Electronic Supplementary Material

Paper title: Where are the strongest associations between autistic traits and traits of ADHD? Evidence from a community-based twin study

Journal: European Child & Adolescent Psychiatry

Authors: Mark J. Taylor1, Tony Charman2, & Angelica Ronald1

Corresponding Author: Mark J. Taylor, Genes Environment Lifespan Laboratory, Centre for Brain and Cognitive Development, Department of Psychological Science, Birkbeck, University of London, UK (email: )

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DeFries-Fulker Analysis

Table S1 Phenotypic group correlations

Outcome
Selection Variable
Variable / CAST
Social / CAST
Communication / CAST
RRBI / Conners ADHD
Hyperactivity/Impulsivity / Conners ADHD
Inattention
CAST Social / ----- / ----- / ----- / .24 / .31
CAST Communication / ----- / ----- / ----- / .60 / .53
CAST RRBI / ----- / ----- / ----- / .40 / .27
Conners ADHD Hyp/Imp / .25 / .55 / .28 / ----- / -----
Conners ADHD Inatt. / .34 / .55 / .24 / ----- / -----

CAST: Childhood Autism Spectrum Test; CAST Social: CAST social difficulties subscale; CAST Communication: CAST communication atypicalities subscale; CAST RRBI: CAST repetitive, restrictive behaviours and routines subscale

Conners ADHD: Conners’ Parent Rating Scale; Conners ADHD Hyp/Imp: Conners ADHD hyperactivity/impulsivity subscale; Conners Inatt: Conners inattention subscale

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Table S2 Transformed means used in univariate and bivariate analyses

Univariate Analyses
Monozygotic Twins / Dizygotic Twins
Measure / Proband Mean / Co-twin Mean / Proband Mean / Co-twin Mean
CAST Social / 1.00 / .69 / 1.00 / .31
CAST Communication / 1.00 / .79 / 1.00 / .37
CAST RRBI / 1.00 / .73 / 1.00 / .35
Conners ADHD Hyp/Imp / 1.00 / .85 / 1.00 / .41
Conners ADHD Inattention / 1.00 / .70 / 1.00 / .24
Bivariate Analyses
Monozygotic Twins / Dizygotic Twins
Selection -> Outcome / Proband Mean / Co-twin Mean / Proband Mean / Co-twin Mean
Social -> Hyp/Imp / 1.00 / .27 / 1.00 / .16
Social -> Inatt. / 1.00 / .30 / 1.00 / .15
Comm. -> Hyp/Imp / 1.00 / .54 / 1.00 / .29
Comm. -> Inatt. / 1.00 / .47 / 1.00 / .22
RRBI -> Hyp/Imp / 1.00 / .41 / 1.00 / .23
RRBI -> Inatt. / 1.00 / .31 / 1.00 / .16
Hyp/Imp -> Social / 1.00 / .21 / 1.00 / .15
Hyp/Imp -> Comm. / 1.00 / .49 / 1.00 / .19
Hyp/Imp -> RRBI / 1.00 / .24 / 1.00 / .12
Inatt. -> Social / 1.00 / .32 / 1.00 / .13
Inatt. -> Comm. / 1.00 / .49 / 1.00 / .14
Inatt. -> RRBI / 1.00 / .21 / 1.00 / .08

CAST: Childhood Autism Spectrum Test; Social: CAST social difficulties subscale; Comm.: CAST communication atypicalities subscale; RRBI: repetitive, restricted behaviours and interests

Conners ADHD: Conners’ Parent Rating Scale; Hyp/Imp: Conners ADHD hyperactivity/impulsivity subscale; Inatt: Conners ADHD inattention subscale

Note that for univariate analyses, both proband and co-twin means are for the selection measure; for bivariate analyses, the proband mean is for the selection measure, while the co-twin mean is for the outcome variable

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Table S3 Group heritability and bivariate heritability estimates from DeFries-Fulker analyses

Group Heritability
Subscale / Group Heritability / 95% CI
CAST Social / .69 ^ / .68-.69
CAST Communication / .79 ^ / .77-.79
CAST RRBI / .73 ^ / .67-.73
Conners ADHD Hyp/Imp / .85 ^ / .81-.85
Conners ADHD Inattention / .70 ^ / .70-.96
Bivariate Heritability
Selection Variable / Outcome Variable / Bivariate Heritability / 95% CI
CAST Social / Conners ADHD Hyp/Imp / .22:.24 / .16-.25
CAST Social / Conners ADHD Inattention / .29:.31 / .22-.33
CAST Communication / Conners ADHD Hyp/Imp / .50/.60 / .45-.52
CAST Communication / Conners ADHD Inattention / .47^:.53 / .44-.47
CAST RRBI / Conners ADHD Hyp/Imp / .35:.40 / .29-.36
CAST RRBI / Conners ADHD Inattention / .29:.27 / .26-.33
Conners ADHD Hyp/Imp / CAST Social / .12:.25 / .06-.14
Conners ADHD Hyp/Imp / CAST Communication / .49^:.55 / .49-.51
Conners ADHD Hyp/Imp / CAST RRBI / .23:28 / .18-.26
Conners ADHD Inattention / CAST Social / .32^:.34 / .31-.32
Conners ADHD Inattention / CAST Communication / .49^:.55 / .49-.77
Conners ADHD Inattention / CAST RRBI / .21^:24 / .18-.21

^ Indicates that the estimate was capped at the transformed MZ co-twin mean

Note that bivariate heritability estimates are presented as the ratio of bivariate heritability to the phenotypic group correlation

CAST: Childhood Autism Spectrum Test; CAST Social: CAST social difficulties subscale; CAST Communication: CAST communication atypicalities subscale; CAST RRBI: CAST repetitive, restricted behaviours and interests subscale; Conners ADHD: Conners’ Parent Rating Scale; Conners ADHD Hyp/Imp: Conners ADHD hyperactivity/impulsivity subscale; Conners ADHD Inattention: Conners ADHD inattention subscale

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