Epidemiology and Genetics

This is an epidemiological study of 8-year-old children and a clinic-based study of high-functioning 9- to 16-year-old outpatients with AS/HFA.

Project information

Project duration


Funded by

Multiple sources

Project funder

Finland’s Slot Machine Association via the Finnish Association for Autism and Asperger’s Syndrome
the Alma and K. A. Snellman Foundation via the Autism Association of Northern Ostrobothnia
the Eija and Veikko Lesonen Foundation, Oulu, Finland, via the Child Psychiatric Research Foundation, Finland
the National Alliance for Autism Research, U.S. (Principal investigator [PI]: David Pauls)
the Emil Aaltonen Foundation, Finland
the Jusélius Foundation, Finland

Project coordinator

University of Oulu

Contact information

Project leader

Project description


Autism spectrum disorders (ASDs), defined as pervasive developmental disorders (PDDs) in DSM-IV and ICD-10, become manifest in childhood, ranging from a severe form, autism, to milder forms, Asperger syndrome (AS) and pervasive developmental disorder not otherwise specified (PDD-NOS)/atypical autism. AS is also specified in the criteria of Gillberg and of Szatmari et al. Different diagnostic criteria sets of AS and overlapping and inaccuracy in the criteria of PDDs have caused confusion in research as well as in clinical practice. An epidemiological study gave us the possibility to consider the prevalence and diagnostic criteria sets of ASDs.

The publication of DSM-5 in 2013 was one of the most anticipated events in the mental health field. In DSM-5, PDDs were replaced by one diagnosis called ASD. The first DSM-5 draft criteria for ASD were posted in 2010 and the revised draft in 2011. Our thoroughly diagnosed epidemiological data gave us the opportunity to compare DSM-IV and the DSM-5 draft criteria posted in 2010 and to be the first to publish the results.

In clinical practice, a screening instrument is an aid in recognizing the features of ASD in order to speed up the diagnostic process. The Autism Spectrum Screening Questionnaire (ASSQ) was developed in Swedish as a screen for ASDs and it has been translated into many languages. The ASSQ was translated into Finnish in the 1990s and has been used in clinical settings in Finland ever since. Validation studies show broad variability of established cut-off scores in different languages and cultures. The Finnish ASSQ had not been validated and the cut-off scores had not been determined prior to the present work.

A high degree of psychiatric comorbidity is associated with ASDs. However, in DSM-IV and ICD-10, many psychiatric comorbidities are excluded, which is a source of confusion. Studies concerning comorbid psychiatric disorders in ASDs have mainly revealed one or just a few comorbidities, and only relatively few investigators have dealt with the whole spectrum of comorbid psychiatric disorders in ASDs or have used standardized instruments. To our knowledge, only one population-based study has previously dealt with psychiatric comorbid disorders in subjects with ASDs. In addition, psychiatric comorbidity in ASDs was previously an unstudied research area in Finland.


The main aims of the present study were to estimate the prevalence of AS according to four sets of diagnostic criteria, estimate the prevalence of autism and ASDs according to DSM-IV criteria, and reveal overlaps and inaccuracies concerning the diagnostic criteria of ASDs, evaluate the DSM-5 draft criteria for ASD posted in 2010, compare the results with those obtained using DSM-IV criteria, and present modifications to the DSM-5 draft criteria, assess the validity of and provide optimal cut-off scores for the Finnish ASSQ for clinical settings and total population screening, and estimate the prevalence and identify the types of comorbid psychiatric disorders associated with AS/high-functioning autism (HFA), and rate the overall level of functioning in relation to psychiatric comorbidity in children/adolescents with AS/HFA.

Subjects, methods and measures

This is an epidemiological study of 8-year-old children and a clinic-based study of high-functioning 9- to 16-year-old outpatients with AS/HFA. The epidemiological target population (n = 4,422) was rated via the ASSQ by parents and/or teachers and a screened subgroup was examined using the Autism Diagnostic Interview-Revised (ADI-R), Autism Diagnostic Observation Schedule (ADOS), Asperger Syndrome (and high-functioning autism) Diagnostic Interview (ASDI), Wechsler Intelligence Scale for Children-Third Edition (WISC-III), school day observations, and medical records in order to estimate the prevalence of ASDs, find out lacks in the diagnostic criteria of ASDs, evaluate DSM-5 draft criteria for ASD, and assess the cut-off scores for the Finnish ASSQ. Comorbid psychiatric disorders were identified using the Schedule for Affective Disorders and Schizophrenia for School-Age Children, Present and Lifetime Version (Kiddie-SADS) and overall level of functioning rated in 50 subjects with AS/HFA using the Children’s Global Assessment Scale (CGAS).


First publication in the year 2007.


Personal grants were awarded by the Rinnekoti Research Foundation/Finnish Brain Foundation, Espoo, Finland, the Alma and K. A. Snellman Foundation, Oulu, Finland, the Child Psychiatric Research Foundation/Finnish Brain Foundation, Finland, the Finnish Medical Foundation, Finland, the Thule Institute, University of Oulu, Finland, the University Pharmacy Fund, University of Oulu, Finland, the Graduate School of Circumpolar Wellbeing, Health and Adaptation, University of Oulu, Finland, the Finnish Medical Society Duodecim, Finland, the Oulu Medical Research Foundation, Oulu, Finland, and the Child Psychiatric Research Foundation of Oulu Area, Finland. Professor (deceased) Harriet Forsius awarded her personal award via the Child Psychiatric Research Foundation, Finland.

Principal Investigator

Marja-Leena Mattila, M.D., defended her doctoral thesis “Autism spectrum disorders. An epidemiological and clinical study” in 2014 and graduated as a Ph.D. with distinction from the Faculty of Medicine, University of Oulu, Finland. She received the Best Doctoral Thesis Award from the Faculty of Medicine, University of Oulu, Finland, in 2014 and the Best Doctoral Thesis Award from the Alma and K. A. Snellman Foundation in years 2011­–2016. Her thesis was also ranked as the fifth best Doctoral Thesis by Finnish Medical Journal in 2014. A young investigator stipend for the best publication in psychiatry and psychopharmacology in the academic year 2006–2007 was awarded by AstraZeneca.

Marja-Leena Mattila is an active member of the PEDEGO Autism study group of Child Psychiatry. She has authored 37 original peer reviewed publications and a review article, presented her study findings in scientific congresses and national schoolings, reviewed manuscripts for international publications and translated several internationally recognized measures in ASDs into Finnish. She also performs expert assignments and works in clinical practice as a pediatrician.

Project results

Main Results

The prevalence of AS was 2.5/1,000 according to DSM-IV, 2.9/1,000 according to ICD-10, 2.7/1,000 according to Gillberg & Gillberg, and 1.6/1,000 according to Szatmari et al. The prevalence of autism was 4.1/1,000 and that of ASDs 8.4/1,000 (DSM-IV). DSM-5 draft criteria were less sensitive to detect AS/HFA. For 7- to 12-year-old children (IQ > 50), the optimal cut-off scores were 30 in clinical settings and 28 in total population screening using parents’ plus teacher’s summed ASSQ scores. Comorbid psychiatric disorders were common (prevalence 74%) and often multiple; behavioral disorders in 44%, anxiety disorders in 42%, and tic disorders in 26%. Oppositional defiant disorder, depressive disorder and anxiety disorders as comorbidities indicated significantly lower level of functioning.