Table 1.

List of primary twin studies on Autism Spectrum Disorder included and excluded in the meta‐analysis

Source	Country	Systematic recruitment; (n pairs)	Blind to zygosity & cotwin status; DZ OS included	Basis for diagnosis	Diagnostic criteria	Reason for exclusion	Ascertainment type
Included studies
3. Steffenburg et al. (1989)	Nordic Regions	Yes; included triplets; (21)	No; No	Records and interview; ABC, the Lotter checklist & DIPBEC	DSM III R	Autistic Disordera	CA
5. Le Couteur et al. (1996)**	United Kingdom	Yes; included triplets; (48)	No; No	Records and interview; ADI, ADOS	ICD 10 & DSM IV	Broader Phenotypeb, Autistic Disorderc, Atypical Autismd	CA
6. Taniai et al. (2008)	Japan	Yes; (45)	Yes; Yes	Records and semistructured interview; CARS‐TV	DSM IV	Autistic Disorderc, Asperger Syndromee, PDD‐NOSf	CA
8. Lichtenstein et al. (2010)	Sweden	Yes; (7982)	n/a; Yes	Records and telephone interview; A‐TAC	DSM IV	Pervasive Developmental Disordersg	RPA
9. Hallmayer et al. (2011)	United States	Yes; (192)	n/a; Yes	Records and interview; ADI, ADOS	DSM IV	Autistic Disorderc, ASDh	IA (π = .96)
12. Nordenbæk et al. (2014)	Denmark	Yes; (36)	Yes; No	Records and interview; ADOS, DISCO	ICD 10 & DSM IV	Autistic Disorderc, Asperger Syndromee, PDD‐NOSf	IA (π = .76)
13. Colvert et al. (2015)	United Kingdom	Yes; (127)	Yes; Yes	Records and interview; CAST, DAWBA, ADI, ADOS, Best‐estimate Diagnosis	ICD 10 & DSM IV	Autism Spectrum Disorderh, Broad Spectrum Disorderi	CA
Excluded studies						Reason for exclusion
1. Folstein and Rutter (1977)**	United Kingdom	Yes	Yes; No	Records and interview	Criteria of DSM released in 1980	Superseded by Le Couteur et al., 1996;	CA
2. Ritvo et al. (1985)*	United States	No	Not specified; Yes	Records and interview	DSM III	Biased Systematic Recruitment	IA (π = .86)
4. Bailey et al. (1995)**	United Kingdom	Yes; included triplets	Yes; No	Records and interview	ICD 10	Superseded by Le Couteur et al., 1996;	CA
7. Rosenberg et al. (2009)*	United States	No	No; Yes	Voluntary Registry: care‐giver reported diagnosis of ASD	DSM IV TR	Did not meet Systematic Recruitment criterion	n/a*
10. Frazier et al. (2014)*	United States	No	No; Yes	Voluntary Registry: care‐giver reported diagnosis of ASD	DSM IV TR	Did not meet Systematic Recruitment criterion	n/a*
11. Sandin et al. (2014)**	Sweden	Yes	n/a; Yes	National Registry Diagnostic information	ICD‐10	No concordance information, and, Lichtenstein et al. (2010) already reported on at least part of this twin sample	CA

CA = complete (or double) ascertainment (π = 1); IA = incomplete ascertainment (0 < π < 1), π is calculated; RPA = random population ascertainment; DSM = Diagnostic and Statistical Manual of Mental Disorders; ICD = International Classification of Diseases.

Study number is assigned chronologically based on publication year. Studies 1 and 4 (**) were excluded on the basis that they were superseded by a more recent study of the same research group; study 11 (**) did not provide twin concordance data, additionally, Lichtenstein et al. (2010) have already reported on twins included in study 11. Studies 2, 7 and 10 (*) did not meet criterion of Systematic Recruitment.

Age range: Studies 1–3: 5–23 years, 26 years, 2–23 years, respectively; Studies 4–6: age range of final sample not specifically reported; Studies 7–12: 4–11 years, 9–12 years, 11–14 years, 6–14 years, 3–14 years and 12–15 years, respectively.

Definition of diagnostic outcomes:

^aAutistic Disorder (DSM II R): onset prior to age 5. Criteria: 8 out of 16 items across three categories: at least two difficulties in social interaction category and at least one difficulty in communication and restricted, repetitive and stereotyped patterns of behaviour categories.

^bThe Broad Phenotype: measured behavioural domains of communication impairment and social dysfunction. Meeting the cut off for either deficit alone or in combination was required for diagnosis. RRB's not included in this criterion.

^cAutistic Disorder (DSM IV): onset prior to age 3. Criteria: at least two difficulties in social interaction category and at least one difficulty in communication and restricted, repetitive and stereotyped patterns of behaviour categories. Delays or abnormal functioning in at least one of the following: social interaction, social language communication, and symbolic/imaginative play.

^dAtypical Autism: atypical clinical features and a loosened age criterion than used in Autistic Disorder (DSM IV) diagnosis.

^eAsperger Syndrome (DSM IV): core triad of symptoms criteria as in Autistic Disorder. However, no clinically significant delay is observed in areas of language development, cognitive development, age‐appropriate self‐skills or adaptive behaviour (other than in social interaction).

^fPervasive Developmental Disorder‐ Not Otherwise Specified (DSM IV): presentations that do not meet the criteria for autistic disorder because of late age of onset, atypical symptomatology, or subthreshold symptomatology, or all of these. This category includes ‘atypical autism’.

^gPervasive Developmental Disorders or Autism Spectrum Disorder (DSM IV): includes conditions of Autistic Disorder, PDD‐NOS, Asperger Disorder, Rett's Disorder, and Childhood Disintegrative Disorder.

^hASD: included individuals with Autistic Disorder diagnosis and those who met a broader definition of ASD based on published criteria for combining information from the ADI‐R and ADOS (see Hallmayer et al., 2011 for more details).

ⁱThe Broad Spectrum: classification not based on any validated algorithms but includes individuals that have just missed diagnostic threshold cut offs for ASD diagnosis and exhibit high levels autism traits [see Colvert et al., 2015 for more details].