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J Autism Dev Disord
DOI 10.1007/s10803-008-0558-6
ORIGINAL PAPER
The Childhood Autism Spectrum Test (CAST): Sex Differences
Joanna G. Williams Æ Carrie Allison Æ Fiona J. Scott Æ Patrick F. Bolton Æ
Simon Baron-Cohen Æ Fiona E. Matthews Æ Carol Brayne
Springer Science+Business Media, LLC 2008
Abstract The Childhood Autism Spectrum Test (CAST)
(formally known as the Childhood Asperger Screening Test)
identifies autism spectrum conditions by measuring social
and communication skills. The present study explored the
sex distribution of scores. The CAST was distributed to
11,635 children aged 4–9 years in Cambridgeshire primary
schools (UK). 3,370 (29%) were returned. The median score
was significantly higher in boys (median test, P \ 0.001)
(Boys, median = 5 (IQR: 3,8); girls, median = 4 (IQR:
2,6)). There was a predominance of boys (n = 81 (79.4%)
over girls (n = 21 (20.6%)) amongst those scoring C 15.
Exclusion of children with ASC did not significantly affect
the results. We conclude that different profiles of social and
communication skills in boys and girls must be taken into
account when measuring these skills in the general
population.
Keywords Communication Social behaviour
Autism Asperger syndrome Sex differences
The Childhood Autism Spectrum Test (CAST): Sex
Differences
Fiona Scott is now freelance.
This study examines sex differences in The Childhood
Autism Spectrum Test (CAST) (Scott et al. 2002 ). The
CAST is a 37 item parental self-completion questionnaire
developed to detect subtler manifestations of Autism
Spectrum Conditions (ASC), including Asperger Syndrome
(AS) in primary school children. The CAST measures
difficulties and preferences in social and communication
skills covering: initiation and maintenance of conversation
and specific language difficulties; social interaction with
adults and peers, including eye contact; choice of play
activities; presence of rigid or repetitive behaviours; choice
of interests and sharing interests with others.
Results using the CAST suggest that those with a
diagnosis of AS score significantly higher than controls
(Scott et al. 2002 ). The cut-point for concerns of possible
ASC is at 15 (out of a maximum of 31, as 6 items are not
scored) or higher. At that cut-point sensitivity of the CAST
is 100%, specificity 97%, and positive predictive value
50% (Williams et al. 2005 ). The test–retest reliability is
good, indicated by a Spearman’s rho correlation of 0.82
(Williams et al. 2006 ). The stability of scores around the
screening cut-point is moderate, indicated by a Spearman’s
rho correlation of 0.67 (Allison et al. 2007 ). An important
J. G. Williams C. Brayne
Department of Public Health and Primary Care,
University of Cambridge, Cambridge, UK
Present Address:
J. G. Williams
Bath and North East Somerset Primary Care Trust, Bath, UK
C. Allison F. J. Scott S. Baron-Cohen
Autism Research Centre, Department of Psychiatry,
University of Cambridge, Cambridge, UK
P. F. Bolton
MRC Centre for Social, Genetic & Developmental Psychiatry,
Department of Child Psychiatry, The Institute of Psychiatry,
Decrespigny Park, Denmark Hill, London SE5 8AF, UK
F. E. Matthews
MRC Biostatistics Unit, Institute of Public Health, University
Forvie Site, Robinson Way, Cambridge CB2 2SR, UK
C. Allison ( & )
Autism Research Centre, University of Cambridge, Douglas
House, 18b Trumpington Road, Cambridge CB2 8AH, UK
e-mail: cla29@cam.ac.uk
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J Autism Dev Disord
aspect of developing any screening test for ASC is an
understanding of whether there are any differences
between the sexes, due to the consistent finding of a higher
prevalence of ASC in boys (Ehlers and Gillberg 1993 ;
Wing 1981 ), and the growing evidence of sex differences
in traits related to ASC and in typical development.
The Extreme Male Brain (EMB) theory (Baron-Cohen
1999 , 2002 ; Baron-Cohen and Hammer 1997 ) has been
suggested as a possible explanation for the predominance of
males to females in studies of ASC. This theory proposes that
males and individuals with ASC are impaired in their ability
to empathize, whilst being at least average or superior in
tasks that require systemizing. Evidence supporting this
theory comes from the finding that individuals with ASC are
better than typical individuals on systemizing or pattern-
recognition tasks on which males out-perform females, such
as the Embedded Figures Task (Jolliffe and Baron-Cohen
1997 ) or the Intuitive Physics Test (Lawson et al. 2004 ).
Conversely, individuals with ASC perform worse on
empathy tasks on which females out-perform males, such as
the Faux Pas Test (Baron-Cohen et al. 1999 ) or the Reading
the Mind in the Eyes task (Baron-Cohen et al. 1997 ).
Typically developing boys and girls have different pat-
terns of behaviour and development in social skills and
communication. Despite considerable overlap, sex differ-
ences have been found in the amount of eye contact made
by 12 month old human infants (Lutchmaya et al. 2002 ),
types of play (Knickmeyer et al., in press), approaches to
friendship formation (Baron-Cohen and Wheelwright
2003 ), in the degree of empathy shown to others (Baron-
Cohen and Wheelwright 2004 ), choice of topics to talk
about, and chosen focus of attention (Baron-Cohen 2003 ).
Such differences are in part accounted for by differences in
foetal testosterone levels (Chapman et al. 2006 ; Auyeung
et al. 2006 ; Knickmeyer et al. 2005 ).
Whilst various tests have been developed to assess a range
of social and communication skills and to screen for ASC and
related social and communication difficulties (Williams and
Brayne 2006 ), few studies have examined sex differences.
One exception to this was a study of the Autism Spectrum
Screening Questionnaire (ASSQ). The ASSQ was adminis-
tered to a large population sample of 7–9 year olds and
showed significantly higher mean scores for boys. Girls
scored lower than boys across the whole score distribution,
indicating that girls had less difficulties than boys (Posserud
et al. 2006 ). Another study used the Social Responsiveness
Scale (SRS) to examine autistic traits in children aged
7–15 years old (Constantino and Todd 2003 ). Again, boys
scored on average 25% higher than girls on this measure.
Sex differences in the number of autistic traits in the
general population was also examined using the Autism
Spectrum Quotient (AQ), a self-administered questionnaire
covering social skills, attention switching, attention to
detail, communication, and imagination. A higher fre-
quency of autistic traits was found in males, indicated by a
higher score on this measure (Baron-Cohen et al. 2001 ).
Child and adolescent versions of the AQ have been
developed, and sex differences were found on both ver-
sions, with typical males scoring higher than females
(Auyeung et al. in press; Baron-Cohen et al. 2006 ). This
pattern of sex differences in autistic traits has been con-
firmed in children as young as 18 months, using the Q-
CHAT (a revised version of the Checklist for Autism in
Toddlers (Allison et al. in press; Baron-Cohen et al. 1992 ;
Baron-Cohen et al. 1996 ). Since these measures have all
shown a sex difference in the distribution of scores, it is
important to examine whether the CAST also reveals a
different psychometric profile in girls and boys, and if so,
what the possible explanations for this may be.
The aim of the study reported below was to examine
score distributions on the CAST by sex and age for the total
population as well as typically developing boys and girls,
as part of a further examination of the utility of the CAST
as a screening instrument for ASC.
Methods
Procedure
The CAST questionnaires in this analysis were from the
SCORE (Social Communication Research and Epidemiol-
ogy) study. The study had full ethical approval from the
Cambridge Local Ethics Committee. 136 mainstream pri-
mary schools, including independent schools but not
special schools (for this phase), in Cambridge City, East
and South Cambridgeshire and Fenland in the UK, were
invited to participate in the research, of whom 68% agreed.
These schools cover a broad cross-section including urban
and rural areas across the county. There were no noted
differences in geographical distributions of schools that
participated versus schools that refused. Each school was
asked to distribute the CAST to all children in years 1 to 4
inclusive (typically age 5–9 years) to take home to their
parent or guardian. Distribution spanned a 15-month per-
iod. The questionnaires were returned to the research team
using a Freepost envelope.
Basic personal information, including the child’s date of
birth, sex, and school, was requested. The child’s school
year was calculated based on date of birth and date ques-
tionnaire completed or returned. Questionnaires were
excluded if the child was more than one year older or
younger than the target age range. These criteria led to
some children aged 4 or 10 years remaining in the sample.
Information was collected about which parent completed
the CAST. The scoring range is from 0 to 31, with the
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J Autism Dev Disord
remaining 6 items being control questions on general
development which are not scored (see Table 1 for direc-
tion of scoring). Differences in CAST scores were
investigated using total score and within three score groups
(B11, 12–14, C15) as in previous studies examining the
questionnaire’s performance (Williams et al. 2005 , 2006 ).
The questionnaire included a series of items about previous
diagnoses of medical conditions or special needs.
questionnaires for analysis. There were an equal number of
boys (n = 1,667) and girls (n = 1,667) in the final sample.
This compares to a proportion of boys of this age of 51.2%
in the Cambridgeshire population. There was no significant
difference between the number of boys and girls excluded
from the sample (chi-squared, P = 0.259). 2,718 (81.5%)
had complete data on scoring CAST questions. 585
(17.6%) had between 1 and 4 missing questions, and 31
(0.9%) had between 5 and 27 missing. 41 of the 3,334
questionnaires had age missing and were excluded from
analyses based on age, leaving a sample size of 3,293.
Analysis
Initially missing data were recoded as zero to give an
observed score. Differences between scores according to
age and sex were investigated. The distribution of scores
was described using medians, inter-quartile ranges (IQR)
and ranges, since the score distribution was extremely
skewed. Whether the two samples by sex were from the
same underlying distribution was tested using Kolmogo-
rov–Smirnov test for equality of the distribution. The
median test assessed the null hypothesis that the samples
were drawn from populations with the same median.
Pearson chi-squared tests were used to test differences
between proportions. A test for trend of the proportion of
boys and girls across the three score groups was carried out
using logistic regression. The score data were transformed
to a normal distribution and the effect of sex and age was
examined using ANOVA and linear regression.
Three sensitivity analyses were carried out:
Distribution of Scores in the Whole Sample
The median score was 4 (IQR: 2,7, range: 0,29) (n = 3,334).
Boys had a higher median score (Median: 5; IQR: 3,8; range:
0,29) than girls (Median: 4; IQR: 2,6; range: 0,29). This
difference between the score distributions was highly
significant (Kolmogorov–Smirnov P \ 0.001; median test,
P \ 0.001), (Fig. 1 ). The shapes of the score distributions
were different, with a longer tail at the upper end of the
distribution for boys, whereas only one girl scored above 24,
shown on the cumulative graph in Fig. 2 . The differences
between the scores increased with increasing score. The
difference between the two distributions is constant on the
logarithm scale, showing that differences between the boys
and girls increases with increasing score. Data were trans-
formed to normality using the zero-skewness log with a
transformation (ln (score +2.022091). The gender difference
had a regression coefficient of b =-0.21. There was a
significant difference in the proportion of girls and boys
across three score groups (chi-squared, P \ 0.001) with
more boys in the higher score groups (test for trend,
P \ 0.001) (Table 2 ).
The distribution of scores was very similar across the ages
represented (Table 3 )(P = 0.31). The differences between
the sexes was still observed (b =-0.21) and there was no
indication of an interaction between age and sex (P = 0.30).
(1) Missing data. To calculate the maximum score that
could have been received had all items been
answered, missing data were recoded to one. The
analyses were repeated using observed score for boys
and maximum score for girls, to model the most
extreme effect of missing data on any observed sex
difference in scores.
(2) ASC. Analyses were repeated using observed scores
having excluded children with a reported previous
diagnosis of an ASC.
(3) All special needs. Analyses were repeated having
excluded children with any reported special need,
medical condition, or developmental difficulty.
Sensitivity Analyses
Results
(1) Missing data
There was no change in the results using the maximum
score including missing data. The co-efficient for the sex
difference remained unchanged (b =-0.21) The number
of boys in the highest score group (C15) remained high
(n = 93 boys (78.2%); n = 26 girls (21.8%)).
Response and Data Completeness
Of 11,635 questionnaires distributed, 3,370 (29%) were
returned. 36 questionnaires were excluded: age and sex
were not reported for 8 children, 24 fell outside the study
age range, and 4 were from schools outside the study,
siblings of those invited into the study. This left 3,334
(2) ASC
37 children were reported to have a diagnosis of ASC, of
whom 27 (73.0%) were boys. Excluding these children did
not change the effects of the sex difference (b =-0.20).
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J Autism Dev Disord
Table 1 Endorsement of each question by sex (n = 3334)
Q
Question wording
Scoring
response
Boys
Girls
No Score
N(%)
Score
N(%)
Missing
N(%)
No Score
N(%)
Score
N(%)
Missing
N(%)
1
Does s/he join in playing games with
other children easily?
No
1,492
(89.5)
167
(10.0)
8
(0.5)
1,556
(93.3)
99
(5.9)
12
(0.7)
2
Does s/he come up to you spontaneously
for a chat?
No
1,606
(96.3)
53
(3.2)
8
(0.5)
1,631
(97.8)
35
(2.1)
1
(0.1)
3
Was s/he speaking by 2 years old?
Not
scored
Yes = 1,451
(87.0)
No = 209
(12.5)
7
(0.4)
Yes = 1,563
(93.8)
No = 97
(5.8)
7
(0.4)
4
Does s/he enjoy sports?
Not
scored
Yes = 1,453
(87.2)
No = 197
(11.8)
17
(1.0)
Yes = 1,473
(88.4)
No = 174
(10.4)
20
(1.2)
5
Is it important to him/her to fit in with the
peer group?
No
1,312
(78.7)
332
(19.9)
23
(1.4)
1,415
(84.9)
221
(13.3)
31
(1.9)
6
Does s/he appear to notice unusual details
that others miss?
Yes
654
(39.2)
980
(58.8)
33
(2.0)
663
(39.8)
953
(57.2)
51
(3.1)
7
Does s/he tend to take things literally?
Yes
675
(40.5)
942
(56.5)
50
(3.0)
733
(44.0)
883
(53.0)
51
(3.1)
8
When s/he was 3 years old, did s/he
spend a lot of time pretending
(e.g., play-acting being a superhero,
or holding teddy’s tea parties)?
No
984
(59.0)
668
(40.1)
15
(0.9)
1,318
(79.1)
338
(20.3)
11
(0.7)
9
Does s/he like to do things over and over
again, in the same way all the time?
Yes
1,205
(72.3)
436
(26.2)
26
(1.6)
1,230
(73.8)
411
(24.7)
26
(1.6)
10 Does s/he find it easy to interact with
other children?
No
1,476
(88.5)
167
(10.0)
24
(1.4)
1,540
(92.4)
113
(6.8)
14
(0.8)
11 Can s/he keep a two-way conversation
going?
No
1,590
(95.4)
68
(4.1)
9
(0.5)
1,622
(97.3)
37
(2.2)
8
(0.5)
12 Can s/he read appropriately for his/her
age?
Not
scored
Yes = 1,445
(86.7)
No = 209
(12.5)
13
(0.8)
Yes = 1,552
(93.1)
No = 105
(6.3)
10
(0.6)
13 Does s/he mostly have the same interests
as his/her peers?
No
1,445
(86.7)
200
(12.0)
22
(1.3)
1,486
(89.1)
163
(9.8)
18
(1.1)
14 Does s/he have an interest which takes up
so much time that s/he does little else?
Yes
1,438
(86.3)
215
(12.9)
14
(0.8)
1,579
(94.7)
79
(4.7)
9
(0.5)
15 Does s/he have friends, rather than just
acquaintances?
No
1,499
(89.9)
155
(9.3)
13
(0.8)
1,569
(94.1)
87
(5.2)
11
(0.7)
16 Does s/he often bring you things s/he is
interested in to show you?
No
1,556
(93.3)
105
(6.3)
6
(0.4)
1,599
(95.9)
64
(3.8)
4
(0.2)
17 Does s/he enjoy joking around?
No
1,588
(95.3)
71
(4.3)
8
(0.5)
1,569
(94.1)
91
(5.5)
7
(0.4)
18 Does s/he have difficulty understanding
the rules for polite behaviour?
Yes
1,396
(83.7)
250
(15.0)
21
(1.3)
1,519
(91.1)
138
(8.3)
10
(0.6)
19 Does s/he appear to have an unusual
memory for details?
Yes
892
(53.5)
745
(44.7)
30
(1.8)
1,044
(62.6)
594
(35.6)
29
(1.7)
20 Is his/her voice unusual (e.g. overly adult,
flat, or very monotonous)?
Yes
1,554
(93.2)
96
(5.8)
17
(1.0)
1,582
(94.9)
73
(4.4)
12
(0.7)
21 Are people important to him/her?
No
1,601
(96.0)
48
(2.9)
18
(1.1)
1,627
(97.6)
29
(1.7)
11
(0.7)
22 Can s/he dress him/herself?
Not
scored
Yes = 1,642
(98.5)
No = 20
(1.2)
5
(0.3)
Yes = 1,656
(99.3)
No = 5
(0.3)
6
(0.4)
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J Autism Dev Disord
Table 1 continued
Q
Question wording
Scoring
response
Boys
Girls
No Score
N(%)
Score
N(%)
Missing
N(%)
No Score
N(%)
Score
N(%)
Missing
N(%)
23 Is s/he good at turn-taking in
conversation?
No
1,285
(77.1)
349
(20.9)
33
(2.0)
1,442
(86.5)
198
(11.9)
27
(1.6)
24 Does s/he play imaginatively with other
children, and engage in role-play?
No
1,511
(90.6)
144
(8.6)
12
(0.7)
1,625
(97.5)
35
(2.1)
7
(0.4)
25 Does s/he often do or say things that are
tactless or socially inappropriate?
Yes
1,296
(77.7)
345
(20.7)
26
(1.6)
1,412
(84.7)
237
(14.2)
18
(1.1)
26 Can s/he count to 50 without leaving out
any numbers?
Not
scored
Yes = 1,422
(85.3)
No = 216
(13.0)
29
(1.7)
Yes = 1,463
(87.8)
No = 178
(10.7)
26
(1.6)
27 Does s/he make normal eye-contact?
No
1,595
(95.7)
63
(3.8)
9
(0.5)
1,644
(98.6)
14
(0.8)
9
(0.5)
28 Does s/he have any unusual and repetitive
movements?
Yes
1,533
(92.0)
121
(7.3)
13
(0.8)
1,609
(96.5)
53
(3.2)
5
(0.3)
29 Is his/her social behaviour very one-sided
and always on his/her own terms?
Yes
1,420
(85.2)
216
(13.0)
31
(1.9)
1,511
(90.6)
138
(8.3)
18
(1.1)
30 Does s/he sometimes say ‘‘you’’ or ‘‘s/he’’
when s/he means ‘‘I’’?
Yes
1,540
(92.4)
118
(7.1)
9
(0.5)
1,570
(94.2)
94
(5.6)
3
(0.2)
31 Does s/he prefer imaginative activities
such as play-acting or story-telling,
rather than numbers or lists of facts?
No
777
(46.6)
834
(50.0)
56
(3.4)
1,075
(64.5)
554
(33.2)
38
(2.3)
32 Does s/he sometimes lose the listener
because of not explaining what s/he is
talking about?
Yes
1,048
(62.9)
588
(35.3)
31
(1.9)
1,157
(69.4)
485
(29.1)
25
(1.5)
33 Can s/he ride a bicycle (even if with
stabilisers)?
Not
scored
Yes = 1,591
(95.4)
No = 70
(4.2)
6
(0.4)
Yes = 1,624
(97.4)
No = 43
(2.6)
0
34 Does s/he try to impose routines on
him/herself, or on others, in such
a way that it causes problems?
Yes
1,501
(90.0)
152
(9.1)
14
(0.8)
1,567
(94.0)
91
(5.5)
9
(0.5)
35 Does s/he care how s/he is perceived by
the rest of the group?
No
1,247
(74.8)
388
(23.3)
32
(1.9)
1,367
(82.0)
275
(16.5)
25
(1.5)
36 Does s/he often turn conversations to
his/her favourite subject rather than
following what the other person wants
to talk about?
Yes
1,174
(70.4)
469
(28.1)
24
(1.4)
1,358
(81.5)
296
(17.8)
13
(0.8)
37 Does s/he have odd or unusual phrases?
Yes
1,475
(88.5)
183
(11.0)
9
(0.5)
1,557
(93.4)
101
(6.1)
9
(0.5)
In the highest score group (C15), the number of boys (58
(80.6% of score group)) still far exceeded the number of
girls (14 (19.4%)).
need, there was a significantly higher proportion of boys
(30% of boys versus 19% of girls) (chi-squared
P \ 0.001). Having excluded children with any special
need or medical condition (n = 819) and those with
missing special needs responses (n = 26), 2,489 remained.
The median scores dropped by one in both sexes, to 4 for
boys (IQR: 3,7; range 0,28) (n = 1,162) and 3 for girls
(IQR: 2,5; range: 0,20) (n = 1,353), but there was still a
highly significant difference between the sexes (median
test, P \ 0.001). The number of boys still exceeded the
number of girls in the highest score group with 21 (81% of
group) boys and 5 (19%) girls scoring C15. The coefficient
(3) All special needs
819 children were reported to have one or more special
needs: 269 (8.1%) language delay; 27 (0.8%) ADHD; 33
(1.0%) dyspraxia; 468 (14.0%) hearing or visual difficul-
ties; 37 (1.1%) ASC; 31 (0.9%), a physical disability; 13
(0.4%) a medical condition (e.g. Down’s syndrome,
chromosomal abnormality); and 172 (5.2%) other special
needs. Of the 819 reported to have some kind of special
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