The aim of this study was to examine whether ADHD is a dimensional trait in the adult general population. We studied whether an increased number of ADHD symptoms was associated with higher comorbidity, exposure to risk factors (childhood abuse and parental psychopathology), and disability. We ascertained whether even low numbers of ADHD symptoms were associated with an increased burden of disease. Data were used from the second wave of the Netherlands Mental Health Survey and Incidence Study-2 (NEMESIS-2, N=5,303). NEMESIS-2 is a nationally representative face-toface survey on mental health of the Dutch general population. ADHD symptoms, mental comorbidity, and disability were assessed using the Adult ADHD Self-Report Scale Screener, the Composite International Diagnostic Interview version 3.0, and the Medical Outcomes Study Short Form Health Survey, respectively. Dose-response relationships were found between the number of ADHD symptoms and Axis I and II mental disorders; exposure to risk factors; and mental and physical disability. Our study supports the notion that ADHD is a dimensional trait in the adult general population. Even low numbers of symptoms were associated with an increased burden of disease, and therefore these should be identified and treated.

Introduction

Many psychiatric disorders exist on a continuum of dimensional traits. These include panic disorder (Batelaan et al., 2007), psychotic disorder (van Os et al., 2009), and borderline personality disorder (Ten Have et al., 2016). Some somatic disorders like diabetes (Weyer et al., 2001), blood pressure (Faraone et al., 2009), and Alzheimer’s dementia (Morris et al., 2001), can have been described in a similar way. The dimensional model is best understood using an iceberg metaphor: at the tip of the iceberg are the known cases with a full-blown disorder, representing one end of the dimension. The largest part of the iceberg is below the waterline, which depicts persons who have low numbers of symptoms or are asymptomatic, representing the other end of the dimension (Lund and Jensen, 1989). Attention-Deficit/Hyperactivity Disorder (ADHD) is also considered a dimensional trait. This is supported by genetic, taxometric, and neuroimaging studies (Mohamed et al., 2015). The crossnational prevalence range of a diagnosis of ADHD in adults (i.e., the tip of the iceberg) is 2.8-3.4% (Fayyad et al., 2007; Fayyad et al., 2017). In contrast to the tip of the iceberg that has received increasing attention in epidemiological research in the last decade; the part under the waterline has been studied less. However, some epidemiological studies have investigated the relationship between low numbers of ADHD symptoms and comorbidity (Das et al., 2012; Estevez et al., 2014), disability (Das et al., 2012; Estevez et al., 2014), lifestyle habits (Weissenberger et al., 2018), violence (Gonzalez et al., 2013), suicidality (Stickley et al., 2016), loneliness (Stickley et al., 2017), satisfaction with life (Oerbeck et al., 2015), emotional empathy (Groen et al., 2017), screen time (Montagni et al., 2016) or dental anxiety (Carlsson et al., 2013), amongst others. In general, these studies found that those with low numbers of ADHD symptoms occupy an intermediate position between those with no ADHD symptoms and those with full threshold ADHD. For example, the study by Das and collegues (2012) found that a higher number of ADHD symptoms was associated with increased comorbidity and disability. However, it remains unclear whether low numbers of ADHD symptoms are associated with the same risk factors (e.g., parental psychopathology and childhood abuse) as full-blown ADHD (Lindblad et al., 2011; Sugaya et al., 2012).