Setting and sample

We recruited participants from 12 mainstream schools and four special schools for children with Intellectual Disability (ID) in the Vellore educational district, Tamilnadu, Southern India. The schools represent the higher (Private ICSC board schools), middle (Private matriculation board schools); lower socio-economic (Public state board schools) backgrounds and they represent the literate late childhood population in India.

Sample size estimation

We calculated sample size to identify correlation (rho = 0.3) between ToM and IQ. Keeping alpha error, beta error and a priori power at 5%, 20%, and 80% respectively, the sample size needed was 84, for a two-tailed evaluation. As we anticipated 20% drop out between the two points of assessment, we recruited at least 100 children in this study.

Selection criteria

We included children of 8 to 11 years of age with their caregivers if they were willing to participate in the study. We excluded children with psychiatric morbidity, long-term physical illness, neurological deficits or disabilities, special sensory deficits, severe behavioral problems, long-term use of any medication, children who had already received prior training for ToM tasks and those unwilling to provide verbal assent or informed consent to participate in this study.

Materials

Theory of mind

The Unexpected contents Theory of Mind Task has a set of questions on the child's description of the appearance, reality, representational change and false belief variables with one point for every correct response [21]. We used this measure to screen the ability of the children to participate in further ToM assessment and to gain additional information on ToM. Detailed assessment of ToM was done using the Picture Sequencing Task (PST). The choice of non-verbal illustrative PST as the tool for ToM assessment alleviated the consequences of two major confounders, general language ability and working memory. PST measures the ToM ability by assessing false belief reasoning and general sequencing ability by employing 14 sequences, which included two practice, four false belief, four mechanical and four social script sequences. Each sequence consists of a series of four pictures made up of black and white sketches and some sample pictures are given in Figure 1. Averaging the mean scores of mechanical and social script sequences provided the General Sequencing Ability. The final score in PST is calculated by subtracting the general sequencing ability from the mean score of false belief sequences and therefore can be represented with a negative score [7]. The PST has also been used in children to study ToM by Langdon [22] who also provided us with the measure to be used in this study.

Figure 1 Sample pictures from the false belief domain in picture sequencing task. Full size image

Intelligence

The Binet-Kamat Scale of intelligence [23] is the Indian adaptation of the 1934 version of Stanford-Binet Scale of Intelligence. Some of tests, items and materials were amended to suit Indian conditions, such as Indian coins, typically Indian pictorial scenes, vocabulary and Indian concepts. This intelligence scale assessed the child's skills in nine domains: language, meaningful memory, non-meaningful memory, conceptual thinking, verbal reasoning, non-verbal reasoning, numerical reasoning, visuomotor coordination and social intelligence.

Adaptive Behavior

The Vineland Adaptive Behavior Scale (VABS) assesses the social competence of children with or without disabilities from birth to 19 years of age under four domains and 11 sub-domains [24].

Psychopathology

The Strength Difficulty Questionnaire (SDQ) has 25 items that screen for psychological strengths and psychiatric disorders among children as well as categorize individuals with low needs; some needs and high needs [25]. It identifies children with conduct disorder, hyperactivity, depressive and anxiety disorders needing various interventions [26].

Interview and assessment

APR, a qualified psychiatrist approached the children and their primary caregivers to enroll in this cross sectional study, according to the protocol approved by the hospital's Institutional Review Board. He obtained written informed consent from the primary care giver and verbal assent from the child before data collection. The interview and assessment protocol during the first day of evaluation required approximately two hours to complete. It consisted of three sections: (i) a face-to-face interview with open ended questions documenting the socio demographic data (ii) a structured assessment to screen for psychopathology using SDQ and for the ability to participate in ToM assessment using Unexpected Contents Theory of Mind task (iii) an in-depth ToM assessment with PST. During the second day of evaluation, SY, an experienced psychologist independently measured the intelligence and adaptive behavior using the BKT and the VABS respectively. Thus, we assessed 105 children of 8–11 years of age who satisfied the selection criteria and consented to participate in the study.

Data analysis

Preliminary checks of skewness and kurtosis and the one sample Kolmogorov Smirnoff tests verified that our data were suitable for parametric analysis. Firstly, we analyzed the socio-demographic data, cognitive profile, adaptive behaviors and ToM details for all the participants using descriptive statistics. Secondly, we grouped the participants into three IQ-based groups based on the conventional nomenclature [27]: below average (IQ < 90); average (IQ 90–110); and above average (IQ > 110) intelligence. We compared the groups using Chi-square tests for categorical variables and one-way ANOVA for continuous variables. Thirdly, we analyzed the linear correlation between the ToM and various domains of general intelligence using Pearson's correlation coefficient tests. Finally, we also conducted the multiple linear regression analysis with ToM as the dependant variable to account for the effects of possible confounders. All tests used two-tailed analysis and a P value of less than 0.05 was considered significant. We analyzed our data using the statistical software package, SPSS 16.0.