Autistic-like traits

Sammanfattning: Introduction Autism spectrum disorders (ASDs) are neurodevelopmental disorders characterized by restrictions in social interaction, understanding, and communication, by stereotyped patterns of repetitive behaviors, and by narrow interests. ASDs, which affect about 1% of the population, are predominantly genetic, but no single explanation has been found. On the contrary, a multitude of developmental trajectories are possible. In relatives of individuals with ASDs, autistic-like traits (ALTs, i.e. traits that are less pronounced than, but qualitatively similar to, ASD symptoms) occur more frequently than expected by chance. When studied in the general population, ALTs have shown high internal consistency, dimensional distributions, and hereditary influence. It is, however, unclear if ASDs are the extreme end of a continuum of ALTs, if ALTs are psychiatrically relevant, and if ALTs share etiological factors with ASDs. Methods This thesis employs three nation-wide twin studies to pursue the following aims: 1. To establish the distribution of ALTs and provide estimates of the genetic and environmental effects involved. 2. To describe the relationships between ALTs and other types of mental problems, including shared etiology. 3. To clarify whether ALTs are influenced by increasing paternal age, which is a known risk factor for ASDs. 4. To determine whether there is a demarcation in the genetic effects between ASDs and ALTs. Results 1. ALTs are dimensionally distributed in 46% of all 9- and 12-year-old twins, and genetic effects account for 68% of the variation in ALTs. 2. Increasing levels of ALTs are related to an increased risk of concomitant mental health problems in both adults and children. In addition, common genetic and environmental etiological factors were found behind ALTs and phenotypically different mental health problems. 3. Increasing paternal age increases the risk for ALTs and ASDs alike. 4. No demarcation could be discerned between the genetic effects on ASDs and ALTs, implying a continuum from ASDs to ALTs predominantly affected by genes. Conclusion Taken together, the results of this thesis suggest that ASDs can be viewed as the extreme end of ALTs, or that ALTs are truly a ‘shadow’ of ASDs in persons with sub-threshold problems with social interaction, communication, and behavioral flexibility, and that ALTs may be crucial for understanding mental health problems and for scientific attempts to identify etiological factors behind ASDs.