The diagnostic value of the AQ

Baron-Cohen et al. [16] tested the diagnostic value of the questionnaire by comparing the results of four groups: individuals with Asperger syndrome or High Functioning Autism (AS/HFA, n = 58), controls selected randomly from 500 adults who had completed the AQ (n = 174), students of Cambridge University (n = 840) and winners of the UK Mathematics Olympiad (n = 16). Mean total AQ score in the AS/HFA group was 35.8 versus 17.6 in the student group and 16.4 in the adult controls. The discriminative qualities of the AQ were confirmed in other studies [17,18,19,20,21].

Studies using the AQ have been conducted in a variety of geographical locations and cultural contexts, including Scotland [22,23], Japan [24,25], the Netherlands [13,26,18], the USA [17,27], Canada [28,29], Italy [8] and China [30]. For example, the Japanese study [31] included 3 groups of participants: adults with AS/HFA (n = 57), an adult control group (n = 194) and students (n =1050). The results mostly confirmed those reported by Baron-Cohen [16]. Total AQ in the Japanese AS/HFA sample was 37.9 (compared to 35.8 in the British sample), 18.5 in the control group (compared to 16.4) and 20.7 in the student group (versus 17.6).

In a study conducted by Lombardo et al. [19], mean total AQ score in the group of adults with AS/HFA was 33.93, compared to 16.50 in the control group matched for age, sex and IQ. Woodbury-Smith et al. [21] have shown that the 26+ threshold score displays the best screening properties, yielding 83% of correct diagnoses. Ketelaars et al. [18] reported even lower total AQ and subscale scores for participants with AS/HFA. Mean total AQ in this relatively small sample (n = 15) was 22.5, and for non-ASD participants (n = 21) 21.8. Similarly, in one of the most recent studies, conducted by Bishop and Seltzer [17] on a group of 65 participants with AS, the mean total AQ was 24.62, i.e. lower than in earlier studies. In that particular study only 11 participants (17% of the sample) met the diagnostic cut-off of 32 proposed by Baron-Cohen et al. [16], while 24 (37%) met the screening cut-off of 26 proposed by Woodbury-Smith et al. [21].

Sex and areas of study differences in autistic traits

In the first study on the use of the AQ questionnaire, Baron–Cohen et al. [16] reported sex differences: mean total AQ score was higher in male students than in female students. Males also scored higher on all AQ subscales, with the exception of attention to detail. The authors interpreted these findings as evidence for the hypothesis that significantly more males than females in the general population demonstrate autistic traits of moderate severity. In the AS/HFA group there were no sex differences either in the total AQ nor in the subscales. This study showed also other interesting differences. Students of science scored higher than humanities and social sciences students, and students of math scored higher than other students of science. These results confirmed the hypothesis that autistic traits may be associated with scientific skills [32].

Similar sex and area of study-based differences in the AQ were found in research conducted in the Netherlands. In the first such study, Hoekstra et al. [13] tested twins aged 18 years, their siblings and their parents. The study employed an alternative scoring method for the questionnaire: instead of scoring responses as 0 or 1, a 1 to 4 points scale was used (1 for “definitely disagree”, 2 for “slightly disagree”, 3 for “slightly agree” and 4 for “definitely agree”). Males scored higher in the total AQ than females. In the parents’ group, the mean score of fathers was also higher than that of mothers. In a subsequent study [26] there were four groups of participants: university students (comprising students of humanities, social sciences and natural/technical sciences (n = 961)), a general population sample (n = 302), 18-year-old twins including their siblings (n = 117) and psychiatric patients (n = 36). A factor analysis of AQ scores yielded two factors: social interaction and attention to detail. For both factors there was a relationship between area of study and AQ scores, as well as sex x area of study interaction. Science students scored higher than students of humanities and social sciences. No effect of sex was found in the analysed faculties, although it was present in the total AQ score as well as in one of the two factors revealed in the factor analysis, namely social interaction. No such effect was found for attention to detail.

Wheelwright et al. [33] and Woodbury-Smith et al. [21] also found no sex differences in AQ scores in the AS/HFA group. In the study cited above conducted by Wakabayashi et al. [31], men scored higher than women in the students, controls and AS/HFA groups, although in the AS/HFA group the difference was not statistically significant. Austin [22] stated that in the general population sample, males scored higher than females in total AQ, social skill, communication and imagination. Thus, sex differences in AQ scores appear to be cross-cultural independent. The effect of sex was confirmed on a large sample of Japanese students by Kunihira et al. [24]. Males scored higher than females in all of the AQ subscales excepting attention to detail: this subscale displayed no main effect of sex.

Not all researchers, however, have confirmed the presence of sex differences in AQ scores. They were not found by Hurst et al. [27,34], and in the [27] study females scored even slightly higher than males in both total AQ and the majority of subscales (except imagination), although the differences were not statistically significant.

The question of sex differences in autistic traits is a common theme in research studies using the AQ questionnaire. According to the Extreme Male Brain Theory of Autism by S. Baron-Cohen [35], the mental functioning of people with autism features a number of typically male brain characteristics at extreme intensity, including poorer empathizing and better systemizing. There is a debate about the apparent disparity in the incidence of autism in males and females [36,37,38]. Exploration of sex differences in the severity of autistic traits in various groups may contribute to a better understanding of the relationship between sex and autism. Moreover, studies in different geographical locations and cultural contexts may provide valuable insight into the nature of sex differences in the context of findings indicating that particular nations may differ in the masculinity-femininity dimension [39,40].

The present study

To date, no standardized instrument which could be useful in ASD screening purposes has been made available in Poland. The diagnosis of ASD is clinically based, made in accordance with the ICD-10 criteria [2] by a psychiatrist, in cooperation with a clinical psychologist and - depending on the particular characteristics and problems experienced by the individual undergoing diagnosis – occupational therapist, neurologist, pediatrician and educator [41]. The process of adaptation and validation of the standardized methods commonly used around the world in the diagnosis of ASD, such as ADOS-2 [42], ADI-R [43] or SCQ [44] is just beginning. The preparation and validation of the AQ for the Polish-speaking population is important for both scientific and ASD screening purposes. Thus, the first aim of the study was to examine the properties of the Polish version of the AQ, particularly to estimate the internal consistency reliability, test-retest reliability and discriminative value of this questionnaire.

The second aim of the study was to explore the distribution of autistic traits in three samples derived from the Polish population, and to examine whether patterns of sex and areas of study differences reported for autistic traits measured by the AQ in the original British study [16] would be reproduced in Poland. A large sample of Polish students (n = 2819) as well as a smaller high functioning ASD group (n = 60) and controls matched in terms of sex, age, level of education and place of residence (n = 60) were tested using the Polish version of the AQ. The study procedure was modelled primarily on that of Baron-Cohen et al. [16] as well as subsequent studies conducted in other countries [26]. Taking into account that some earlier studies featured somewhat smaller groups of females with AS/HFA (e.g. in the Baron-Cohen et al. [16] study there were 45 males and 13 females, and in the Ketelaars et al. [18] study there were 12 males and only 3 females), and that some researchers made no gender-based distinctions [26], the present study (involving 39 males and 21 women) may provide additional information on sex differences in autistic traits in people with high functioning ASD.

Participants and procedure

The study included 2939 participants belonging to three groups. The first group (Students) was composed of 2819 students from nine institutions of higher education: universities, technical universities, medical universities, colleges of education and higher schools of economics in northern, central and western Poland, aged 18-30 years (M = 20.72, SD = 1.68). The Students group included 1328 males (age: M = 20.33, SD = 1.48) and 1491 females (age: M = 21.07, SD = 1.78). Participants were divided into four subgroups based on their areas of study:

• students of humanities (558 in total, 501 females and 57 males). The group included students of Polish studies, applied linguistics, law, history, French studies, classical studies, Russian studies and modern languages. Mean age of participants was 21.38 years, SD = 1.60 (M = 21.39, SD = 1.62 for females; M = 21.32, SD = 1.48 for males).

• medicine students (428 in total), including 259 females and 169 males. They were attending the faculties of medicine, dentistry and nursing. Mean age of participants was 21.53 years, SD = 2.07 (M = 21.51, SD = 2.16 for females; M = 21.56, SD = 1.94 for males).

• students of natural sciences and engineering (1496 in total), including 492 females and 1004 males. Their faculties included physics, mathematics, biology, biotechnology, food technology, ship construction, engineering (environmental, production), chemistry, mechanical engineering, electronics and telecommunications. Mean age in this group was 20.06 years, SD = 2.22 (M = 20.20, SD = 0.98 for females; M = 19.99, SD = 1.09 for males).

• students of social sciences. This group comprised 337 participants, including 239 females and 98 males. They were students of psychology, pedagogy (general and special education), resocialization and management. Mean age of participants was 21.54 years, SD = 2.22 (M = 21.70, SD = 2.21 for females; M = 21.14, SD = 2.19 for males).

The second group (high functioning ASD group) consisted of 60 high functioning individuals with ASD aged 17-44 years (M = 22.10, SD = 5.87). The group included 39 males (aged 18 to 35 years, M = 20.94, SD = 4.73) and 21 females (aged 18-44 years, M = 24.24, SD = 7.2). Participants were contacted through institutions and organisations providing assistance to individuals with ASD in Poland, as well as psychiatric clinics providing diagnoses of ASD. For each participant from this group the first inclusion criterion was a formal diagnosis of Asperger syndrome (n = 53) or childhood autism (n = 7), issued in accordance with legal regulations applied in Poland by psychiatrists, based on the ICD-10 diagnostic criteria [2]. This diagnosis is made on the basis of a clinical observation and structured interview with parent/caregiver. The interview concerns many topics related to early childhood development, later development and behavioural problems, including age at onset of developmental problems, social skills, language and communication, stereotyped and repetitive patterns of behaviour, sensory problems, the course of motor development and particular interests. Usually, a clinical psychologist is also involved in the diagnostic process. At the time of data collection no validated Polish version of ADOS, ADI-R or SCQ was available, so we could not use them as sources of information on symptoms of ASD in participants. To increase the accuracy of qualification to the ASD group, all of the subjects were assessed for presence of ASD according to ICD-10 criteria by two independent clinicians with experience in diagnosing ASD. Only these individuals who met the diagnostic criteria for ASD in the course of two independent evaluations run by two clinicians were included in the study. Moreover, subjects with IQ below average (IQ < 90) as measured by the Wechsler Adult Intelligence Scale-Revised [45], in Polish adaptation by Brzeziński et al. [46], were excluded. All subjects had successfully completed primary education at mainstream schools (9 years of education). During the study, 24 participants were attending secondary or post-secondary schools (general high schools, technical and vocational secondary schools), 16 were attending universities (areas of study included computer science, physics, finance and medicine), and 11 had graduated. Individuals suffering from depression and substance abuse were excluded. All subjects were Polish native speakers. Seven participants were permanent residents of villages, while the others lived in cities.

The third group (Controls) comprised 60 individuals from the general population (39 males and 21 females) who had not been diagnosed with ASD, selected from a larger group of 396 adults who had completed the AQ. They were matched to the participants in the ASD group in terms of four criteria: sex, age (≤ 6 month difference), level of education and place of residence (village/city). Mean age of participants was 22.57 years (SD = 6.27); for males M = 21.87 (SD = 5.70) and for females M = 24.85 (SD = 7.18).

Ethical approval was granted by an Ethics Committee of the Faculty of Psychology, University of Warsaw, prior to recruiting participants. In case of participants under 18 years of age, written consent from their parents or legal guardians was obtained. Participants aged 18 and above were informed about the voluntary and anonymous character of their participation in the study and about their right not to participate. Obtaining written consent from adult participants in non-invasive studies that involve no manipulation and are anonymous is not required in Poland.

Questionnaires with ≥10% of information missing (i.e. no responses to five or more statements) were treated as incomplete and excluded from analysis. This was the case for 68 questionnaires in the Students group and for two questionnaires in the ASD group. Since the distribution of the missing data across the items in the remaining cases (<10% of information missing) approach random distribution, and the missingness did not exceed 0.5% of the data (521 out of 146 650), the missing values have not been included in the analyses, with no further corrections. The total number of incomplete questionnaires included in the analysis was 226 in Students group (40 missing four items, 50 missing three items, 65 two and 81 one item) and 1 in ASD group (one missing item).

Instrument

Internal Consistency and Test-Retest Reliability

The internal consistency of the AQ was calculated using Cronbach’s α coefficient in the total sample. The results were thus: for total AQ = .71; social skill = .71; attention switching = .49; attention to detail = .60; communication = .50; imagination = .45. The Cronbach’s α coefficients for the total AQ scores were also assessed for each group in the study and were as follows: in the Students sample: .66, in the ASD sample: .86 and in the control sample: .75. The internal consistiencies for the subscale were estimated at: α = .70/.72/.60 (social skill), α = .62/.76/.64 (communication), α = 57/.60/.57 (attention to detail), α = .53/.73. /53 (attention switching) and α = .55/.65/.54 (imagination).

In order to assess the test-retest reliability the study was conducted on a group of 80 students. After a 1-2 month time interval the measurement was repeated. Sixty eight participants returned the AQ the second time (85%). Scores from the first and second AQ measurements did not differ statistically (t = 0.39, df = 67, p = 0.7). The Pearson r coefficients were as follows: r = .76 for total AQ, r = .78 for social skill, r = .62 for attention to detail, r = .65 for attention switching, r = .72 for communication and r = .67 for imagination.

Discriminating power of items

In order to estimate the discriminating power of items in the Polish version of AQ, the point-biserial coefficient (r_pb) was used [48]. The results of this analysis are presented in Table 2.

As shown in Tab. 2, the r_pb value of almost all items was > .3, and in two items (4 and 16) it was >.28. Thus, it can be assumed that discriminating power of each item is acceptable.

Moreover, the percentages of subjects with ASD and Controls scoring a point in each AQ item were calculated (Table 3). Only for item 29 (I am not very good at remembering phone numbers) was the percentage of subjects with ASD and Controls scoring on it the same. As for all the other items, the percentage of people with ASD was higher than Controls. This indicates that AQ was useful in distinguishing participants with ASD from Controls in this study.

Distribution of AQ scores and determining a useful cut-off

The range of total AQ scores in the ASD group was from 11 to 48 points, in the Control group from 6 to 35 points, and in the Students group from 2 to 40 points.

The distribution of total AQ scores by group is presented in Figure 1. In order to establish a useful cut-off that would discriminate the groups with as many true positives and as few false positives as possible, the distribution of cumulative rates of individual total AQ values for the ASD group, Controls and Students was analyzed (Tab. 4). As shown in Tab. 4, 32+ points (cut-off in the British study) were scored by 53.33% of participants in the ASD group, 5% in the Control group and 1.56% in the Students group. As much as 80% of the ASD group scored ≥ 25 points, which compares to less than 11.7% of the Control group and 9.22% of the Students group. This cut-off seems to have better screening properties for females (respectively: 86%, 10%, 5%) than males (72%, 13%, 14%). For females, the best score for distinguishing females with ASD from Controls and Students is a score of 28+ (respectively: 76%, 3%, 1.88%), while for males its screening properties are weaker (54%, 10%, 7.6%).

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Figure 1. Distribution of total AQ scores in the ASD group, Controls and Students.

https://doi.org/10.1371/journal.pone.0075236.g001

Group and sex differences on the AQ

A two-way ANOVA was used to compare the scores of males and females in the study group. The first factor, Group, assumed one of three values: Students, ASD group and Controls, and the second factor was Sex. To avoid Type I error, a Bonferroni correction was applied. Therefore, the alpha level was set at 0.001 for all analyses. Results are shown in Tab. 5. Post-hoc Scheffe statistics showed that the ASD group scored higher than the other groups for all measured variables. Group effect size was significantly larger than the size of the remaining effects for total AQ (η²= .11) and for the subscales. The only exception was imagination, where the effect of sex was slightly larger than the effect of group. As far as attention to detail, there was only an effect of group. There were no differences in total AQ, social skill, attention switching and imagination between Controls and Students. There were differences in attention to detail (Students scored higher) and communication (Controls scored higher). The interaction group x sex effects were so weak that we abandoned efforts at their interpretation.

The main effect of sex was revealed for five analysed variables: total AQ (males: M = 18.42, SD = 6.17; females: M = 15.57, SD = 5.60), social skill (males: M = 2.35, SD = 2.33; females: M = 1.74, SD = 1.93), attention switching (males M = 4.88, SD = 2.04; females M = 4.54, SD = 2.01), communication (males: M = 2.52, SD = 1.93; females: M = 1.75, SD = 1.65) and imagination (males: M = 3.20, SD = 1.87; females: M = 2.19, SD = 1.64).

In the Students group, males scored higher than females in five analysed variables: total AQ (t(2819) = -13.63, p < .001), social skill (t(2819) = -7.61, p < .001), attention switching (t(2819) = -4.41, p < .001), communication (t(2819) = -11.99, p < .001) and imagination (t(2819) = -15.86, p < .001). The difference for attention to detail was not significant.

The only statistically significant difference between males and females in the ASD group was found for communication (t(60) = 2.59, p < .05), in which males scored higher than females. In the Controls there were no differences between the results of males and females, although mean scores in total AQ and subscales were slightly higher for males than females.

Areas of study differences in Students group

Descriptive statistics for Students of different faculties are presented in Tab. 6. The results for males and females of different areas of study were compared to the total AQ and five AQ subscales using a one-way ANOVA. The factor, area of study, had four values: humanities, medicine, science and social science. As in the initial analyses, to avoid Type I error, a Bonferroni correction was applied and the alpha level was set at 0.001. The results of the analyses are shown in Tab. 7. As seen in the data in Tab. 7, area of study effects were present for total AQ and all subscales. Post-hoc Scheffe tests revealed that science students scored higher than all other groups in total AQ and imagination, and higher than social science and medicine students in social skill, attention switching, attention to detail and communication. The humanities students scored higher than medicine and social science students in all measured variables except imagination. It should be noted, however, that the area of study factor accounted for only 2% to 7% of variance in the measured variables.