Clinical Psychology and People with Intellectual Disabilities

Eric Emerson, Chris Hatton, Kate Dickson, Rupa Gone, Amanda Caine, Jo Bromley

• Publisher: Wiley
• Year: 2012
• ISBN:
  9781119945291
• Edition: 2
• Topic: Psychology
• Subtopic: Clinical Psychology
• Index: Psychology

Clinical Psychology & People with Intellectual Disabilities provides trainee and qualified clinical psychologists with the most up-to-date information and practical clinical skills for working with people with intellectual disabilities.

• Represents an invaluable training text for those planning to work with people with intellectual disabilities
• Includes coverage of key basic concepts, relevant clinical skills, and the most important areas of clinical practice
• All chapters have been fully updated with the latest evidence. New chapters cover working professionally, working with people with autism and addressing aspects of the wider social context within which people with learning disabilities live.
• Beneficial to related health and social care staff, including psychiatrists, nurses, and social workers

Part 1

SETTING THE SCENE

Chapter 1

INTELLECTUAL DISABILITIES – CLASSIFICATION, EPIDEMIOLOGY AND CAUSES

Chris Hatton

INTRODUCTION

Epidemiology has been defined as ‘the study of the distribution and determinants of health, disease, and disorder in human populations’ (Fryers 1993). Although intellectual disability can be argued to be neither a disease nor a disorder, understanding the epidemiology of intellectual disability is of fundamental importance for service planning. Quite simply, to provide a needs-led service you have to know how many people with intellectual disabilities there are, what services they are likely to need, and whether there will be any changes in the need for services in the future.

However, determining the epidemiology and causes of intellectual disabilities is at best an inexact science. As ‘intellectual disability’ is socially constructed, what it means, how it is measured, and therefore who counts as having an ‘intellectual disability’ has varied over time (Trent 1995; Wright and Digby 1996) and across cultures and countries (Emerson et al. 2007; Jenkins 1998). Current professionally driven conceptualisations of ‘intellectual disability’ as largely a deficit in intelligence (Wright and Digby 1996) often have little resonance for people labelled with intellectual disability or their families (Finlay and Lyons 2005; Jenkins 1998). Therefore, before looking more closely at the literature concerning epidemiology and causes, we must first look at how people are currently classified as having an ‘intellectual disability’.

CLASSIFICATION

As mentioned above, ‘intellectual disability’ is socially constructed. The classification system used will determine who counts as having an ‘intellectual disability’, with obvious consequences when considering epidemiology and causes. In high-income English speaking countries, over the last 100 years classification systems have largely located intellectual disability as a series of deficits within the individual; typically in terms of deficits in intelligence and ‘adaptive behaviour’ (the behaviours necessary to function within society) that become apparent before cultural norms of adulthood (Emerson et al. 2007) – the so-called ‘medical model’. In more recent times, the social model of disability (where it isn’t a person’s ‘impairment’ that disables them, but the oppressive organisation of society that acts to create disability) has presented a fundamental challenge to traditional classification systems (Thomas 2007).

Classification systems have changed in different ways to meet the challenge laid down by the social model of disability. For example, the American Association on Mental Retardation (AAMR), now renamed the American Association on Intellectual and Developmental Disabilities (AAIDD), produced the most recent revision of their classification system in 2010 (AAIDD 2010), presented in Box 1.1; similar (although less precise) definitions are used by the Department of Health (Department of Health 2001). This revision still locates intellectual disability as largely a function of individual deficits, although in their guidance they do state that adaptive skills are a result of the ‘fit’ between a person’s capacities and their environment. In a supportive environment a person may be able to function perfectly well (thus not meeting the criteria for intellectual disability) – in a less supportive environment the same person may have problems and meet criteria for intellectual disability.

Box 1.1 AAIDD 2010 Definition of ‘Intellectual Disability’

‘Intellectual disability is characterized by significant limitations both in intellectual functioning and in adaptive behaviour as expressed in conceptual, social, and practical adaptive skills. This disability originates before age 18.’

OPERATIONAL DEFINITIONS:

‘Intellectual functioning: an IQ score that is approximately two standard deviations below the mean, considering the standard error of measurement for the specific assessment instruments used and the instruments’ strengths and limitations.’

‘ Adaptive behavior: performance on a standardized measure of adaptive behavior that is normed on the general population including people with and without ID that is approximately two standard deviations below the mean of either (a) one of the following three types of adaptive behavior; conceptual, social, and practical or (b) an overall score on a standardized measure of conceptual, social, and practical skills.’

Important elements of the definition:

…significant limitations …

Intellectual disability is defined as a fundamental difficulty in learning and performing certain daily life skills. There must be significant limitations in conceptual, social and practical adaptive skills, which are specifically affected. Other areas (e.g. health, temperament) may not be.

…in intellectual functioning…

This is defined as an IQ standard score of approximately 70 to 75 or below (approximately two standard deviations below the mean), based on assessment that includes one or more individually administered general intelligence tests.

…and in adaptive behavior…

Intellectual functioning alone is insufficient to classify someone as having an intellectual disability. In addition, there must be significant limitations in adaptive skills (i.e. the skills to cope successfully with the daily tasks of living.

…originates before age 18…

The 18th birthday approximates the age when individuals in this society (i.e. USA) typically assume adult roles. In other societies, a different age criterion might be more appropriate.

A more thoroughgoing attempt to incorporate social model ideas into medical model classification systems has come from the World Health Organization’s International Classification of Functioning, Disability and Health (ICF) (World Health Organization 2001). This classification system attempts to describe intellectual disability in terms of interactions between the person’s impairment (i.e. intellectual ability), their potential capacity and their actual performance across a range of activities, taking into account the person’s environmental, cultural and personal context.

Whichever classification system is used, there are a number of issues regarding classification which are likely to arise when working in services for people with intellectual disabilities.

Levels of Intellectual Disability

Although some classification systems do not define levels of intellectual disability and regard the labels attached to levels of intellectual disability as misleading (AAIDD 2010), the concept of different degrees of severity of intellectual disability is commonly used in policy and practice in the UK. These classifications are typically based on standardised IQ scores. A typical system is that of the International Classification of Diseases (or ICD), produced by the World Health Organisation:

Mild	50–70
Moderate	35–49
Severe	20–34
Profound	<20

For many purposes (such as epidemiological studies), all people with IQ<50 are classified as people with severe intellectual disabilities. While these labels of levels may assist heuristically in getting a sense of a person’s likely capabilities and support needs, they do not map reliably on to capabilities that are potentially important for the clinician, such as capacity to give informed consent or capacity to participate effectively in clinical interventions requiring significant linguistic, memory or other cognitive capabilities. There is no substitute for individual assessment of a person’s individual profile of capabilities and support needs.

Cultural and Linguistic Diversity

‘Intellectual disability’ is socially constructed, and can be regarded as a product of specific English-speaking cultures at a particular point in history (Emerson et al. 2007). This is particularly important when considering the reliance of epidemiological research on IQ tests, which can dramatically over-estimate prevalence rates of intellectual disability amongst minority ethnic communities (Hatton 2002; Leonard and Wen 2002). There are also highly likely to be cultural differences in perceptions of which behaviours are considered to be adaptive (Jenkins 1998).

Present Functioning

‘Intellectual disability’ is not necessarily a life-long trait or condition, and depending on people’s circumstances and responses to them they may not be regarded as having intellectual disabilities throughout their lives. Indeed, many people with ‘mild’ intellectual disabilities (but see AAIDD 2010) have only intermittent and time-limited contact with services, usually to assist at times of crisis.

Classification in Service Settings

Formal classification systems like the ones outlined above, with their associated assessment tools, are rarely used in existing services to make decisions about whether a person has intellectual disabilities. Also, because such assessments are made by professionals within services, decisions about whether a person has intellectual disabilities are frequently influenced by the availability of services and the professional’s judgement of what is in the best interests of the individual. Many factors can impact upon this decision; financial, political, ideological, and administrative.

Consequently, there may be people within intellectual disability services who would not meet systematic classification criteria (e.g. people who were institutionalised many years ago). It is also highly likely that there are people not in contact with intellectual disability services who would meet standard classification criteria. Services are increasingly tightening eligibility criteria to decide who is eligible for intellectual disability services and to ‘prioritise’ (i.e. ration) service provision. These eligibility criteria vary widely between different services, and use widely different methods of assessment.

EPIDEMIOLOGY

The general epidemiological literature generally has two ways of counting the number of people with a particular disorder in a given population, prevalence and incidence (see Box 1.2), although as the above discussion will have made clear this is a very inexact science when applied to people with intellectual disabilities.

Box 1.2 Definitions of prevalence and incidence

Prevalence is the number of cases, old and new, existing in a population at a given point in time or over a specified period.

Incidence refers to the number of new cases of a disorder arising in a population in a stated period of time.

(Richardson and Koller 1985)

Prevalence

Epidemiological studies of the prevalence of intellectual disabilities of children and adults across the world’s high income countries are becoming more common (see Leonard and Wen 2002; McLaren and Bryson 1987; Murphy et al. 1998; Roeleveld et al. 1997 for reviews). Prevalence estimates for the world’s middle and low income countries are more sparse and varied for a number of reasons, but rates may be higher than those found in high income countries (Durkin et al. 2006; Emerson and Hatton 2007; Maulik et al. 2011). Much of the variance in prevalence rates reported across studies can be accounted for by methodological factors, including:

1) Sampling method. Studies which use total population samples, and assess all members of a population for intellectual disability, typically report much higher overall prevalence rates than studies using administratively defined populations (i.e. those currently using services for people with intellectual disabilities or those known to services). This discrepancy is much less for studies of the prevalence of severe intellectual disabilities.

2) Classification criteria. As discussed earlier, classification systems for deciding whether a person has an intellectual disability vary over time and across different geographical areas, and different researchers have used more or less stringent criteria for classifying people with intellectual disabilities.

3) Assessment method. Reliance on IQ alone almost inevitably results in higher prevalence rates than those using IQ and adaptive behaviour assessment methods. Other factors, such as the skills of the professional conducting the assessment and the language and culture of people being tested (and those doing the testing) will all influence the prevalence rate reported.

People with Mild Intellectual Disabilities (i.e. IQ 50 or 55 to 70)

Studies of high income countries (see Leonard and Wen 2002; McLaren and Bryson 1987; Murphy et al. 1998; Roeleveld et al. 1997) report the following findings (see Durkin et al. 2006, for information on low and middle income countries):

1) General prevalence rates (i.e. across all ages) of mild intellectual disabilities from 3.7 to 5.9 per 1,000 based on administratively defined populations (i.e. those known to services), with total population studies reporting much higher prevalence rates (based on IQ asse...

Table of contents

• Cover
• Series page
• Title page
• Copyright page
• Dedication
• ABOUT THE EDITORS
• LIST OF CONTRIBUTORS
• PREFACE
• Part 1: Setting the Scene
• Part 2: General Clinical Issues
• Part 3: Working With …
• Index