ADHD, autism spectrum disorder, temperament, and character: Phenotypical associations and etiology in a Swedish childhood twin study

Abstract

Objective

To explore the links between neurodevelopmental disorders – attention deficit hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) – and personality in a population-based, genetically sensitive study of children.

Method

A population-based sample of 1886 twins aged 9 and 12, enriched for childhood mental health problems, was recruited from the Child and Adolescent Twin Study in Sweden (CATSS). Parents were interviewed over the telephone using the Autism-Tics, AD/HD and other Comorbidities (A-TAC) inventory, and in a second step they rated their children according to the Junior Temperament and Character Inventory (JTCI).

Results

ADHD was strongly correlated with novelty seeking, while ASD was correlated positively with harm avoidance and negatively with reward dependence. The strongest associations between personality traits and neurodevelopmental disorders were negative correlations between the character dimensions of self-directedness and cooperativeness and ADHD and ASD alike. Cross-twin cross-trait correlations between ADHD, ASD, and personality dimensions in monozygotic twins were more than double those in dizygotic twins, indicating a strong genetic effect behind the phenotypic covariation between neurodevelopmental disorders and personality.

Conclusions

Neurodevelopmental disorders are linked specifically to particular temperament profiles and generally to hampered development of the self-governing strategies referred to as “character.” Poor self-agency and cooperation may be core functional outcomes in the separation of children with handicapping conditions from those with traits only reminiscent of neurodevelopmental disorders. The associations between neurodevelopmental disorders and personality are at least partly due to genetic effects influencing both conditions. As a consequence, personality must be broadly considered in neuropsychiatry, just as neuropsychiatric disorders and their genetic, neurodevelopmental, and cognitive susceptibilities have to be in personality research and clinical treatment.

Introduction

Attention deficit hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) are childhood-onset neurodevelopmental disorders, stemming from aberrant brain development affecting specific cognitive or social abilities [1]. Deficient executive control of attention, activity, and emotional regulation has been proposed as the key cognitive mechanism behind the ADHD phenotype [2], while deficits in the cognitive–emotional integration of social interaction (empathy), communication skills, and flexibility form the core of ASD [3]. The notion that ADHD and ASD are related to deviant neurodevelopment is supported by evidence of strong, partially overlapping, genetic influences [4], an increased prevalence of ASD and ADHD in children with medical disorders affecting brain development (e.g., fetal alcohol syndrome, cerebral paresis, and chromosomal disorders), overrepresentation of neurological symptoms (such as epilepsy and motor dyscoordination) in children with neurodevelopmental disorders, and the fact that both conditions are more common in boys than in girls, [1]). In fully developed clinical forms, ADHD affects about 5% of all children [5], and ASD about 1% [6]. Broader phenotypes or sub-threshold conditions were first reported for autism in systematic family studies [7], while recent population studies and twin analyses support the notion that the same genetic effects are involved in both narrow and broad definitions of the conditions [8], [9].

The conceptualization of ADHD and ASD as persistent alterations of mental functions with early childhood onset points to analogies with personality disorders and personality traits. In adults, ADHD has been associated with the Cluster B “dramatic” personality disorders, especially borderline personality disorder (BPD) [10], [11], [12] and antisocial personality disorder (APD) [13], [14]. In children and adolescents, clinical descriptions of ADHD include traits analogous to those in BPD and APD, such as antisocial attitudes towards others, impulsiveness, and emotional dysregulation [1]. In contrast, ASD has been linked to the “withdrawn” Cluster A disorders, such as schizoid personality disorder [15], and to traits implying avoidance and rigidity [3]. Systematic assessments of all types of personality disorders in adults diagnosed with ASD and/or ADHD show less clear-cut patterns that involve all three clusters of personality disorders, often in the form of several overlapping diagnoses [16].

Cloninger's biopsychosocial model, a dimensional assessment of personality, has been useful in studies of the relationship between personality and neurodevelopmental disorders [17]. This model distinguishes between four dimensions of temperament and three of character. The former are defined as individual differences in basic emotional drives and patterns of reaction to stimuli, and the latter as individual differences in higher cognitive processes identifying a person's style of mental self-government [17], [18]. The temperament dimensions include novelty seeking (impulsive and irritable vs. rigid and stoical), harm avoidance (pessimistic and anxious vs. optimistic and risk-taking), reward dependence (sociable and warm vs. aloof and cold), and persistence (persevering and ambitious vs. easily discouraged and lazy). The character dimensions are defined as self-directedness (responsible and resourceful vs. blaming and inept), cooperativeness (helpful and principled vs. hostile and opportunistic), and self-transcendence (intuitive and insightful vs. concrete and conventional). The original theory suggested a substantial genetic influence on the temperament dimensions, while culture and social learning were held to be more important to the character dimensions. Quantitative genetic studies have since shown a more complex picture, and the impact of genetics seems to be at least equally important in the development of the character dimensions as in dimensions of temperament [19], [20].

In studies of children with ADHD, novelty seeking has been high and persistence low, combined with immature character (low self-directedness and cooperativeness) [21], [22], [23] and, in some studies, with low reward dependence [21], [23]. High novelty seeking in ADHD has been linked specifically to impulsivity/hyperactivity rather than to problems with inattention [24]. Similar patterns of personality traits have been found in adults diagnosed with ADHD [16], [25]. Adults with ASD have also shown very low self-directedness and cooperativeness, but with a different temperament profile, including high harm avoidance and, at least in the absence of coexisting substance-use disorders, low novelty seeking and reward dependence [16], [25]. The finding of character immaturity in both disorders is consistent with the model in which the presence of a personality disorder is indicated by very low self-directedness and cooperativeness (defined as equal to or less than 2 standard deviations below average), while the type of disorder is indicated by the temperament configuration [26].

This study aimed to assess in a twin child population the previously described phenotypic associations between neurodevelopmental disorders and personality traits. The specific aims of this paper were

• to test whether the previously reported temperament and character profiles of ADHD (from studies on both children and adults) and ASD (in adults) could be replicated in a large, population-based study of children;

• to assess whether the presence of immature character (low self-directedness and/or cooperativeness) reported in both ADHD and ASD depends on the severity of the neurodevelopmental disorder; and

• to test whether phenotypic associations are due to common etiological factors acting on both neurodevelopmental disorders and personality.