10 Chapter 1 highest prevalence of ASD can be found in the United States, with studies reporting numbers varying from 1:36 (Maenner et al., 2023) to 86 per 10,000 (Anorson et al., 2021). Elsewhere in the world, the estimated prevalence of ASD is lower, with the disorder affecting 59 per 10,000 people in Europe and 47 per 10,000 in Oceania (Anorson et al., 2021). ASD is more commonly diagnosed in boys than in girls. The male-to-female ratio is often assumed to be 4:1, although more recent research considered the ratio closer to 3:1 (Loomes et al., 2017). It is presumed that that girls may present less overt symptoms, may be more likely to mask their social deficits and/or that a gender-based interpretation bias of social difficulties lead to a higher risk for girls being misdiagnosed, diagnosed at a later age or being overlooked (Lai et al., 2015; Hodges, Fealko & Soares, 2020). Individuals who are diagnosed with ASD often cope with lifelong negative consequences on individual, family, and society level which emphasizes the need for global, suitable, and easy access interventions. The neurodiversity perspective The neurodiversity concept has garnered more attention in recent years. It emphasizes the wide natural variation of the human brain, and as has been stated by Baron-Cohen (2017) that “there is no single way for a brain to be normal, as there are many ways for the brain to be wired up and reach adulthood.” Within the concept of neurodiversity, autism is considered as a variation of the human brain with its owns strengths in thinking and behaving (Silberman, 2015). Following this discussion, Baron-Cohen (2017) offers a different approach towards psychiatric conditions such as autism by introducing the four D’s: difference, disability, disease, and disorder. “Difference” meaning that people with autism just differ from the norm set by neurotypical people, whereas “disease” refers to a medical condition with known underlying causal mechanism and pathophysiology. When a parent expresses concerns regarding their child’s development with the child facing challenges in one or more areas of development, autism can be conceived as a “disability”. When there are significant impairments in functioning and there is need for treatment, we speak of autism as a “disorder”. Please note, in this thesis, we focus on the latter two ‘D’s i.e. disability and disorder. Precursors and early clinical manifestations of autism in infants and toddlers As there is no single diagnostic biomarker for ASD, the clinical diagnosis is based on standardized assessments of both observable behaviors and information gathered from developmental history. As defined by the DSM-5 criteria (see Box 1.1.) symptoms of ASD must be present during the early developmental period (American Psychiatric Association, 2013). Research provides solid evidence that the clinical manifestations of ASD can emerge during the first two years of life (Cleary et al., 2023; Zwaigenbaum et al., 2019). In addition to the early clinical manifestations outlined in the DSM-5, extensive research has been conducted on early behavioral precursors through both retrospective and prospective studies.
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