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Executive functions and adaptive behaviour in individuals with Down syndrome

Abstract Background Previous research has explored executive functions (EFs) and adaptive behaviour in children and adolescents with Down syndrome (DS), but there is a paucity of research on the relationship between the two in this population. This study aims to shed light on the profile of EFs and adaptive behaviour in DS, exploring the differences by age and investigating the relationship between these two domains. Method Parents/caregivers of 100 individuals with DS from 3 to 16 years old participated in the study. The sample was divided into preschoolers (3–6.11 years old) and school‐age children (7–16 years old). Parents/caregivers completed either the Preschool Version of the Behaviour Rating Inventory of Executive Function (for children 2–6.11 years old) or the Second Edition of the same Inventory (for individuals 7 + years old). Adaptive behaviour was assessed with the Vineland Adaptive Behaviour Scale – Interview, Second Edition. Results Findings suggest that individuals with DS have overall difficulties, but also patterns of strength and weakness in their EFs and adaptive behaviour. The preschool‐age and school‐age children's EF profiles differed slightly. While both age groups showed Emotional Control as a relative strength and Working Memory as a weakness, the school‐age group revealed further weaknesses in Shift and Plan/Organise. As concerns adaptive behaviour, the profiles were similar in the two age groups, with Socialisation as a strength, and Communication and Daily Living Skills as weaknesses, but with a tendency for preschoolers to obtain intermediate scores for the latter. When the relationship between EFs and adaptive behaviour was explored, Working Memory predicted Communication in the younger group, while in the older group the predictors varied, depending on the adaptive domains: Working Memory was a predictor of Communication, Inhibit of Daily Living Skills, and Inhibit and Shift of Socialisation. Conclusion As well as elucidating the EF profiles and adaptive behaviour in individuals with DS by age, this study points to the role of EFs in adaptive functioning, providing important information for targeted interventions.


Abstract
Background Previous research has explored executive functions (EFs) and adaptive behaviour in children and adolescents with Down syndrome (DS), but there is a paucity of research on the relationship between the two in this population. This study aims to shed light on the profile of EFs and adaptive behaviour in DS, exploring the differences by age and investigating the relationship between these two domains. Method Parents/caregivers of 100 individuals with DS from 3 to 16 years old participated in the study. The sample was divided into preschoolers (3-6.11 years old) and school-age children (7-16 years old). Parents/caregivers completed either the Preschool Version of the Behaviour Rating Inventory of Executive Function (for children 2-6.11 years old) or the Second Edition of the same Inventory (for individuals 7 + years old). Adaptive behaviour was assessed with the Vineland Adaptive Behaviour Scale -Interview, Second Edition. Results Findings suggest that individuals with DS have overall difficulties, but also patterns of strength and weakness in their EFs and adaptive behaviour. The preschool-age and school-age children's EF profiles differed slightly. While both age groups showed Emotional Control as a relative strength and Working Memory as a weakness, the school-age group revealed further weaknesses in Shift and Plan/ Organise. As concerns adaptive behaviour, the profiles were similar in the two age groups, with Socialisation as a strength, and Communication and Daily Living Skills as weaknesses, but with a tendency for preschoolers to obtain intermediate scores for the latter. When the relationship between EFs and adaptive behaviour was explored, Working Memory predicted Communication in the younger group, while in the older group the predictors varied, depending on the adaptive domains: Working Memory was a predictor of Communication, Inhibit of Daily Living Skills, and Inhibit and Shift of Socialisation. Conclusion As well as elucidating the EF profiles and adaptive behaviour in individuals with DS by age, this study points to the role of EFs in adaptive functioning, providing important information for targeted interventions.

Background
Down syndrome (DS) is the most common genetic cause of intellectual disability. In the majority of cases, it is caused by an extra chromosome 21 (trisomy 21) (Strippoli et al. 2019). Its estimated incidence is about one per 1000-1100 population (World Health Organisation, Genomic Resource Centre 2015). Although DS is associated with general developmental delays, specific vulnerabilities have been identified in various aspects of cognition (Chapman and Hesketh 2000;Lanfranchi et al. 2010). Difficulties with executive functions (EFs) have been demonstrated (Lanfranchi et al. 2010;Daunhauer, Fidler, Hahn et al. 2014;Daunhauer et al. 2017) and found associated with adaptation in academic, home and community settings in school-aged children Daunhauer et al. 2017), and with adaptive behaviour and employment in adulthood (Tomaszewski et al. 2018).

Executive functions
The umbrella term EFs describes a set of higher order cognitive processes that are important for completing goals (Stuss and Benson 1986;Welsh et al. 1991;Zelazo et al. 1997). Several abilities have been classified as EFs, including working memory (i.e. the ability to keep information in mind and mentally work on it), shifting (i.e. the ability to transition from one task to another), planning and organisation (i.e. the ability to identify and select the steps required to obtain a goal), cognitive flexibility (i.e. the ability to switch from one cognitive framework to another), monitoring (i.e. the ability to check, update and keep track of information about more than one task and to recognise when the next step of a task or a switch to another task is required) and emotional control (i.e. the ability to experience, express and modulate emotional experiences) (Pennington and Ozonoff 1996;Miyake et al. 2000;Friedman et al. 2006). These functions are thought to be related, but distinct, as suggested by low correlations between various EF tasks (Miyake et al. 2000). Several studies have conducted laboratory tests to analyse EFs in individuals with DS (e.g. Kogan et al. 2009;Lanfranchi et al. 2010;Daunhauer and Fidler 2013). The findings suggest impairments with respect to participants' mental age in various EFs, such as verbal and visuospatial working memory, the verbal component of inhibition, shifting and planning skills, and sustained attention (e.g. Lanfranchi et al. 2010;Borella et al. 2013;Carney et al. 2013;Costanzo et al. 2013;Esbensen et al. 2019). More recently, a growing number of studies have focused on assessing EFs in daily living situations. A measure widely used in this context is the Behaviour Rating Inventory of Executive Function (BRIEF; Gioia et al. 2000) or its version for preschoolers, BRIEF-P (Gioia et al. 2003). These rating scales are completed by parents and/or teachers to assess EF-related behaviour at school and at home. The BRIEF-P was conceived for use with children aged between 2 and 5 years. Items are grouped into five subscales, assessing Inhibition, Shifting, Emotional Regulation, Working Memory and Planning/Organisation. Although it is intended for preschool-age children, several studies have also applied it to older children with DS (using it outside the normative range), judging the items more appropriate for their mental age (Lee et al. 2011;Daunhauer, Fidler, Hahn et al. 2014;Pritchard et al. 2015). These studies demonstrated that, when used with this population, the BRIEF-P is reliable, stable and sensitive to age, and it detects a profile of impairment consistent with the one found in studies using laboratory measures (Lee et al. 2011;Liogier d'Ardhuy et al. 2015). The BRIEF is the version for older children (6 to 18 years old) developed to explore types of behaviour more typical of school-age children. It includes three additional clinical subscales: Self-Monitoring, Initiation and Task Monitoring. Previous studies using the BRIEF for older children and adolescents with DS showed that it retained its psychometric properties and appropriateness when used with this population (e.g. Esbensen et al. 2019). Proxy-report measures (like the BRIEF and BRIEF-P) have the advantage of ecological validity and may serve as a more accurate measure of 'successful EF goal pursuit' in everyday life than laboratory-based, direct assessments (Toplak et al. 2013). Indirect measures of EFs also seem to be particularly appropriate for individuals with DS, avoiding methodological issues frequently encountered with laboratory tests, such as problems with understanding instructions or floor effects (Pulina et al. 2019). Previous studies using the BRIEF and BRIEF-P found an overall impairment in EFs in individuals with DS (Daunhauer, Fidler, Hahn et al. 2014;Lee et al. 2011Lee et al. , 2015Loveall et al. 2017), but also a particular profile with relative strengths and weaknesses. Studies focusing on preschoolers found this age group relatively stronger in Emotional Control and Shift and weakest in Working Memory (Loveall et al. 2017). The strongest skills in school-age children appear to be Emotional Control and Organisation of Materials, while the weakest concern Working Memory, Monitor, Plan/Organise and Shift (Daunhauer, Fidler, Hahn et al. 2014;Lee et al. 2011Lee et al. , 2015Loveall et al. 2017). Not many studies have looked at age-related differences in the EFs of individuals with DS (Lee et al. 2015;Loveall et al. 2017). In one cross-sectional study, Lee et al. (2015) explored the BRIEF profile for EFs and the effects of age in a sample of individuals from 4 to 24 years old. They found that EF difficulties in individuals with DS remain much the same throughout childhood and into young adulthood (up to 24 years old), suggesting a stable BRIEF profile over time. On the other hand, a cross-sectional study by Loveall et al. (2017) comparing preschoolers (2-5 years old) with school-age children (6-18 years old) found this profile only partially stable. In both groups, they identified Emotional Control as a relative strength, Working Memory as a weakness and Inhibit somewhere in between. The picture changed in some aspects over time, however. Shift went from being a strength in preschoolers to a weakness in the school-aged group. Plan/Organise was an intermediate-level ability in the preschool group but became a weakness in the school-age children.

Adaptive behaviour
The term 'adaptive behaviour' refers to the conceptual, practical and social skills that individuals use in their everyday lives (Schalock et al. 2010). Conceptual skills involve both receptive and expressive language, reading, writing, math reasoning and understanding the concepts of time and money. Social skills involve awareness of others' thoughts and feelings, friendship skills, the ability to respect social rules, and social judgement. Practical skills involve personal care, job responsibilities, money management and work task organisation (Schalock et al. 2010). Difficulties with aspects of adaptive behaviour are a part of the definition of intellectual disability, making adaptive behaviour a crucial dimension to consider in individuals with DS (Schalock et al. 2010).
It is important to assess adaptive behaviour to see how individuals function, and take appropriate steps to improve their autonomy, and their quality of life as a consequence. Previous studies exploring adaptive behaviour in DS used the Vineland Adaptive Behaviour Scales, Second Edition (VABS-II, Sparrow et al. 2005), which assess skills indirectlyusually by means of parental reports in the case of children. The skills considered include communication (e.g. understanding and expressing language), daily living skills (e.g. hygiene and household chores), socialisation (e.g. forming relationships and coping) and motor skills (e.g. going up and down the stairs or using scissors). When this tool has been used to assess children and adolescents with DS between 1 and 17 years old, it has identified a profile characterised by strengths in Socialisation and weaknesses in Communication and Motor Skills (Dykens et al. 2006;Will et al. 2018;Spiridigliozzi et al. 2019). A global impairment in adaptive behaviour emerges for infants and toddlers with DS (5-45 months old) by comparison with typically developing children. The former have difficulties across all domains already in the first year of life , with their standard scores declining as they grow older Spiridigliozzi et al. 2019). Young children with DS also show a deceleration in adaptive trends as they grow up, the most pronounced discrepancies between DS and typical development involving motor and communication skills . The profile remains fairly stable over time, although toddlers seem to present a more varied picture (with strengths in Socialisation and weaknesses in Communication), while 12-year-olds show a flatter profile (Van Duijn et al. 2010). The profile seems to persist through adolescence and young adulthood, before a decline in Communication occurs beyond the age of 22 (Spiridigliozzi et al. 2019).
It is well known that EFs are fundamental to planning, organising and monitoring everyday activities, and also for adaptive behaviour. Previous studies on typically developing individuals found a relationship between EFs and adaptive behaviour, demonstrating that the former contributes to the latter. For example, working memory, shifting and inhibition have been found to be related to communication (Mazuka et al. 2009;Kaushanskaya et al. 2017). Inhibitory control also relates significantly to adaptive behaviour in early childhood, and to socialisation in particular, helping an individual to avoid inadequate responses and to adjust to social norms (Diamond 2013;Benavides-Nieto et al. 2017). A relation between EFs and adaptive behaviour has also been demonstrated in individuals with autism spectrum disorders (e.g. Gilotty et al. 2002;Gardiner and Iarocci 2018) or intellectual disabilities (Gligorović and Buha 2014). To our knowledge, only one such study is available on DS (Sabat et al. 2020), which focuses on adolescents aged 12-17 years. EFs (working memory, inhibition and cognitive flexibility) were assessed using laboratory tasks, and adaptive behaviour (conceptual, social and practical) was rated by parents and teachers. Working memory predicted conceptual adaptive behaviour rated by parents, while inhibition and flexibility predicted conceptual adaptive behaviour rated by teachers. This difference may stem from the fact that different settings (home vs. school) make different demands on the child. It is important to explore these relationships in more depth to clarify which skills are worth targeting in early intervention for children with DS to improve their adaptive behaviourand their quality of life as a result.

The present study
Given the above considerations and previous literature, the aim of the present study is to shed further light on the strengths and weaknesses in the EFs and adaptive behaviour of preschoolers and school-age children with DS. The relationship between the two domains is also explored. In particular, the present study with DS poses the following research questions.  (Sabat et al. 2020), a relationship between EFs and adaptive behaviour can be expected. In addition, because adaptive behaviour requires more complex skills for school-age children than at preschool age, there are expected to be more correlations in the older group than in the younger one.
Shedding more light on the EF profiles and adaptive behaviour of children/adolescents with DS, identifying any age-related differences, and understanding the relationship between the two domains in DS can elucidate developmental patterns and support targeted interventions to improve long-term outcomes.

Participants
One hundred parents/caregivers of individuals with DS took part in the study, after giving their informed consent. They were recruited during the annual follow-up of the individuals with DS at the Unit of Neonatology of St. Orsola-Malpighi Polyclinic in Bologna, Italy. The sample was divided into two groups based on the children's age and education level: preschoolers (aged between 3 and 6.11 years) and school-age children (between 7 and 16 years old). In the school system in Italy, as in the vast majority of European countries, children start school at 6 years of age. However, as all children with DS in our country are included in mainstream schools, it is not infrequent for parents to decide, by agreement with the child's clinician, to keep a child in kindergarten a year longer to give them more time to acquire the fundamental skills needed in first grade at primary school. That is why participants in our group of school-age children were 7 or more years old. There were 40 children in the preschooler group and 60 in the school-age group. Participants' characteristics are shown in Table 1. No differences emerged between the groups in terms of sex, race, or the mothers' or fathers' education.

Behaviour Rating Inventory of Executive Function -Preschool Version
The BRIEF-P (Gioia et al. 2003) is a standardised rating scale designed to measure EFs in children 2-5.11 years old. It was completed by the parents or caregivers for the children with DS. The BRIEF-P presents a series of 63 statements regarding a child's behaviour. For each statement, parents are asked to rate how often (never = 1, sometimes = 2, or often = 3) each type of behaviour has been a problem in the previous 6 months. Higher scores indicate more severe problems. The BRIEF-P yields T-scores (M = 50, SD = 10), which are standardised scores based on the age and sex of the individual being described. There are five scales: Inhibit, Shift, Emotional Control, Working Memory and Plan/ Organise, which together give rise to three index scales: Inhibitory Self-Control (ISCI = Inhibit + Emotional Control), Flexibility (FI = Shift + Emotional Control) and Emergent Metacognition (EMI = Working Memory + Plan/Organise). A composite score is obtained as well, called the Global Executive Composite score. The BRIEF-P parent form has a good internal consistency (0.80-0.95) and a good test-retest reliability (0.78-0.90; Gioia et al. 2003).
Following a procedure already used in the field (e.g. Lee et al. 2011;Daunhauer, Fidler, Hahn et al. 2014;Pritchard et al. 2015), the BRIEF-P was used in the present study for children from 3 to 6.11 years old (preschoolers), as the items in the BRIEF-P are more appropriate than those in the BRIEF for children attending preschool. Raw scores from each of the scales and indexes were used to generate age-referenced and sex-referenced normative Tscores. In this study, CA was used to generate age-referenced T-scores. However, considering that normative data are up to 5.11 years old, for children aged between 6 and 6.11 years, T-scores were calculated referring to the normative data for the oldest age range, that is, 4-5.11 years.

Behaviour Rating Inventory of Executive Function -Second Edition
The BRIEF 2 (Gioia et al. 2000) is a standardised rating scale designed to measure EFs in individuals aged 6-18 years. The BRIEF 2 parent form consists of 86 items. The rating format and T-score norms are the same as for the BRIEF-P. The BRIEF 2 contains 36  Edgin et al. 2010;Loveall et al. 2017). There are also published studies showing correlations between the BRIEF scales and laboratory-based assessments of EFs, minimal floor performance, and adequate test-retest reliability in DS (Edgin et al. 2010;Liogier d'Ardhuy et al. 2015). Esbensen et al. (2019) demonstrated, moreover, that the BRIEF and its subscales generally perform in a psychometrically sound manner when applied to children with DS.

Vineland Adaptive Behaviour Scales, Second Edition -Survey Interview Form
The VABS-II -Survey Interview Form (Sparrow et al. 2005) is a semi-structured interview for parents/caregivers of individuals aged from birth to 90 years. It investigates adaptive behaviour across four domains: Communication, Daily Living Skills, Socialisation and Motor Skills (for ages 0-6 years only). The Communication domain contains three subdomains assessing how well an individual understands language (Receptive), produces language (Expressive) and understands how to use letters and words, as well as how to read and write (Written). The Daily Living Skills domain contains three subdomains concerning an individual's skills in eating, dressing and hygiene (Personal), household tasks (Domestic), and time and money management, technology and job-related skills (Community). The Socialisation domain contains three subdomains covering an individual's relationships (Interpersonal Relationships), recreational skills (Play and Leisure), and how an individual demonstrates sensitivity and responsibility (Coping Skills). The Motor Skills domain includes two subdomains concerning fine and gross motor skills. Items are scored on a 0-2 scale indicating the frequency with which an individual uses a given skill autonomously: usually (2), sometimes (1) or never (0). Raw scores are converted into standard scores (M = 100; SD = 15), and a composite standard score, the Adaptive Behaviour Composite, is obtained from the standard scores in the four domains.
High internal consistencies have been reported across all VABS-II domains (rs = 0.70-0.95), and a high inter-rater reliability has been reported for the Survey Interview Form (rs = 0.68-0.95).

Procedure
The present data were collected as part of a broader project aiming to explore the correlation between genotype and phenotype in DS. All participants were admitted to the Unit of Neonatology of St. Orsola-Malpighi Polyclinic in Bologna, Italy, and the study was proposed during routine annual follow-up visits for children with DS. Written consent was obtained from the participating parents/caregivers before they were interviewed in a quiet room at the Department of Developmental Psychology and Socialization in Padova, Italy, by a psychologist who first administered the Vineland-II, then participants completed the BRIEF-P or BRIEF 2 questionnaire under the psychologist's supervision, and the psychologist was available to explain any items they found unclear.

Analysis plan
Descriptive statistics, Student's t-test, repeated-measures ANOVAs and regression-based curve estimates were used in the analyses.
First, to answer the question of whether preschoolers and school-age children with DS have the same strengths and weaknesses in EFs, descriptive statistics were calculated on T-scores, and the percentages of clinically elevated scores were recorded. Student's t-tests were used to see how the children and adolescents with DS compared with the normative group to identify similarities and differences with respect to typical development. Repeated-measures ANOVAs were run to describe the profile of strengths and weaknesses in EFs separately for each group (since the BRIEF-P and BRIEF 2 scales partially differ), considering first the indices and then the scales. In this analysis, the Greenhouse-Geisser adjustment was applied to the P values (reported as P gg ) when the assumption of sphericity was violated. Post-hoc t-tests were twotailed, and the P values were corrected for the analysis of multiple comparisons using the Bonferroni method (i.e. the alpha value was divided by the number of comparisons). Cohen's d was calculated to establish the magnitude of the effects, where the rule of thumb for effect sizes was as follows: d (0.01) = very small, d (0.2) = small, d (0.5) = medium, d (0.8) = large, d (1.2) = very large and d (2.0) = huge (Sawilowsky 2009). Only the five scales that the two tools have in common (Inhibit, Shift, Emotional Control, Working Memory and Plan/Organise) were considered when comparing the EF profiles of the preschoolers and school-age children, using repeated-measures ANOVAs.
As done for EF, for the question of whether preschoolers and school-age children with DS have the same strengths and weaknesses in adaptive behaviour, descriptive statistics were calculated on standardised scores, along with the percentages of clinically elevated standardised scores for preschoolers and school-age children.
To elucidate the strengths and weaknesses in adaptive behaviour in the two groups, and to compare the two profiles, a repeated-measures ANOVA was run with Scale as the within factor and Group as the between factor.
Finally, to explore whether the relationship between EFs and adaptive behaviour is the same in preschoolers and school-age children with DS, bivariate correlations were run separately for the two groups, to examine the relationship between the EF indexes (BRIEF-P and BRIEF 2) and the children's adaptive behaviour (Vineland-II). Regression models were used to explore the combined effect of the EFs considered on adaptive behaviour. For each model, age was entered first, followed by the indexes, and each Vineland-II scale was the outcome variable. Following a procedure already used in the field (e.g. Esbensen et al. 2021), when an index was found associated with a Vineland-II scale, further analyses were run to detect which scale of the index showed the strongest association.

Executive functions
Executive functions in the preschooler group Table 2 shows the descriptive statistics, the percentages of clinically elevated T-scores (>65) and the results of Student's t-test comparing the mean Tscore of the group with the norm of 50.
Among the children with DS aged 3-6.11 years, almost half of the sample had clinically elevated T-scores for Working Memory and the Emergent Metacognition Index (47.5% and 40%, respectively). In contrast, only 5% of the sample had clinically elevated 38 T-scores obtained from the Global Executive Composite scores were significantly above the norm of 50. Of the three indexes, however, only the Emergent Metacognition Index was significantly above the norm, suggesting that the main EF deficit in DS at this age concerns Emergent Metacognition. T-scores for the five scales were also compared with the norm, using a Bonferroni correction (α 0.05/5 = 0.01). Some subscales indicated significant difficulties, suggesting a profile of strengths and weaknesses, the former in Emotional Control and the latter in Inhibit, Working Memory and Plan/Organise.
Then, the EF profiles were investigated by running two different repeated-measures ANOVAs. In one, the three indexes were dependent variables and Index was the within-group variable. In the other, the seven scales were dependent variables, and Scale was the within-group variable. T-scores were considered for both analyses. A significant effect of Index emerged (F 2,78 = 44.21, P < 0.001, η p 2 = 0.53), and subsequent post-hoc analyses showed higher scores in Emergent Metacognition than in Inhibitory Self-Control  Figure 1 shows the profiles, considering both the indexes and the scales. Table 3 shows the descriptive statistics, the percentages of clinically elevated T-scores (>65) and the results of Student's t-test comparing the mean T-score of the group with the norm of 50. For children with DS aged 7-16 years, the highest percentages of clinically elevated scores emerged for Shift (37%) and Task-Monitor (32%), and the lowest percentages being for Emotional Control (10%) and Organisation of Materials (12%).

Executive functions in the school-age group
T-scores obtained from the Global Executive Composite scores were significantly above the norm of 50, and so were those for the three indexes concerning Behaviour Regulation, Emotional Regulation and Cognitive Regulation. The T-scores for each of the nine scales were also compared with the norm of 50, using Bonferroni's correction (α 0.05/9 = 0.006). The scores were significantly higher than 50 for almost all the scales (Inhibit, Self-Monitor, Shift, Initiate, Working Memory, Plan/Organise and Task-Monitor), but not for Emotional Control or Organisation of Materials.
Here again, the EF profiles were investigated by running two different repeated-measures ANOVAs. In one, the three indexes were dependent variables, and Index was the within-group variable. In the other, the nine scales were dependent variables, and Scale was the within-group variable. T-scores were considered for both analyses.

Executive functions: profile comparison
Because the BRIEF-P and BRIEF 2 have five scales in common (Inhibit, Shift, Emotional Control, Working Memory and Plan/Organise), the EF profiles of the two groups could be compared with a 5 × 2 ANOVA, with Scale as the within factor and Group as the between factor. The main effect of Scale (F 3.38,321.41 = 45.34, P gg < 0.001, η p 2 = 0.31) and the ScaleXGroup interaction (F 3.38,321.41 = 9.73, P gg < 0.001, η p 2 = 0.09) were significant, but the main effect of Group was not.  Table 4 shows the corresponding post-hoc analyses, and Fig. 3 is a graphical representation of the data. Between-group comparisons showed a significant difference in Shift, the older group having higher scores. Within-group comparisons indicated that both groups had Emotional Control as a strength and Working Memory as a weakness, while it was only in the school-age group that difficulties also emerged in Shift and Plan/Organise.

Adaptive behaviour
Adaptive behaviour in the preschooler group Table 5 shows the descriptive statistics, the percentages of clinically low standardised scores (<70) and the results of Student's t-test comparing the mean T-scores of the group with the norm of 100.
Standardised scores obtained from the Adaptive Behaviour Composite scores were significantly below the norm of 100. Among the children with DS aged 3-6.11 years, far more than half of the sample had clinically low standardised scores on Communication, Daily Living Skills and Motor Skills (85%, 70% and 87.5%, respectively). In contrast, only 20% of the sample showed clinically low standardised scores for Socialisation.

Adaptive behaviour in the school-age group
Table 5 also shows the descriptive statistics, percentages of clinically low standardised scores (<70) and results of Student's t-test comparing the mean T-scores of this group with the norm of 100. Standardised scores from the Adaptive Behaviour Composite scores were significantly below the norm of 100. Among these older children with DS, most of the sample had clinically low standardised scores in all three domains: Communication, Daily Living Skills and Socialisation (85%, 86.7% and 70%, respectively).

Adaptive behaviour: group comparison
The two age groups were compared using a repeated-measures ANOVA, where Scale was the within factor and Group the between factor (refer to Fig. 4 for a graphical representation). The main effects of Scale (F 2,196 = 59.00, P < 0.001, η p 2 = 0.37) and Group were significant (F 1,98 = 13.06, P < 0.001, η p 2 = 0.12), and so was the ScaleXGroup interaction (F 2,196 = 4.20, P = 0.016, η p 2 = 0.04).
Post-hoc analyses on the effect of Scale showed that Socialisation was the highest score, and it differed significantly from Communication (Mdiff. = 14.06, P < 0.001, d = 0.93) and Daily Living Skills (Mdiff. = 11.15, P < 0.001, d = 0.81). Table 6 shows the post-hoc analyses run after the significant interaction. Based on Bonferroni's correction, the alpha levels were adjusted to 0.016 (i.e. 0.05/3) for the comparisons between the groups on each scale, and to 0.008 (i.e. 0.05/6) for comparisons between the scales within each group.
Between-group comparisons indicated significant differences in Daily Living Skills and Socialisation, with the younger group obtaining higher standardised scores. Within-group comparisons showed a similar picture for the two groups, with Communication and Daily Living Skills differing significantly from Socialisation, which emerged as a relative strength.

Correlations between VABS-II and BRIEF-P/BRIEF 2
The relationship between the VABS-II scales and the BRIEF-P and BRIEF 2 indexes was investigated, and the bivariate correlations between the VABS-II standard scores and the BRIEF T-scores in the two age groups are reported ( Table 7 for BRIEF-P, Table 8 for BRIEF 2).
In the younger group, the only significant (moderate) correlation that came to light was between the Emergent Metacognition Index and Communication. This correlation is explained mainly by the significant correlation between Working Memory and Communication. In the older group, on the other hand, all three indexes (Behaviour Regulation, Emotion Regulation and Cognitive Regulation), and almost all the scales correlated significantly with the three adaptive behaviour indexes (Communication, Daily Living Skills and Socialisation).

Regression analyses
Simple linear regressions were run for both groups to examine the role of EFs (BRIEF-P and BRIEF 2) on the children's adaptive behaviour (Vineland-II). For each model, age was entered first, followed by the indexes (ISCI, FI and EMI for BRIEF-P, and BRI, ERI and CRI for BRIEF 2), while each Vineland-II scale (Communication, Daily Living Skills and Socialisation) was the outcome variable. When an index emerged as a predictor, further analyses were run to detect which scale of the index was most predictive.
For the younger group, of all the EF indexes and adaptive behaviour scores, the Emergent Metacognition Index was the only significant predictor (β = À0.33, P = 0.04) for Communication. Within the subscales comprising this index (Working Memory and Plan/ Organise), Working Memory emerged as the significant predictor (β = À0.34, P = 0.03). No significant predictors were identified for Socialisation or Daily Living Skills.

Discussion
The aim of this study was to clarify the EF and adaptive behaviour profiles of children/adolescents with DS, explore any differences by age, and analyse the relationship between EFs and adaptive behaviour in this particular population. Two groups of 43 individuals with DS, one aged 3 to 6.11, the other aged 7 to 16 years, were assessed on their EFs (with the BRIEF 2/BRIEF-P) and adaptive behaviour (with the VABS-II).

Executive functions
Generalised EF difficulties were seen in individuals with DS, consistently with the previous literature (e.g. Lee et al. 2011;Loveall et al. 2017). As in other studies (Daunhauer, Fidler, Hahn et al. 2014), there was no difference between the two age groups in terms of the severity of these difficulties described by parents.  demands at school make these difficulties more obvious to parents while they might previously have gone unnoticed (Loveall et al. 2017). Overall, these findings confirm the profile described in the literature (Daunhauer, Fidler, Hahn et al. 2014;Lee et al. 2011Lee et al. , 2015Loveall et al. 2017), suggesting that some EFs remain stable over time, while others vary. It is worth noting that the results of the present study largely replicate those reported by Loveall et al. (2017), although the two studies examined slightly different age ranges due to the two countries where the studies were conducted adopting different policies on the age at which children start school. This seems to support the hypothesis that the environment, rather than age 45  Cross-cultural studies on children aged between 5 and 7 years would be especially helpful in elucidating this matter.

Adaptive behaviour
All the children with DS in the sample had standardised scores that were two standard deviations below the norm in at least one adaptive behaviour domain, and most of the children had deficits in more than one. This is in line with previous reports (e.g. Dykens et al. 2006;Will et al. 2018) and consistent with DS involving intellectual disability. Comparing the two age groups, preschoolers had higher standardised scores than school-age children with DS in Daily Living Skills and Socialisation. The lower scores seen in the older children reflect not a loss of their abilities, but a slower development of these skills than in the typically developing population. Scores for Communication were equally low in both DS groups, in line with the tendency of individuals with this syndrome to be particularly weak in language development at any age (Silverman 2007;Grieco et al. 2015). Similarities and differences also emerged in the two groups' adaptive behaviour. They were both relatively strong on Socialisation and weak on Communication and Daily Living Skills, but preschoolers scored higher for Daily Living Skills than school-age children. These results are in line with previous reports of a relative strength in Socialisation and weakness in Communication (Dykens et al. 2006;Fidler et al. 2006;Van Duijn et al. 2010;Will et al. 2018;Spiridigliozzi et al. 2019) in children with DS. More variability has been reported regarding Daily Living Skills, and it may be that environmental variables (such as living conditions or education) have a role in modulating the development of these skills.

Executive functions and adaptive behaviour
The relationship between EFs and adaptive behaviour differed in the two age groups considered here. The two domains seemed quite independent in preschoolers, with significant relationships only between Communication and Working Memory (the latter predicting the former). For school-age children, on the other hand, correlations emerged between almost all EFs and adaptive behaviour domains, emphasising the important role of EFs in everyday functioning at this age. The Emergent Metacognition Index predicted Communication, with Working Memory emerging as the significant predictor. This can be explained by the relationship between working memory and language, which has been demonstrated in both typical development (Gathercole 2006) and DS (Lanfranchi et al. 2009). The Behaviour Regulation Index, and Inhibit in particular (an area where individuals with DS are weak; e.g. Lanfranchi et al. 2010;Borella et al. 2013), predicted Daily Living Skills. In daily living activities, such as self-care, household tasks, managing money, time and technology, and coping with rules, it is important to inhibit a prepotent response in favour of a more pondered behaviour. The deficit in inhibition exhibited by individuals with DS may therefore play a part in their weak daily living skills. Moreover, Inhibit and Shift were the predictors of Socialisation. Looking at the subscales of the VABS in the socialisation domain (Interpersonal Relationships, Play and Leisure, and Coping Skills), inhibitionin the sense of our ability to monitor our behaviour, stop any inappropriate behaviour and choose a more appropriate responseplays an important part in relations with others, when playing or interacting in other ways. Porter et al. (2007) reported similar findings and associated this result with frontal lobe abnormalities. Shiftingor the ability to switch spontaneously to a different action, thought or person in response to situational changesis also essential when interacting with others. It is used when playing games, for instance, to switch from one activity to another, and when social rules have to be applied to interpret a context (which can change rapidly) and decide what behaviour to adopt. Our results are consistent with findings in typically developing samples (e.g. Benavides-Nieto et al. 2017;Kaushanskaya et al. 2017;Fogel et al. 2020), and in adolescents with DS (Sabat et al. 2020), suggesting that EFs support adaptive behaviour in DS in much the same way as in typical development.
Finally, it is worth noting the different involvement of EFs in adaptive behaviour at different ages. The preschoolers' adaptive behaviour demanded basic skills, such as understanding simple, common words, eating finger food or showing interest in others, that do not seem to be particularly supported by EFs. The older group's adaptive behaviour needed to be more complex and articulated, and this probably meant a greater involvement of their EFs.

Limitations
Some limitations of this study need to be acknowledged. First, the data reported are crosssectional, not longitudinal, and this prevents us from giving a more detailed account of how EFs, adaptive behaviour and their interaction change over time.
Comparing two cohorts might also have led to the possibility of other variables, besides age, contributing to group differences. Future work should track samples longitudinally to obtain a more precise picture of their developmental trajectories. Second, the present study relied on indirect measures, and parents may have different expectations depending on their children's age and their own mental representations of themaspects that may have influenced the information they provided. It would be helpful to replicate the findings from this study using direct measures to track the developmental trajectories of EFs in individuals with DS.

Implications
Understanding the relative strengths and weaknesses in the EFs and adaptive behaviour of individuals with DS, how they are related, and how they change over time could help us to pinpoint areas where extra support may be needed. Our results support the conviction that interventions targeting EFs are particularly important for individuals with DS, not only for their cognitive development but also to support their adaptive behaviour. For instance, knowing that EFs and adaptive behaviour are associated in school-age children suggests that it might be important to target EFs in early intervention to support the abilities that are associated with adaptive behaviour in older children. Moreover, considering the association between EFs and adaptive behaviour, it would be interesting to envisage an intervention programme that involves exercises embedded in everyday life activities to foster EFs.