (No) 14–24 (Yes) 4.11 (3.15–5.35) <0.001

1.00 3.91 (3.01–5.09)

<0.001

1.00 2.55 (1.88–3.45)

<0.001

1.00 2.48 (1.83–3.36)

<0.001

Symptom severityb,c Adult ADHD Self-Report Scale Screener 1.00 0–9 2.72 (2.32–3.20) <0.001 10–13 5.04 (3.75–6.78) <0.001 14–17 8.07 (4.71–13.82) <0.001 18–24

1.00 2.66 (2.26–3.14) 4.88 (3.63–6.55) 7.08 (4.14–12.11)

<0.001 <0.001 <0.001

1.00 2.13 (1.79–2.54) 3.11 (2.23–4.32) 4.23 (2.25–7.98)

<0.001 <0.001 <0.001

1.00 2.10 (1.76–2.51) 3.06 (2.20–4.26) 3.81 (2.03–7.15)

<0.001 <0.001 <0.001

P-value

a

Abbreviations: ADHD Attention-deficit/hyperactivity disorder; OR Odds ratio; CI Confidence interval. Model 1: Adjusted for sex, age, ethnicity, income, educational qualifications, marital status, employment status, self-rated health, and ADL difficulty. Model 2: Adjusted for covariates in Model 1 and social support. Model 3: Adjusted for covariates in Model 1 and common mental disorders. Model 4: Adjusted for covariates in Model 1 and common mental disorders and social support. a ADHD was defined as a score of 14 or higher on the 6-item Adult ADHD Self-Report Scale (ASRS) Screener. b ADHD severity was based on the 6-item version of the adult ADHD Self-report Scale (ASRS) Screener. c A significant trend was observed for all models (P < 0.05). Table 3 Common mental disorders and social support as mediators of the association between ADHD symptoms and loneliness.a Mediator Common mental disordersb

Social supportc

Effect

OR (95%CI)

P-value

OR (95%CI)

P-value

Total Direct Indirect

4.37 (3.23–5.92) 2.48 (1.83–3.36) 1.76 (1.58–1.97) *38.5% of the link mediated

<0.001 <0.001 <0.001

2.62 (1.93–3.56) 2.48 (1.83–3.36) 1.06 (1.01–1.11) *5.7% of the link mediated

<0.001 <0.001 0.020

Abbreviation: ADHD Attention-deficit/hyperactivity disorder; OR Odds ratio; CI Confidence interval. a ADHD was defined as a score of 14 or higher on the 6-item Adult ADHD Self-Report Scale (ASRS) Screener. Both models were adjusted for sex, age, ethnicity, income, educational qualifications, marital status, employment status, self-rated health, and ADL difficulty. b Adjusted also for social support. c Adjusted also for common mental disorders.

stronger symptom severity associated with higher odds for reporting loneliness. As with the previous analysis, adjusting for social support and mental disorders attenuated the ORs but the estimates across all symptom severity categories (OR 2.10–3.81) remained significant even in the fully adjusted model. CMDs and social support were significant mediators (as evidenced by the significant indirect effect) in the association between ADHD and loneliness explaining 38.5% and 5.7% of the link, respectively (Table 3). 4. Discussion This study found that ADHD symptoms and symptom severity were strongly associated with loneliness even after full adjustment for possible confounding factors, with symptom severity being linked with loneliness in a dose-response fashion. This association was partially mediated by the presence of CMDs and to a much lesser extent, social support. Importantly, even those adults with fewer ADHD symptoms (in Stratum II) were significantly more likely to be lonely. The results of our study are in line with two studies among adults which have suggested that ADHD and loneliness are linked (Michielsen et al., 2015; Weiss & Hechtman, 1993) but not with two other studies among children and adolescents which found no association between ADHD/ADHD symptoms and loneliness (Diamantopoulou et al., 2005; Houghton et al., 2015). It is uncertain what underlies this age difference and whether it relates to different methodologies (measurement and classification issues) and/or the specific population samples studied. However, it is possible that age might be an important factor in this association. When explaining the non-association observed for children it has been suggested that despite having greater peer problems, those with ADHD symptoms might be able to associate with other children who have similar disruptive behavior (Diamantopoulou et al., 2005) or diagnoses (Houghton et al., 2015) and that this could be protective against feelings of loneliness. It is also possible that ‘positive illusory bias’ might play a role i.e., that children with ADHD overestimate the extent of their social acceptance (Diamantopoulou et al., 2005; Hoza, Pelham, Dobbs, Owens, & Pillow, 2002; Mikami, 2010). For adults however, recent research has shown that not only do ADHD/ADHD symptoms exist across the entire life course (Guldberg-Kjär, Sehlin, & Johansson, 2013; Michielsen et al., 2012) but that the effects of symptoms may be cumulative, as seen for example, in the more solitary lives of some older adults with ADHD who have alienated

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those people closest to them over time (Brod et al., 2012). In short, the accumulated experience of relational difficulties and rejection across the life span may act to ‘correct’ any illusory bias, while associating with others who are similar may be more difficult in adulthood given the prescribed adult societal roles (e.g. spouse/worker). Having said this, given that the few studies to date have been restricted to certain age groups, more research is needed among all age groups on the relation between ADHD/ADHD symptoms and loneliness to determine whether age is important for this association. Earlier research has shown that depression and other mental disorders are associated with loneliness (Meltzer et al., 2013) and that psychiatric comorbidity in adults with ADHD is high (Sobanski, 2006). This may be why CMDs accounted for over one-third of the association between ADHD symptoms and loneliness. Although it is uncertain what links ADHD, CMDs and loneliness it is possible that personality characteristics might play a role. Greater neuroticism and lower selfmastery have recently been reported in adults with ADHD (Michielsen et al., 2014), while both have also been found to be important in the association between depression and loneliness (Peerenboom, Collard, Naarding, & Comijs, 2015). In addition, while worse social functioning in the presence of persisting symptoms may explain why ADHD has been linked to poorer quality relationships (Bruner, Kuryluk, & Whitton 2015), being never married/divorced, and having a lower number of family network members (Michielsen et al., 2015), in our study a proxy measure of these factors, social support, had only a modest effect on the association between adult ADHD symptoms and loneliness, while an additional analysis showed that marital status did not have a significant mediational effect (explaining only 2.7% of the association [data not shown]). The results from the mediational analysis suggest therefore that other factors might also be important for the observed association. For example, it is possible that for some children the effects of peer rejection which has been linked to childhood hyperactivity/impulsivity and inattention (together with the more aggressive behavior that can occur among children with ADHD) (Nijmeijer et al., 2008), may stretch across the life course by undermining the development of social skills and thus impairing social functioning in many life domains (Mrug et al., 2012). In terms of the present study a social skills deficit might be especially important as it may lead to loneliness by inhibiting different aspects of social functioning such as selfintroductions, being friendly, friendship formation and taking part in social activities (Heinrich & Gullone, 2006). Decreased social skills might also be a factor in reduced self-esteem (Shaw-Zirt, Popali-Lehane, Chaplin, & Bergman 2005), which has been observed among individuals with ADHD at both younger (Canu & Carlson, 2007) and older ages (Michielsen et al., 2014), and which is one of the central cognitive characteristics of loneliness (Heinrich & Gullone, 2006). It is also possible that the mood instability (Skirrow, McLoughlin, Kuntsi, & Asherson, 2009) and emotional dysregulation that are seen in many adults with ADHD (Shaw, Stringaris, Nigg, & Leibenluft, 2014) might have consequences for increased loneliness. Barkley and Murphy (2010) have shown that symptoms of emotional impulsiveness such as impatience, being hot-tempered and quick to anger as well as emotional overreactivity and a high level of excitability all impact detrimentally on social relations with others both inside the home and at work. The exact way in which poorer mood and more volatile emotions might be linked for example, to worse family functioning (Skirrow & Asherson, 2013) is uncertain, although it is possible that different pathways might exist. A recent study has indicated for instance, that ADHD couples may use more negative conflict resolution behaviors (complaints and criticism) which may undermine relationships (Canu, Tabor, Michael, Bazzini, & Elmore, 2014). Volatile and unpredictable moods/behaviors might also be one factor in the greater reluctance to engage with individuals with ADHD that was observed in an earlier study among university students (Canu, Newman, Morrow, & Pope, 2008), which might also act to isolate those with ADHD/ADHD symptoms if replicated more widely in the general population. This study has several limitations. Although an earlier study has indicated that adults with ADHD provide better information about their own condition than other informants, it also highlighted that they tend to underreport symptom severity (Sandra Kooij et al., 2008). This might have introduced bias into the current study, especially if any underreporting of symptoms was also affected by the degree of loneliness. In addition, we used a single-item measure to obtain information on loneliness. In contrast, a recent study that examined social functioning among older adults with ADHD/ADHD symptoms in the Netherlands which used a scale measure of loneliness (the de Jong Gierveld scale) that was able to differentiate between different forms of loneliness, found that although emotional loneliness was linked to ADHD/ADHD symptoms, a statistically significant association between ADHD symptoms and social loneliness was mediated by depressive symptoms (Michielsen et al., 2015). This highlights the importance of using more detailed measures of loneliness in future research on this topic, especially as single-item measures may be susceptible to random measurement errors. In addition, we also lacked information on other conditions such as autism spectrum disorder (ASD) which may have affected the findings in the current study. More specifically, not only does ASD seem to overlap with ADHD in some adults (Hofvander et al., 2009), but it has also been linked to loneliness in children and adolescents (Bauminger & Kasari, 2000; Lasgaard, Nielsen, Eriksen, & Goossens, 2010), while an earlier study found that young adults with a greater number of autistic characteristics also reported greater loneliness (Jobe & White, 2007). 5. Conclusion In this study a strong relation was observed between ADHD symptoms and loneliness, which was only partially mediated by the presence of CMDs. Importantly, this association was even seen among individuals with fewer ADHD symptoms which accords with the results from another study which found that lower levels of symptom severity are associated with impairment across several life domains (Das et al., 2012). Given that loneliness may be becoming more prevalent in contemporary society (Cacioppo, Grippo, London, Goossens, & Cacioppo 2015), and that it can have an extremely harmful

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impact on health and well-being, an important task for future research is to determine the nature of this association more precisely, including the specific mechanisms that link adult ADHD symptoms with loneliness. Acknowledgements We would like to thank the National Center for Social Research and the University of Leicester who were the Principal Investigators of this survey. In addition, we would also like to thank the UK data archive, the National Center for Social Research and other relevant bodies for making these data publically available. They bear no responsibility for this analysis or interpretation of this publically available dataset. AK’s work was supported by the Miguel Servet contract financed by the CP13/00150 project, integrated into the National R + D + I and funded by the ISCIII – General Branch Evaluation and Promotion of Health Research – and the European Regional Development Fund (ERDF-FEDER). References Asherson, P., Akehurst, R., Kooij, J. 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