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CHAPTER NINE
Epidemiology of Mental Illness and
Maladaptive Behavior in Intellectual
Disabilities
Johannes Rojahn
*
and Lisa J. Meier
*
Contents
1. Introduction 240
2. Intellectual Disability 241
3. Psychopathology in Intellectual Disability 241
3.1. Mental illness 241
3.2. Maladaptive behavior 242
3.3. Maladaptive behaviors as behavioral equivalents 242
4. Epidemiology 243
4.1. Prevalence 259
4.2. Incidence 263
5. Risk Factors and Correlates 264
5.1. Levels of intellectual disability 265
5.2. Chronological age 267
5.3. Sex 270
5.4. Comorbidity with autism spectrum disorder 271
5.5. Genetic disorders—behavioral phenotypes 272
5.6. Social–environmental factors 274
5.7. Psychological coping skills 275
6. Conclusions 276
References 280
Abstract
The following chapter discusses a literature review on the epidemiology of
psychopathology in intellectual disabilities (ID), with a focus on research
published since 2000. Substantial differences in prevalence and incidence
*
Department of Psychology, George Mason University
International Review of Research in Mental Retardation, Volume 38 Ó2009 Elsevier Inc.
ISSN 0074-7750, DOI 10.1016/S0074-7750(08)38009-4 All rights reserved.
239
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estimates across publications due to methodological discrepancies are a
concern but some of the more recent studies from Australia and some
European countries (foremost the UK) show noticeable technical improve-
ments. In summary, prevalence of global mental illness across all ID levels,
sex, and age ranged from roughly 16% to 54%. Psychopathology in children
with ID was 4.8 and 4.5 times higher than in typically developing comparison
groups. Maladaptive behavior prevalence ranged from 0.1% to 23% (aggres-
sive behavior 6% to 32%, self-injurious behavior 4% to 21%, and destructive
behavior 2% to 19%). While the presence of a mental illness does not seem
to be associated with cognitive functioning levels, maladaptive behavior is
strongly negatively correlated, especially self-injurious and stereotyped beha-
vior. Age does not affect the presence of mental illness while challenging
behaviors increase from childhood to early or mid adulthood, before dimin-
ishing as people age. While sex does not predict general mental illness,
evidence points to a male predominance in aggressive and destructive beha-
vior. Comorbid autism spectrum disorder (ASD) is a risk factor for concurrent
mental illness and challenging behaviors. There is growing evidence that
certain forms of psychopathology may be phenotypic for certain chromoso-
mal conditions. Vulnerability for psychopathology in ID, as in the general
population, is a function of complex interactions of often still poorly under-
stood factors that include neurobiological substrates, personal characteris-
tics, and socioeconomic circumstances.
1. Introduction
For centuries, psychiatric disorders in individuals with intellectual
disability (ID) were ignored. Maladaptive behaviors, especially severe
ones, of course were never overlooked but it took a long time before
the possibility of a conceptual link between some of those behaviors and
psychiatric disorders began to dawn. Indeed, mental illness was consid-
ered by some to be incompatible with ID due to insufficient ego
strength or lack of cognitive abilities (Matson, 1988a). It was arguably
not until the pivotal Isle of Wight total population prevalence study by
Rutter, Graham, and Yule (1970) that professionals and researchers
began to appreciate and acknowledge the fact that major mental illnesses
such as mood disorders, anxiety disorders, and schizophrenia could occur
alongside ID. Rutter et al.’s (at that time) surprising discovery has since
been corroborated. However, the issue of psychopathology is complex,
particularly when it is comorbid with ID. The purpose of this chapter is
to review the literature on the epidemiology of psychopathology in ID,
to summarize what we know at this point, to identify gaps in our
methodology, and to point out future directions. First, key concepts of
ID and psychopathology must be defined.
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2. Intellectual Disability
There is a broad international consensus
1
about the three core char-
acteristics of ID: (1) significantly subaverage intellectual functioning as
determined by an intelligence quotient (IQ) score of two standard devia-
tions below the mean (70–75 or below); (2) significant deficits in adaptive
functioning, and (3) the onset before the age of 18 years. Definitions do
diverge slightly as to the classification of functional levels or needed sup-
ports. The range of severity is typically defined as mild (IQ 50–55 to 70),
moderate (IQ 35–40 to 50–55), severe (IQ 20–25 to 35–40), and profound
(IQ below 20–25). Most epidemiological studies show prevalence rates of
1–1.5%. For instance, the 1994/1995 National Health Interview Survey—
Disability Supplement found a prevalence rate of ID below 1% of the U. S.
population (University of Minnesota, 2000, April).
3. Psychopathology In Intellectual Disability
In this chapter, the term psychopathology is used as an umbrella term
that covers all major forms of mental illness, as well as maladaptive
behaviors, whether or not they are symptoms of a psychiatric disorder.
3.1. Mental illness
Mental illness includes the clinical disorders (Axis I) and the personality
disorders (Axis II) of the Diagnostic and Statistical Manual of Mental Disorders,
Fourth Edition, Text Revision (DSM-IV-TR; American Psychiatric Associa-
tion, 2000). While the multiaxial DSM-IV-TR has widely accepted diag-
nostic criteria for the Axis II condition of mental retardation/intellectual
disabilities, its usefulness for diagnosing other psychiatric disorders in the ID
population has been questioned. Recently, NADD (National Association for
the Dually Diagnosed) published the Diagnostic Manual-Intellectual Disability:
a Textbook of Diagnosis of Mental Disorders in Persons with Intellectual Disability
(DM-ID; NADD, 2007). In association with the American Psychiatric
Association, NADD modified and adjusted DSM-IV-TR criteria for major
psychiatric disorders so that the diagnostic criteria are more appropriate for
individuals with ID and also suggests strategies on how to use the multiaxial
system with this special population. Its usefulness and psychometric
1
Including the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR) (American Psychiatric
Association, 2000), the Definition, Classification and System of Supports adopted by the American Association on Intellectual and
Developmental Disabilities (AAMR/AAID) (Lucksson et al., 2002), the Diagnostic Criteria for Psychiatric Disorders for Use with Adults
with Learning Disabilities/Mental Retardation (DC-LD; Royal College of Psychiatrists, 2001), the International Classification of Diseases and
Related Health Problems (ICD-10; World Health Organization, 1994), and the International Classification of Functioning, Disability, and
Health (ICF; World Health Organization, 2001).
Epidemiology of Mental Illness and Maladaptive Behavior 241
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properties are yet to be evaluated. A similar attempt had been made by the
Royal College of Psychiatrists (2001) with the development of the Diagnostic
Criteria for Psychiatric Disorders for Use with Adults with Learning Disabilities/
Mental Retardation (DC-LD), which was designed to complement the
ICD-10 for individuals with mild ID (World Health Organization, 1992).
3.2. Maladaptive behavior
As a group, individuals with ID are prone to develop some form of
maladaptive behavior during their lifetime. The most common maladap-
tive behaviors in this group include aggression, destructive behavior, self-
injurious behavior, and stereotyped behavior, oppositional behavior,
overly demanding behavior, eloping (wandering), and sexually inap-
propriate behavior. Such behaviors can escalate to a point when they
become serious safety concerns. In a clinical context, behavior problems
are often defined informally and ad hoc because terms such as “aggressive
behavior”seem to have high face validity. As we shall demonstrate,
however, it can make a difference how behavior problems are defined
when trying to determine their prevalence and their relationship to
psychiatric disorders. Especially for research purposes, it is important to
use standardized instruments and operationalized definitions (for a more
in-depth discussion on the assessment of maladaptive behavior, see
Rojahn, Hoch, Whittaker, & Gonza´lez, 2007). This is especially true
for epidemiological research.
The Royal College of Psychiatrists (2001) established criteria that need
to be met before a behavior can be diagnosed as a “behavior problem”: (1)
it has to be serious enough as to require clinical assessment and interven-
tions, (2) it is not a direct consequence of other psychiatric disorders (e. g.,
pervasive developmental disorders), drugs or physical disorders (which is
not always easy to determine), (3) it has either a significant negative impact
on someone’s quality of life, or presents health and/or safety risks, and (4) it
is persistent and pervasive across a range of personal and social settings.
3.3. Maladaptive behaviors as behavioral equivalents
One hypothesis as to why maladaptive behaviors are so prevalent in indivi-
duals with ID is that some of them may be a behavioral equivalent, or an
atypical manifestation of psychiatric disorders. The term was coined by
Sovner and Hurley (1982a,b). They argued that because individuals with
ID function differently from those of average or above intelligence, the
clinical features of their psychological illnesses could also present differently.
For example, some have suggested that self-injury, aggression, or screaming
might be behavioral equivalents of depression in individuals with severe or
profound ID (Marston, Perry, & Roy, 1997; Reiss & Rojahn, 1993). Charlot
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(2005) pointed out that, in order to decide whether a behavior should be
considered a behavioral equivalent, one has to follow the course of the
behavior and the other symptoms of the psychiatric disorder. In mood
disorders, for example, onset and duration of the behavior equivalents must
be synchronized with the episodic onset and duration of the mood shifts.
Although the idea of behavioral equivalents is intriguing and clinically
plausible, strong empirical evidence for their existence is still lacking. A few
studies have supplied data consistent with the general notion of behavioral
equivalents—namely that challenging behaviors are differentially associated
with one or more specific psychiatric conditions (e. g., Felce, Kerr, &
Hastings, 2009; Hemmings, Gravestock, Pickard, & Bouras, 2006; Holden
& Gitlesen, 2003; Kishore, Nizamie, & Nizamie, 2005; Rojahn, Matson,
Naglieri, & Mayville, 2004), while other did not find such associations
(e. g., Tsiouris, Mann, Patti, & Sturmey, 2003). But even among those
studies that found associations between behavior problems and psychiatric
conditions, considerable inconsistencies were found across studies. In addi-
tion, studies that showed the strongest association between behaviors and
specific psychiatric conditions were those in which the behaviors were part
of the diagnostic criteria for that disorder (e. g., stereotyped behavior linked
to pervasive developmental disabilities). The relationship between psychia-
tric disorders and behavior problems might be stronger for individuals with
lower levels of functioning (Felce et al., 2009). At this stage, the safest
conclusion would be that some maladaptive behaviors can be nonspecific
indicators of emotional distress associated with psychiatric conditions,
rather than atypical symptoms of specific disorders.
4. Epidemiology
Although most experts acknowledge that people with ID are vulner-
able to a wide range of psychiatric disorders, reliable epidemiological data
remain elusive. This is in part due to a host of quandaries such as (1) the
difficulty in reaching consensus on definitions and classifications of many
psychiatric disorders in individuals with ID, (2) the unique challenge of
diagnosing individuals with limited capacity to reflect upon and report their
thoughts, feelings and experiences, (3) difficulty in drawing representative
samples that allow comparisons between studies, and (4) the screening and
assessment differences across epidemiological studies that result in very
different results and make it difficult to compare outcomes. Differences
between studies can be found in sampling, assessment instruments, and data
reporting. For instance, among 23 publications that we surveyed for this
chapter, 22 different assessment instruments were used either as single
instruments or in various combinations (see Table 9.1). Striking evidence
Epidemiology of Mental Illness and Maladaptive Behavior 243
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Table 9.1 Summary of international epidemiological studies on psychopathology in intellectual disabilities since the year 2000 listed in
alphabetical order by author
Authors Reference population
and sampling procedure
Definition of the
concept and
assessment tools
Results
a
1. Cooper et al.
(2009a)
Location: Glasgow, UK
Sampling method:
Individuals with ID living
within the Greater
Glasgow Health Board
area.
Phase 1: 2002–2004
Phase 2:2004–2006
N
phase 1
=1023
N
phase 2
=651
Ages: 16 years and older
ID: mild to profound
PAS-ADD
Checklist
(screen)
PPS-LD:
•DC-LD
Point prevalence of
self-injurious behavior: 4.9%
Two-year incidence of
self-injurious behavior: 0.6%
Two-year remission rate: 38.2%
2. Cooper et al.
(2007)
b
Location: Glasgow, UK
Sampling method:
Individuals with ID living
within the Greater
Glasgow Health Board
area 2002–2004.
N
t1
=184 (time 1)
N
t2
=131 (2 years later)
Ages: 16–82 years
PAS-ADD
Checklist
PPS-LD (which
includes four
different types
of diagnosis):
•Clinical diagnosis
•DC-LD
Point prevalence of mental
illness at time1 (as a function
of diagnostic measure):
52.2%—clinical diagnosis
45.1%—DC-LD
10.9%—DCR-ICD-10
11.4%—DSM-IV-TR
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ID: profound •DCR-ICD-10
•DSM-IV-TR
Point prevalence of problem
behavior at T
1
(as a function
of diagnostic measure):
39.1%—clinical diagnosis
33.7%—DC-LD
0.0%—DCR-ICD-10
0.0%—DSM-IV-TR
Two year incidence of mental
illness (varied by measure):
13.0%—clinical diagnosis
12.2%—DC-LD
6.1%—DCR-ICD-10
2.3%—DSM-IV-TR
Two-year incidence of problem
behavior (as a function of
diagnostic measure):
6.1%—clinical diagnosis
6.1%—DC-LD
0.0%—DCR-ICD-10
0.0%—DSM-IV-TR
(Continued)
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Table 9.1 (Continued )
Authors Reference population
and sampling procedure
Definition of the
concept and
assessment tools
Results
a
3. Cooper et al.
(2009b)
Location: Glasgow, UK
Ages: 16 years and older
Sampling method:
Individuals with ID living
within the Greater
Glasgow Health Board
area.
Phase 1: 2002–2004
Phase 2:2004–2006
N
phase 1
=1,023
N
phase 2
=651
Ages: 16 years and older
ID: mild to profound
PAS-ADD
Checklist
(screen)
PPS-LD:
•DC-LD
Point prevalence of aggressive
behavior Phase 1: 9.8%
•6.3 %—physically aggressive
•3.0%—destructive
•7.5%—verbally aggressive
Two-year incidence of aggressive
behavior: 1.8%
•0.6 %—physically aggressive
•0.6%—destructive
•1.4%—verbally aggressive
Two-year remission rate: 27.7%
•31.5 %—physically aggressive
•29.4%—destructive
•32.1%—verbally aggressive
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4. Cooper et al.
(2007)
b
Location: Glasgow, UK
Sampling method:
Individuals with ID living within the
Greater Glasgow
Health Board area
between 2002 and 2004
N=1,023
Ages: 16 years and older
ID: mild to severe
PAS-ADD
Checklist
(screen)
PPS-LD (which
includes four
different types
of diagnosis):
•Clinical diagnosis
•DC-LD
•DCR-ICD-10
•DSM-IV-TR
Point prevalence of mental illness
(as a function of diagnostic
measure):
40.9%—clinical diagnosis
35.2%—DC-LD
16.6%—DCR-ICD-10
15.7%—DSM-IV-TR
Point prevalence of problem
behavior (as a function of
diagnostic measure):
22.5%—clinical diagnosis
18.7%—DC-LD
0.1%—DCR-ICD-10
0.1%—DSM-IV-TR
5. Crocker et al.
(2006)
Location: Quebec,Canada
Sampling method:
Individuals with ID receiving
services from three
rehabilitation agencies
N=3,165
Ages: 18 years and older
ID: mild to profound
MOAS (modified
to include
sexual
aggression)
Prevalence of aggressive behavior
over 12 months:
53.9%—total aggressive behavior
•25%—physical
•37.6%—verbal
•25%—property
•25%—self
•9.8%—sexual
(Continued)
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Table 9.1 (Continued )
Authors Reference population
and sampling procedure
Definition of the
concept and
assessment tools
Results
a
6. Dekker and
Koot (2003)
Location: Holland, Netherlands
Sampling method:
Random sample from schools for
children with ID in the
province of Zuid
N=474
Ages: 7–20 years
ID: borderline to moderate
DISC-IV-P
PDD-MRS
Prevalence of mental illness:
(over 1 year)
38.6%—any DSM-IV disorder
21.9%—anxiety disorder
4.4%—mood disorder
25.1%—disruptive behavior
disorder
•Any ADHD 14.8%
•ODD 13.9%
•Conduct disorder 3.0%
14.2%—more than one DSM-IV
disorder
40.7%—DSM-IV disorder
comorbid with PDD
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7. de Ruiter
et al. (2007)
Location: Holland, Netherlands
Sampling method:
Random sample from schools
for children with ID in the
province of Zuid
ID Samples:
N
t1
=953 (1997)
N
t2
=463 (1998)
N
t3=
509 (2002)
Ages: 6–18 years
Mean Ages:
T
1
=11.7 years
T
2
=12.8 yaers
T
3
=15.3 years
ID: borderline to moderate
CBCL
•Withdrawn
•Delinquent
behavior
•Social problems
•Anxious/
depressed
•Aggression
•Thought
problems
•Somatic
complaints
•Externalizing
•Attention
problems
•Internalizing
Prevalence of mental illness:
Not reported
Mean CBCL scores:
8. Einfeld et al.
(2006)
Location: Australia rural
and urban agencies
Sampling method:
Longitudinal study with
four measurements (about
every 3 years) between
(1991 and 2003)
N=507
Ages: 5–19 years (wave 1)
ID: mild to profound
DBC subscales:
•Disruptive
•Self-absorbed
•Communication
Disturbance
•Anxiety
•Social Relating
Disturbance
•Total Behavior
Problem Score
Prevalence of mental illness:
41%—significant DBC-type
psychopathology at first measure
31%—at fourth measure
(Continued)
Age in years ID Non-ID
6–8 39.1 20.5
9–11 33.2 20.2
12–14 33.0 17.7
15–18 29.2 16.6
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Table 9.1 (Continued )
Authors Reference population
and sampling procedure
Definition of the
concept and
assessment tools
Results
a
9. Emerson
(2003)
Location: Great Britain
Sampling method:
Retrospective analysis of 1999 data
files from Office for National
Statistics on mental health of
British children
N
total
=10,438
N
1
=264 with ID
N
2
=10,174 without ID
Ages: 5–15 years
ID: not specified
DAWBA
(ICD-10 and DSM-
IV criteria)
Prevalence of mental illness in
children with and without ID:
Children/adolescence with ID
39%—any psychiatric disorder
9.5%—any emotional disorder
8.7%—any anxiety disorder
1.5%—depression
25.0%—any conduct disorder
7.6%—PDD
8.7%—hyperkinesis
Children/adolescence without ID
8.1%—any psychiatric disorder
4.1%—any emotional disorder
3.6%—any anxiety disorder
0.9%—depression
4.2%—any conduct disorder
0.1%—PDD
0.9% hyperkinesis
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10. Emerson and
Hatton
(2007)
Location: Great Britain
Sampling method:
Retrospective analysis of 1999 and
2004 data files from Office for
National Statistics on mental
health of British children
N
1
=641 with ID
N
2
=17,774 without ID
Ages: 5–15 years
ID: not specified
DAWBA
(ICD-10 and
DSM-IV
criteria)
Point Prevalence of mental
illness:
36%—significant psychopathology
in those with ID
8%—significant psychopathology in
those without ID
11. Emerson et al.
(2001)
Location: District Health
Authorities in UK
Sampling method:
1. All ID service programs identified
(residential, day, respite, etc.)
2. Screening of each setting
3. Personal information on people
with maladaptive behavior
N
1995
=2,189
Age: not reported; children and
adults
ID: not reported
CBS-IS Prevalence of maladaptive
behavior:
12.1%—overall maladaptive
behaviors
•6.7%—aggressive behavior
•3.4%—self-injurious behavior
•4.0%—destructive behavior
•9.8%—other maladaptive
behavior
Prevalence data calculated to
represent percentage of total
Nscreened
(Continued)
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Table 9.1 (Continued )
Authors Reference population
and sampling procedure
Definition of the
concept and
assessment tools
Results
a
12. Gustafsson
and
Sonnander
(2004)
Location: Sweden
Sampling method:
Mental Health Services Register to
identify adults with ID also
receiving out or inpatient
psychiatric care
N=294
Ages: 18 years and older
ID: mild to severe
RSMB
PIMRA
Prevalence of mental illness
(varied by measure):
RSMB—37% of total sample
PIMRA—54% of total sample
13. Holden and
Gitlesen
(2003)
Location: Norway
Sampling method:
All lived at home or local facilities
referred for services in two
counties of Norway
N
total
=165
N
with maladaptive behavior
=105
N
without maladaptive behavior
=60
Ages: 18 years and older
ID: mild to profound
PASS-ADD
Checklist
DSM-IV
Type of Maladaptive
Behavior
•aggression
•destruction
•self injury
•other
Prevalence of mental illness:
Total sample:
21.3%—anxiety
30.0%—depression
8.4%—hypomania
15.5%—psychosis
With maladaptive behavior
27.1%—anxiety
27.1%—depression
9.4%—hypomania
19.8%—psychosis
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Without maladaptive behavior
11.9%—anxiety
32.2%—depression
6.8%—hypomania
8.5%—psychosis
14. Holden and
Gitlesen
(2006)
Location: Hedmark, Norway
Sampling method:
All people, both children and adults,
with administratively defined
mental retardation, living in
Hedmark, Norway
N=904
Ages: 0–89 years
ID: mild to profound
CBS-IS Prevalence of maladaptive
behavior:
11.1%—overall maladaptive
behavior
Forms of maladaptive behavior:
•6.4%—attacking others behavior
•4.4% self-injurious behavior
•2.3%—destructive behavior
•7.1%—other difficult, disruptive
or socially unacceptable behavior
15. Hove and
Havik
(2008a)
Location: Western Norway
Sampling method:
Identified as receiving community
based services for ID in 32
municipalities
N=593
Ages: 18 years and older
ID: mild to profound
P-AID (based on
DC-LD)
Three-month point prevalence of
mental illness:
34.9%—mental disorders
43.0%—mental disorder or behavior
problem
15.9%—anxiety disorders
12.3%—affective disorders
4.6%—psychoses
20.2%—problem behavior specifically
(Continued)
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Table 9.1 (Continued )
Authors Reference population
and sampling procedure
Definition of the
concept and
assessment tools
Results
a
16. Jones et al.
(2008)
b
Location: Glasgow, UK
Sampling method:
Individuals with ID living within the
Greater Glasgow Health Board
area 2002–2004.
N=1,023
Ages: 16–83 years
ID: mild to profound
PAS-ADD
Checklist
DC-LD criteria for
problem
behavior
Point prevalence of problem
behaviors varied by criteria:
22.5%—problem behavior by
psychiatrists’ opinion
18.7%—problem behavior by
DC-LD
17. Lowe et al.
(2007)
Location: South Wales, UK
Sampling method:
All services providers in seven unitary
authorities in the for people with
intellectual disabilities were
screened
N
1
=1,458 individuals screened with
Setting Questionnaire
N
2
=901 after attrition administered
CBS-IS (e. g., no consent)
Ages: 5–93 years
ID: not reported
CBS-IS Prevalence of serious maladaptive
behavior:
10%—of LD population had
seriously maladaptive behavior
Out of N
2
=901
32%—aggressive behavior
21%—self-injurious behavior
19%—destructive behavior
39%—disruptive behavior
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18. Morgan et al.
(2008)
Location: Western Australia
Sampling method:
Cross referenced Mental Health
Information System and
Intellectual Disability Register
N=245,749
Ages: 23–37; 38–52
ID: all levels including borderline
ICD-9
Data extracted from
registers in 2003
Prevalence of mental illness:
31.7%—with ID had a psychiatric
disorder
(1.8%—with a psychiatric disorder
had ID)
19. Myrbakk and
von
Tetzchner
(2008)
Location: Norway
Sampling method:
Administrative sample of individuals
with ID receiving support from
municipality
N
total
=181 with ID
N
1
=71 with behavior problems
N
2
=71 without behavior problems
Ages: 14–70 years
ID: Mild to profound
ABC (moderate or
severe behavior
problems)
RSMB
Mini PAS-ADD
ADD
DASH-II
Prevalence of psychiatric
symptoms in ID individuals with
and without behavior problems:
69%—psychiatric symptoms in the
group with behavior problems
29%—psychiatric symptoms in the
group without behavior problems
(Continued)
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Table 9.1 (Continued )
Authors Reference population
and sampling procedure
Definition of the
concept and
assessment tools
Results
a
20. Smiley et al.
(2007)
b
Location: Glasgow, UK
Sampling method:
Individuals with ID living within the
Greater Glasgow Health Board area.
(Follow up from Cooper, 2007)
N=651
Ages: 16 years and older, time 1
18–80 years, time 2
ID: mild to profound
PPS-LD
PAS-ADD Checklist
Clinical diagnosis
DC-LD
ICD-10
DSM-IV-TR
Two-year incidence of mental
illness varied by measure:
16.3%—clinical diagnosis
14.7%—DC-LD
8.4%—DCR-ICD-10
6.8%—DSM-IV-TR
Two-year incidence of problem
behavior varied by measure:
4.6%—clinical diagnosis
3.5%—DC-LD
0.0%—DCR-ICD-10
0.0%—DSM-IV-TR
21. Tsakanikos
et al. (2006)
Location: London, UK
Sampling method:
20-year retrospective record review
for those referred for mental health
services
N
total
=590
N
men
=295
N
women
=295
Ages: adults
ID: mild to severe
ICD-10
Psychiatric interview
Prevalence of mental illness:
49% of total sample had a psychiatric
disorder
47% of men had a disorder
•17.3% schizophrenia spectrum
•9.2% personality disorder
•7.5% anxiety
•5.8% depressive disorder
•4.1% adjustment reaction
•1.7% dementia
52% of women had a disorder
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•16.3% schizophrenia
spectrum
•4.7% personality disorder
•6.4% anxiety
•7.8% depressive disorder
•7.8% adjustment reaction
•5.8% dementia
22. Tsakanikos
et al. (2006)
Location: London, UK
Sampling method:
Individuals with ID referred to
Specialist Mental Health Service of
London
N
1
=605 ID without Autism
N
2
=147 ID with Autism
Ages: 16–84 years
ID: mild to severe
ICD-10
PAS-ADD Checklist
Prevalence of mental illness in ID
individuals with and without
Autism:
ID with Autism:
16.4%—schizophrenia spectrum
2.9%—personality disorder
4.3%—anxiety
6.4%—depressive disorder
5.0%—adjustment reaction
1.4%—dementia
ID without Autism:
18.5%—schizophrenia spectrum
9.0%—personality disorder
8.1%—anxiety
9.0%—depressive disorder
6.5%—adjustment reaction
4.2%—dementia
(Continued)
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Table 9.1 (Continued )
Authors Reference population
and sampling procedure
Definition of the
concept and
assessment tools
Results
a
23. Tyrer et al.
(2006)
Location: Leicestershire, UK
Sampling method:
Individuals with ID listed on the
Leicestershire register
N=3,065
Ages: 19 years and older
ID: mild to profound
DAS Prevalence of physical aggression
towards others was 14%
a
Unless otherwise specified, percentage in the results section stands for the percentage among those individuals with ID who were identified as cases among those who were actually screened for caseness.
b
Indicates research based on same initial database.ABC =Aberrant Behavior Checklist (Aman & Singh, 1986).
ADD =Assessment of Dual Diagnosis (Matson & Bamburg, 1998).
AIRP Assessment and Information Rating Profile (Bouras, 1995).
CBCL =Child Behavior Checklist (Achenbach, 1991).
CBS-IS =Challenging Behavior Survey—Individual Schedule (Alborz, Emerson, Kiernan, & Qureshi, 1994).
DAS =Disability Assessment Schedule (Holmes, Shah, & Wing, 1982).
DASH-II =Diagnostic Assessment of the Severely Handicapped (Matson, 1995).
DAWBA =Development and Well Being Assessment (Goodman, Ford, Richards, Gatward, & Meltzer, 2000).
DBC =Developmental Behavior Checklist (Einfeld & Tonge, 2002).
DC-LD =Diagnostic Criteria for Psychiatric Disorders for use with Adults with Learning Disabilities/Mental Retardation (Royal College of Psychiatrists, 2001).
DCR-ICD-10 =Diagnostic Criteria for Research - International Classification of Diseases Tenth Revision (World Health Organization, 1993).
DISC-IV-P =Diagnostic Interview Schedule for Children-Parent Version (DSM-IV) (Shaffer, Fisher, Lucas & Comer, 2000; Shaffer, Fisher, Lucas, Dulcan & Schwab-Stone, 2000).
DSM-IV-TR =Diagnostic and Statistical Manual of Mental Disorders: Fourth Edition-Text Revision (American Psychiatric Association, 2000).
ICD-10 =International Classification of Diseases Tenth Revision (World Health Organization, 1992)
Mini PAS-ADD =Mini Psychiatric Assessment Schedule for Adults with Developmental Disability (Prosser, Moss, Costello, Simpson, & Patel, 1997).
MOAS =Modified Overt Aggressive Scale (Kay, Wolkenfeld, & Murrill, 1988).
P-AID =Psychopathology Checklist for Adults with Intellectual Disability (Hove & Havik, 2008b).
PAS-ADD =Psychiatric Assessment Schedule for Adults with Developmental Disabilities (Moss et al., 1998).
PDD-MRS =Scale of Pervasive Developmental Disorder in Mentally Retarded Persons (Kraijer, 1997).
PIMRA =Psychopathology Inventory for Mentally Retarded Adults (Matson, 1988b).
PPS-LD =Present Psychiatric State for Adults with Learning Disabilities (Cooper, 1997).
RSMB =Reiss Screen for Maladaptive Behavior (Reiss, 1988).
Setting Questionnaire (Kiernan & Qureshi, 1986).
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of the dependence of prevalence rates on the definitions of the target
phenomena, classifications systems, and assessment instruments was pro-
vided by Cooper, Smiley, Morrison, Williamson, and Allan (2007; Study 4,
Table 9.1) and Cooper et al. (2007; Study 2, Table 9.1). Prevalence
estimates of psychiatric disorders ranged from 15.7 to 45.1%, depending
on whether DC-LD criteria (Royal College of Psychiatrists, 2001), were
used as opposed to criteria developed for the intellectually typical section of
the population (DSM-IV-TR and ICD-10), or clinical diagnoses.
Accurate diagnoses of mental illness rely heavily upon gaining access to
the patient’s covert processes such as recurrent thoughts, impulses, or
mental images (Mikkelsen, Charlot, & Langa, 2005). A patient’s ability to
report on covert events deteriorates with declining intellectual abilities and
the clinician is often left with interpreting the individual’s observable
behavior (or interpreting reports of the person’s behavior by a third
party.) As a result, psychiatric diagnoses become increasingly problematic
as the intellectual functioning of the individual diminishes (Rush &
Frances, 2000).
To review the recent epidemiological research on psychopathology in
ID, we were not aiming at an exhaustive review. Instead, we primarily
selected studies that (1) screened for a broad range of psychiatric disorders
and behavior problems (exceptions were a few excellent recent studies on
specific types of maladaptive behavior), (2) screened individuals with
unspecified ID, (3) were published in the new millennium, and (4) used
large national, regional, or total population sampling frames. We thus
identified 23 research publications (abstracted in Table 9.1) all of which
were conducted in countries with central health services registries
(Australia, Canada, Holland, Norway, Sweden, England, Scotland, and
Wales). Several manuscripts were separate reports based on the same
comprehensive database [Cooper et al., 2009a (Study 1, Table 9.1); Cooper
et al., 2007 (Study 2, Table 9.1); Cooper et al., 2009b (Study 3, Table 9.1);
Cooper et al., 2007 (Study 4, Table 9.1); Smiley et al., 2007 (Study 20,
Table 9.1)], while others used accumulating data sets at different points in
time [Tsakanikos, Bouras, Sturmey, & Holt, 2006 (Study 21, Table 9.1);
Tsakanikos et al., 2006 (Study 22, Table 9.1)]. In addition to the systematic
summary of these 23 publications presented in Table 9.1, we also discussed
other recent epidemiology studies, but without systematically abstracting
them in a table format.
4.1. Prevalence
In the context of this chapter, prevalence refers to the total number of cases
(e. g., individuals with ID who also have some form of mental illness and/
or severe challenging behavior) in the reference population (e. g., the
population of individuals with ID that was screened) at a given point in
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time. It is important to distinguish between prevalence (proportion of cases
in the reference population) and the proportion of cases with an
additional criterion (e. g., proportion of cases in a certain age group).
4.1.1. Mental illness
According to the studies presented in Table 9.1, the prevalence rate for
mental illness of any kind in individuals across all ID levels, sex, and age
ranged from 15.7% [Cooper et al., 2007 (Study 4, Table 9.1)] to 54.0%
[Gustafsson & Sonnander, 2004 (Study 12, Table 9.1)].
Table 9.2 shows the variability of prevalence estimates of overall psy-
chopathology as a function of assessment criteria. The estimates vary
drastically, depending on the criteria and assessment methods alone. This
is best illustrated by Cooper et al. (2007; Study 2, Table 9.1) with point
prevalence of mental illness ranged from a high of 52.2% when clinical
diagnoses were the criteria to a low of 11.4% when DSM-IV-TR criteria
were applied in the screening.
The prevalence rates of specific mental illness conditions also varied
widely. Anxiety disorders ranged from 1.5% (Salvador-Carulla, Rodriguez-
Blazquez, Rodriguez de Molina, & Perez-Martin, 2000) to 24.4% (Felce
et al., 2009); depression from 1.5% (Emerson, 2003) to 30.0% (Holden &
Gitlesen, 2003); psychotic disorders to 4.6% (Hove & Havik, 2008a) to
26.93% (Salvador-Carulla et al., 2000); and personality disorders from 4.5%
(Felce et al., 2009) to 9.0% (Tsakanikos et al., 2006).
4.1.2. Maladaptive behavior
Because maladaptive behavior, particularly severe aggression and self-
injury, are likely to lead to costly out-of-home placement and to have a
significant impact on the individual, accurately assessing the prevalence and
causes as well as effective treatment is critical to serving this population.
Cooper et al. (2007; Study 4, Table 9.1) found that prevalence rates for
maladaptive behavior of any kind ranged from 0.1% when DSM-IV-TR or
DCR-ICD-10 criteria were used to 22.5% when clinical diagnoses were
used. Again, estimates vary drastically, depending on the screening criteria
and assessment methods. Table 9.3 shows the variability of overall behavior
problems as a function of assessment criteria.
The range of prevalence rates for specific types of behavior problem
were as follows: Aggressive behavior ranged from 6.4% [Holden & Gitle-
sen, 2006 (Study 14, Table 9.1)] to 32% [Lowe et al., 2007 (Study 17,
Table 9.1)]; self-injurious behavior ranged from 4.4% [Holden & Gitlesen,
2006 (Study 14, Table 9.1)] to 21% [Lowe et al., 2007 (Study 17,
Table 9.1)]; and destructive behavior ranged from 2.3% [Holden & Gitle-
sen, 2006 (Study 14, Table 9.1)] to 19% [Lowe et al., 2007 (Study 17,
Table 9.1)].
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260 Johannes Rojahn and Lisa J. Meier
Table 9.2 Varying prevalence rates of overall psychopathology across studies and assessment methods
Assessment criteria
a
Authors Study # in
Table 9.1
Clinical
diagnosis
DC-LD DCR-
ICD-10
DSM-
IV-TR
DBC RSMB PIMRA
Cooper et al.
(2009a)
2 52.2 45.1 10.9 11.4
Cooper et al.
(2007)
4 40.9 35.2 16.6 15.7
Dekker and
Koot (2003)
5 38.6
Einfeld et al.
(2006)
641
Emerson (2003) 7 39 39
Emerson and
Hatton (2007)
83636
Gustafsson and
Sonnander
(2004)
10 37 54
Hove and Havik
(2008a)
13 34.9
Morgan et al.
(2008)
16 31.7
Tsakanikos et al.
(2006)
19 49
a
For the full name of the assessment criteria or instruments, see the legend in Table 9.1.
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Table 9.3 Varying prevalence estimates of problem behaviors across studies and assessment methods
Assessment criteria
a
Authors Study # in
Table 9.1
Clinical
Diagnosis
DC-LD DCR-ICD-10 DSM-
IV-TR
CBS-IS
Cooper et al.
(2009a)
2 39.1 33.7 0 0
Cooper et al.
(2007)
4 22.5 18.7 0.1 0.1
Emerson et al.
(2001)
9 12.1
Holden and
Gitlesen (2006)
12 11.1
Hove and Havik
(2008a)
13 20.2
Jones et al. (2008) 14 22.5 18.7
Lowe et al. (2007) 15 10
Smiley et al.
(2007)
18 4.6 3.5 0 0
a
For the full name of the assessment criteria or instruments, see the legend in Table 9.1.
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4.2. Incidence
Incidence in this context means the number of newly emerging cases of
mental illness or maladaptive behavior during a given time period in
individuals with ID.
4.2.1. Mental illness
To our knowledge, there are no lifelong incidence data on psychopathol-
ogy in ID. For obvious reasons of cost and practicality, longitudinal inci-
dence studies on mental illness in ID are very rare and, those that do exist
cover relatively short time periods. For instance, Smiley et al. (2007; Study
20 in Table 9.1) used a prospective cohort design to measure the incidence
of episodes of mental illness in adults with ID over a 2-year period in
Greater Glasgow, U. K. Incidence of any type of psychopathology varied
between 6.8% when DSM-IV-TR criteria were used to16.3% when clin-
ical diagnoses were considered. It is unclear, however, whether these
estimates referred to newly emerging cases, or active episodes of mental
illness.
Although not a true incidence study, Einfeld et al. (2006; Study 8,
Table 9.1) followed a group of 578 children and adolescents with ID
between 1991 and 2003 as part of the Australian Child to Adult Development
Study. Data were collected at four points in time and complete data were
obtained for 507 individuals. At the first measurement, point prevalence of
psychopathology was 41%. Interestingly, prevalence subsequently
decreased to 31% at fourth measure, despite the fact that only 10% of
individuals had received any mental health treatment during that time.
More specifically, of the five subscale scores of the Developmental Beha-
vior Checklist (DBC; Einfeld & Tonge, 2002), four decreased over time
(disruptive behavior, self-absorbed, communication disturbance, and anxi-
ety). The severity of psychopathology decreased more in boys than in girls
(boys started with higher baseline scores) and decreased more in individuals
with mild ID compared to those with severe or profound ID. Although the
prevalence of psychopathology decreased slowly over time, for most it
persisted well into young adulthood. Another important discovery was
that the overall seriousness of psychopathology was consistent across the
range of ID.
De Reuter, Dekker, Verhulst, and Koot (2007, Study 7, Table 9.1)
conducted a longitudinal, multiple-birth-cohort study on a large sample
of children and adolescents ages 6–18 years using a Dutch version of
the Child Behavior Checklist (CBCL; Achenbach, 1991) comparing chil-
dren with and without ID. Children with ID had higher CBCL
scores on all subscales than typically developing children across all age
groups.
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At this point, our knowledge about the incidence of psychopathology
in individuals with ID is still rudimentary. Longitudinal studies such as the
one described by Smiley et al. (2007) are needed, preferably studies that are
conducted over longer time periods, involving young children that would
allow us to learn about first onset of mental illness in ID. The findings of
Einfeld et al. (2006) and de Ruiter et al. (2007) of a decrease of psycho-
pathology in children and adolescents with ID over time are interesting but
waiting to be replicated.
4.2.2. Maladaptive behavior
Smiley et al. (2007; Study 20, Table 9.1) also examined the incidence of
problem behaviors in adults with ID and found again a considerable varia-
tion as a function of the assessment criteria. Over a 2-year period, incidence
of problem behaviors was reported as ranging from 0% (with DCR-ICD-
10 or DSM-IV-TR) to 3.5% (according to DC-LD criteria), and finally
4.6% (when determined by clinical diagnosis). The study by de Reuter
et al. (2007, Study 7, Table 9.1) is also relevant for the issue of maladaptive
behavior. While children with ID had higher overall CBCL scores than the
control children, the CBCL score differences decreased over time, most
noticeably in social problems and aggression.
Again, more longitudinal research, especially starting at a very early age,
will not only help to identify the process of early development of mala-
daptive behavior (e. g., Richman & Lindauer, 2005; Symons, Sperry,
Dropik, & Bodfish, 2005) but also guide early intervention to prevent the
development of maladaptive behavior (Richman, 2008).
5. Risk Factors and Correlates
In any field of human health and well-being, a main interest is to
identify risk factors that may contribute to later pathology. However, there
is often confusion about the correct terminology of related terms “risk
factors,”“correlates,”and “risk markers”(Witwer & Lecavalier, 2008).
Kazdin, Kraemer, Kessler, Kupfer, and Offord (1997) defined a risk factor
as a precursor event that is associated with a higher probability (risk) of a
certain outcome over the probability of that outcome in individuals who
were not exposed to that event. Risk factors have a causal relationship with
an outcome. Identifying a risk factor is often accomplished first by recog-
nizing a correlational relationship of an event with the outcome (correlate)
when measured at the same point in time. Evidence that an event is not
only correlated with the outcome but is causing it (i. e., that it is a risk
factor) has to be established by longitudinal studies which demonstrate that
the event actually predicted the later outcome. The literature on
264 Johannes Rojahn and Lisa J. Meier
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psychopathology in ID has only a few established risk factors in that
stringent sense. Risk markers, on the other hand, are biological or neurop-
sychological traits that indicate a genetic tendency toward developing an
outcome (Compton, 2005). Risk markers will be discussed in the section
on behavioral phenotypes.
Even though prevalence rates for psychiatric and problem behaviors are
not consistent across studies, the general impression is that individuals with
ID show a higher rate of psychiatric and behavior disorders than the general
population [e. g., Emerson, 2003 (Study 9, Table 9.1)]. There are multiple
possible biological, psychological, developmental, emotional, social, and
environmental factors that may contribute to this increased prevalence,
either uniquely or in interaction with other factors. Harris (2006) proposed
that it is the combined and interactive nature of a number of risk factors and
markers that contribute to psychopathology in ID, including neurobiolo-
gical causes of both psychopathology and ID; cognitive impairments lead-
ing to fewer adaptive coping responses; poor academic performance that
contributes to low self-esteem; unrewarding peer relationships; the amount
of stress that the care of an individual with ID can put on the family;
increased abuse and exploitation; and the lack of appropriate diagnosis and
treatment. More rarely, there may also be a negative circular relationship
among severe psychosocial deprivation, poor nutrition, and lack of stimu-
lation leading to further cognitive, social, and psychological impairment.
This notion is also reflected in the generalized model of dynamic influences
on individual health, adaptation and development as shown in Fig. 9.3.
Although we realize the complex interactive nature of risk factors and
risk markers and the difficulty of isolating the impact of single risk factors,
we will discuss one by one the available evidence for some salient factors
such as level of functioning, age, and sex.
5.1. Levels of intellectual disability
Cognitive functioning as a potential risk factor of psychopathology is
highly associated with language development, communication, coping
strategies with stress, and other adaptive behavior skills and other critical
developmental variables. Although some have deemphasized the impor-
tance of ID levels as a dimension of classification (Luckasson et al., 2002),
it is one of the most robust predictors of developmental outcome in the
population with ID.
5.1.1. Mental illness
With regard to mental illness in general, there is considerable inconsistency
in the literature about the effects of the ID functioning level. One group of
studies showed that cognitively higher functioning individuals with ID
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were more likely to have a mental illness diagnosis than lower functioning
individuals [for instance, Morgan, Leonard, Bourke, & Jablensky, 2008
(Study 18, Table 9.1); Myrbakk & von Tetzchner, 2008b (Study 19,
Table 9.1); Rojahn & Esbensen, 2005]. Koskentausta, Livanainen, and
Almqvist (2007), on the other hand, found a curvilinear relationship
between the presence of mental illness and level of functioning: the risk
of psychopathology was higher in children with moderate ID as compared
to mild or profound ID. Probably the most convincing series of epidemio-
logic studies in this field has been conducted by Cooper and her colleagues.
They found no evidence that level of ID in adults was related to mental
illness (when autism and behavior problems were excluded). The group
with mild ID had the same prevalence of mental illness of 22.4% as the
group with moderate to profound ID [Cooper et al., 2007 (Study 4,
Table 9.1)] (see also Fig. 9.1).
Obviously, more research needs to be conducted to clarify the relation-
ship between level of functioning and mental illness in general as well
specific disorders. One important concern is, as we had mentioned, that
the reliability of psychiatric diagnoses decreases with declining levels of
function (Rush & Frances, 2000). It may also be that mental illness in the
lower functioning portion is manifested mostly by atypical symptoms (i. e.,
behavioral equivalents). Since the concept of behavioral equivalents is still
conceptually vague and lacks strong empirical support, many cases of
mental illness may still be overlooked in the lower function group of
individuals with ID.
0
5
Problem behavior Mental illness
(excluding behavior
problems and autism)
Mild ID (n = 389)
Moderate to profound ID
(n = 625)
10
15
20
25
30
Figure 9.1 Prevalence of problem behaviors and clinical diagnoses of mental illness
as function of level of intellectual dis abiliy (figure based on data presented by Cooper
et al., 20 07).
266 Johannes Rojahn and Lisa J. Meier
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5.1.2. Maladaptive behavior
While the evidence is still patchy as to whether and to what extent the
intellectual functioning level may present a risk factor for mental illness,
the picture is clearer with regard to behavior problems (Rojahn & Tasse´,
1996; Witwer & Lecavalier, 2008). A host of studies show that level of
ID is strongly correlated with the prevalence of challenging behaviors in
general [e. g., Cooper et al., 2009a (Study 1, Table 9.1); Cooper et al.,
2007 (Study 4, Table 9.1); Holden & Gitlesen, 2003 (Study 13,
Table 9.1); Jones et al., 2008 (Study 16, Table 9.1)]. Figure 9.1 shows
the differential relationship between level of functioning and the pre-
valence of mental illness versus the prevalence of maladaptive behavior
that was discovered using the same general methodology by Cooper
et al. (2007; Study 4, Table 9.1).
There is also some evidence that the relationship between level of
functioning and problem-behavior prevalence may differ depending upon
the type of behavior problem. For instance, Emerson et al. (2001) and
Cooper et al. (2009b, Study 3, Table 9.1) found that outwardly directed
aggression was more likely to be shown by individuals with less severe
disabilities. Holden and Gitlesen (2003; Study 13, Table 9.1) also found that
aggression against others and temper tantrums was more common among
those with mild to moderate disability compared to those with more severe
disability while self-injurious behavior was more common in those with
profound ID. Yet these findings are offset by the results presented by
Crocker et al. (2006; Study 5, Table 9.1) and Tyrer et al. (2006; Study
23, Table 9.1) that individuals with severe/profound ID were more likely
to engage in aggression than those with mild/moderate ID.
Figure9.2showstheprevalenceofdifferenttypesofsevereproblem
behavior by level of functioning in a population of 135,282 individuals (ages
0–45 years of age) who received developmental disabilities services from the
states of New York and California (described in Rojahn, Borthwick-Duffy, &
Jacobson, 1993). It illustrates that prevalence of maladaptive behavior in
general was positively correlated with level of functioning, but that aggressive
behavior, relative to self-injury for instance, was more prevalent in the cohort
with mild ID, and less prevalent in those with profound ID.
5.2. Chronological age
The next question we want to address is to what extent chronological age
might be a potential risk factor in ID.
5.2.1. Mental illness
While there have been studies published on the prevalence of mental illness
in ID at different age levels, we have not found a study that compared
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prevalence risks across the lifespan. A study by Dykens, Shah, Sagun, and
King (2002) did examine changes in mental health problem behaviors in a
group of 211 children and adolescents with Down syndrome. They found
that of four age groups (4–6, 7–9, 10–13, 14–19) internalizing behaviors,
and specifically withdrawn behaviors, were significantly higher in the age
groups 10–13 and 14–19 years compared to younger children. In the 14–19
age group, age and gender interacted significantly such that females more
than males scored significantly higher in this age group. Externalizing
behaviors were more prevalent in the 10- to 13-year-old age group.
Age was not found to be a predictor of mental illness prevalence in
adults [Cooper et al. 2007 (Study 2, Table 9.1)] or in children [Dekker &
Koot, 2003 (Study 6, Table 9.1)]. In an institutional sample of ID adults 18
years and older, the highest prevalence of mental illness was found in the
broad age span of 51–94 years, with 77% of those identified with mental
illness coming from this age group. Moss and Patel (1993) found that
prevalence of mental disorders was 11.4% in people age 50 years and up
with moderate to profound ID.
Two studies were found that included age as a factor for prevalence of
specific psychiatric disorders in children. In one study [Emerson, 2003 (Study
9, Table 9.1)] children under the age of 11 years were more likely to be
diagnosed with a pervasive developmental disorder, whereas children age 11
and over were more likely to be diagnosed with depression. In another study
[Emerson & Hatton, 2007 (Study 10, Table 9.1)], children between the ages
0
5
Mild Moderate Severe Profound
Aggression
Destruction
SIB
Stereotypy
10
15
20
25
30
Figure 9.2 The prevalence of different types of problem behaviors by level of
functioning (n=135,282; from state agency administrative data sets from New York
and California described in Rojahn et al., 1993).
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of 5 and 10 years were more likely to be diagnosed with “hyperkinesis”
compared to children age 11–16 years, who were more likely to be diag-
nosed with an emotional disorder. De Reuter et al. (2007) found that while
children with ID demonstrated a greater risk for psychopathology on a parent
rating scale compared to children without ID across all ages, the develop-
mental course between ages 6 and 18 was similar. Interestingly, attention
problems decreased more sharply in children with ID, and in males, social
problems decreased more sharply in children with ID.
5.2.2. Maladaptive behavior
Emerson et al. (2001; Study 11, Table 9.1) studied the age distribution of
maladaptive behavior in two subsequent surveys. In their 1988 survey,
the majority (64%) of the individuals with challenging behavior were
between the ages of 12 and 35 and in the 1995 sample 60% were in that
age bracket. In the 1995 survey sample, prevalence increased up until and
then peaked in the mid-teens, while it increased until the mid-twenties
(1988 sample) before leveling off. Lowe et al. (2007; Study 17, Table 9.1)
found that the greatest proportion of individuals with severe maladaptive
behavior were young adults between 16 and 39 years (40%), followed by
those between 40 and 59 (34%) years, 17% were children younger than
16 years, 9% were 60 years or older. Compared to Emerson et al. (2001),
however, Lowe et al. (2007) found the peak prevalence of general
maladaptive behavior to occur at a somewhat older age (between the
ages of 30 and 60 years).
Holden and Gitlesen (2006; Study 14, Table 9.1) distinguished between
more or less severe problem behaviors and found an interaction effect
between age and behavior problem severity: Demanding challenging beha-
vior increased between the ages of 10 and 20 years and was highest between
the ages of 20 and 40 years. After that the prevalence of demanding
maladaptive behavior decreased. Less demanding maladaptive behavior,
however, was most common in individuals under 20 years, with little
change after that. Maladaptive behavior of any severity was least common
in individuals over 60 years of age. de Ruiter et al. (2007) found that
aggressive behavior in boys with ID decreased sharply between ages 6 and
18 years.
Chronological age clearly is related to the prevalence of general
maladaptive behavior, but like with most correlates or risk factors the
available data vary. One constant finding is that prevalence of challenging
behavior increase until early or mid adulthood, and slowly decreases into
old age—a trend that has been confirmed by several earlier studies that
are not presented in Table 9.1, such as Borthwick (1994),
Oliver, Murphy, and Corbett (1987), Rojahn et al. (1993), and Saloviita,
2000).
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5.3. Sex
5.3.1. Mental illness
Sex as a potential risk marker has been examined in some studies. Cooper
et al. (2007; Study 4, Table 9.1) reported that the prevalence rates of mental
illness of any type based on clinical diagnosis (excluding maladaptive
behavior) were 29.2% for men and 27.3% for women.
Tsakanikos, Bouras, Sturmey, and Holt (2006; Study 21, Table 9.1) found
sex differences in psychiatric comorbidity in adults with mild to severe ID,
both in terms of specific diagnoses and in patterns of referral for specialized
mental health services. Based on ICD-10 criteria, personality disorder was the
most common diagnosis among men, whereas dementia and adjustment
reaction were most common among women. No statistically significant sex
differences were found in depression, anxiety, or schizophrenia.
Lunsky (2003) interviewed 51 men and 48 women with borderline
intellectual functioning through moderate ID and found that women
reported significantly higher levels of depression than men on the adapted
Birleson Depression Scale (BDS; Birleson, 1981), which is consistent with the
trend in the general population. However, when depression was assessed by
a rater using the Reiss Screen for Maladaptive Behavior (RSMB; Reiss, 1988)
and the informant version of the BDS, the difference between men and
women was not significant. Informant ratings were significantly lower than
self ratings for men and for women. The difference between self and other
report scores may suggest that observers were less able to accurately assess
internal symptoms of depression than are the individuals themselves, even
in relatively high functioning individuals.
Witwer and Lecavalier (2008) concluded from their literature review
that adolescent girls had more internalizing symptoms than males, but these
sex differences were not seen in younger children. As for Attention-Deficit/
Hyperactivity Disorder (ADHD), males tend have higher scores than
females on behavior rating scales. In other words, most of the research
that examined the effects of sex on the prevalence of mental illness in ID
seems to be consistent with the relationship between sex and mental illness
in the general population.
5.3.2. Maladaptive behaviors
McClintock, Hall, and Oliver (2003) summed up their meta-analysis by
concluding that men were more likely to show aggression than females.
Likewise, based on their literature review, Witwer and Lecavalier (2008)
surmised that boys and male adolescents were more likely to show dis-
ruptive, antisocial and problem behaviors than their female counterparts.
Male predominance in challenging behavior in general was also found by
Lowe et al. (2007; Study 17, Table 9.1), while Tyrer et al. (2006; Study 23,
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Table 9.1) found that men were more likely to engage in physical aggres-
sion than women. Crocker et al. (2006; Study 5, Table 9.1) found sex
differences in adults with mild to profound ID for certain types of aggres-
sion, but no sex differences in verbal and physical aggression. Men were
found to be more likely to show property destruction and sexual
aggression.
However, other credible studies found opposite sex effects on maladap-
tive behavior. For instance, Cooper et al. (2009a; Study 1, Table 9.1) and
Cooper et al. (2009b; Study 3, Table 9.1) reported that being female
predicted aggressive and self-injurious behavior. Jones et al. (2008; Study
16, Table 9.1) also found that being female predicted problem behaviors.
Hemmings et al. (2006) challenged the notion of sex contributing to the
prediction of any challenging behaviors altogether (aggression, destruction,
self-injury, tantrums, etc.).
In other words, the question of whether sex is a risk marker for
maladaptive behavior has no clear answer as of yet.
5.4. Comorbidity with autism spectrum disorder
Autism spectrum disorders (ASDs) are a group of complex developmental
disabilities characterized by problems with social interaction and
communication, as well as by routines or repetitive behaviors.
5.4.1. Mental illness
An earlier literature review suggested that ASD was correlated with an
increased risk for psychopathology (Witwer & Lecavalier, 2008) and more
recent studies are still consistent with that summary. For instance, inpatients
with ASD and an ID were found to have more mood, anxiety, and other
related symptoms than inpatients with ID but without ASD (Charlot et al.,
2008). Higher rates of psychiatric symptoms in individuals with ID combined
with ASD were found in other studies as well (Hill & Furniss, 2006; LaMalfa
et al., 2007). Hill and Furniss (2006) and LaMalfa et al. (2007) found that
individuals with pervasive developmental disorder/autism had higher scores
on subscales for organic disorder, anxiety, and mania of the Diagnostic Assess-
ment for the Severely Handicapped-II (DASH-II; Matson, 1995). The more
severe the autistic symptoms, the higher the DASH-II subscale scores were.
Tsakanikos et al. (2006; Study 22, Table 9.1), however, found no evidence
that adults with ASD had an increased risk for psychopathology.
5.4.2. Maladaptive behaviors
As stated above, repetitive actions are core symptoms of ASD and are also
referred to as stereotyped behavior, which can also include self-injurious
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behavior. Therefore, since stereotypies and self-injury are by definition
associated with ASD we would expect higher levels of those types of in
individuals with a diagnosis of ASD and ID as compared to individuals with
ID without autism. Several studies have been conducted to investigate
whether individuals with ASD do indeed have higher levels of maladaptive
behavior than those without. Bodfish, Symons, Parker, and Lewis (2000)
compared matched groups of adults with ID with or without autism.
Although the difference was not statistically significant, there was a trend
for the group of individuals with ASD to have a higher proportion of self-
injurious behavior cases (45%) compared to the non-ASD group (25%). In
2006, Hill and Furniss found that children and young adults with severe ID
and with ASD had higher scores on the DASH-II subscales for stereotypies
than the comparison group without ASD. Felce et al. (2009, p. 249) also
found that “the triad of social impairment characteristic of autism spectrum
disorder”contributed independently to the prediction of maladaptive
behavior, and Matson, Wilkins, and Macken (2009) reported that the
severity of ASD was positively related to the number and the intensity of
challenging behaviors. Those with severe ASD exhibited significantly
higher levels of problem behaviors (including self-injury, stereotypies,
aggression, and destruction) than children with mild or moderate ASD.
Matson, Ancona, and Wilkins (2008) also identified increased sleep dis-
turbance in individuals with both ID and ASD as compared to ID alone and
Ashworth, Martin, and Hirdes (2008) found that individuals with ASD had
a higher rate of pica than those with ID alone.
In contrast, no relationships between self-injurious behavior and ASD
were found by Cooper et al. (2009a; Study 1, Table 9.1) nor between
aggressive behavior and ASD by Tyrer et al. (2006; Study 23, Table 9.1).
Lowe et al. (2007; Study 17, Table 9.1) did not find relationships between
any problem behaviors and ASD.
5.5. Genetic disorders—behavioral phenotypes
The concept of the behavioral phenotype was introduced by Nyhan (1972;
cf. Harris, 2006) in his presidential address to the Society for Pediatric
Research and refers to the observation that specific chromosomal, genetic,
or neurodevelopmental disorders may be associated with particular patterns
of behavior.
5.5.1. Mental illness
Chromosomal, genetic, or neurodevelopmental disorders that are asso-
ciated with ID can also be risk markers for specific psychiatric disorders,
or phenotypic mental illness. For instance, persons with Down syndrome
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are at higher risk of Alzheimer’s dementia and depression in later life
(Dykens, 2007).
Many individuals with fragile X syndrome meet criteria for anxiety dis-
orders and in particular social anxiety or shyness (Dykens & Hodapp, 1997;
Sullivan, Hooper, & Hatton, 2007). Signs of depression are often found in
women with fragile X (Freund, Reiss, Hagerman, & Vinogradov, 1992;
Sobesky, Hull, & Hagerman, 1994). Adult females with fragile X also show
social deficits including shyness, social anxiety, and withdrawal and may
meet criteria for schizotypal and avoidant personality disorders.
Prader–Willi syndrome is associated with high rates of affective disorders
with psychotic features (Beardsmore, Dorman, Cooper, & Webb, 1998).
Clarke (1998) estimated the prevalence of psychotic disorders to be about
6.3% in a population of adults with Prader–Willi syndrome. Soni et al.
(2007) found that individuals with Prader–Willi syndrome had psychiatric
illness that most closely resembled an affective disorder, Specifically, those
with the maternal uniparental disomy genetic subtype had a more severe
course of illness than those with the deletion genetic subtype in terms of
risk of recurrence of an affective disorder, more episodes, and higher
prevalence.
Individuals with Williams syndrome are described as quite sociable and
interpersonally sensitive, suggesting a vulnerability to the experience of loss
and sadness. Anxiety, hyperactivity, and inattentiveness are common in
Williams syndrome, including generalized anxiety or worry about future
events (Dykens & Hodapp, 1997). Einfeld, Tonge, and Florio (1997) found
that individuals with Williams syndrome were more likely to be diagnosed
with a psychiatric disorder when compared to a group individuals matched
on age, sex, and level of ID. Increased rates of anxiety, hyperactivity,
preoccupations, and inappropriate interpersonal relating were found as
well as sleep disturbance and hyperacusis.
Furthermore, individuals with ID also have a higher prevalence of
medicalconcernsthatmayhavethesamecauseastheID,suchasheart
conditions in Down syndrome (Gabriel,Loschen,Reeve,Sanderson,&
Charlot, 2007). Because individuals with ID are often poor reporters of
their own health concerns, physical discomfort might contribute to
disruptive behaviors or development of depression or anxiety (Gabriel
et al., 2007).
5.5.2. Maladaptive behaviors
There are several genetic conditions that are associated with severe behavior
problems. For instance, individuals with Lesch–Nyhan syndrome all show
ferocious self-biting and other self-injurious and aggressive behaviors (Nyhan,
2002). Other genetic or chromosomal syndromes that are known to be asso-
ciated with behavior problems include Cornelia de Lange syndrome
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(Hyman et al., 2002; Moss et al., 2005; Oliver et al., 2009; Sarimski,1997), Smith
Magenis syndrome (Finucane, Dirrigl, & Simon, 2001; Dykens, Hodapp, &
Finucane, 2000; Dykens & Smith, 1998; Taylor & Oliver, 2008), Rett syn-
drome (Mount, Hastings, Reilly, Cass, & Charman, 2003; Sansom, Krishnan,
Corbett, & Kerr, 1993), Prader–Willi syndrome (Einfeld, 2005; Oliver,
Woodcock, & Humphreys, 2009; Reddy & Pfeiffer, 2007; Stein, Keating, &
Zar, 1993) and fragile X syndrome (Einfeld, 2005; Einfeld, Tonge, Turner,
Parmenter, & Smith, 1999; Hagerman, 1990; Hagerman & Silverman, 1991).
Behavior problems that are linked to a specific genetic cause may be less
responsive to general behavior modification strategies (Einfeld, 2005).
The most prominent feature of Prader–Willi syndrome is overeating,
which is very common in older children and adults (Dykens & Hodapp,
1997). Infants may show failure to thrive. The onset of overeating or
hyperphagia is associated with an increase in other behavior problems such
as non-food-related repetitive, compulsive, and ritual behaviors, including
skin picking and hoarding, impulsivity, and low activity levels. Obsessive–
compulsive symptoms are common as are other non-food-related repetitive,
compulsive, and ritual behaviors such as skin picking and hoarding
(Beardsmore et al., 1998; Dykens, Leckman, & Cassidy, 1996; Dykens &
Shah, 2003). Sleep disorders are also common in Prader–Willi syndrome,
which are generally related to obesity and sleep apnea.
In a comparison of severity of behavior problems in four common genetic
disorders associated with ID, Einfeld et al. (1999) found that individuals with
Prader–Willi syndrome and Williams syndrome had higher rates of behavior
problems compared to individuals with Down or fragile X syndromes.
5.6. Social–environmental factors
This section briefly summarizes the available data on the potential influence
of social–environmental disadvantage such as poverty, single-parent family,
poor family functioning, maternal poor mental health, and negative life
events (Emerson & Hatton, 2007) on psychopathology in ID.
5.6.1. Mental illness
In a risk factor study with children and adolescents aged 6–18 years with ID
in the Netherlands, Wallander, Dekker, and Koot (2006) found that family
dysfunction, parental mental health, and child physical health have been
shown to predict newly emerging psychopathology. Emerson and Hatton
(2007; Study 10, Table 9.1) looked for associations between three psychia-
tric disorders (conduct disorder, emotional/anxiety disorder, and hyperkin-
esis) and sex, age, and social/environmental factors. Social/environmental
risk factors included poverty, single-parent family, poor family functioning,
maternal poor mental health, and negative life events. Cumulative exposure
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to social disadvantage was related to increased risk of all three types of
disorders for both children with ID and typically developing children;
children with ID had a higher exposure rate to the social/environmental
risk factors than did typically developing children. They also found that if
they statistically controlled for the increased exposure to all forms of social
disadvantage in children with ID, the increased risk of psychiatric disorders
among these children decreased significantly.
Socioenvironmental risk factors for psychiatric disturbance in children
with ID were also investigated by Koskentausta et al. (2007) on a sample of
75 children in Finland with ID between the ages of 6 and 13 years. They
found that the risk of psychopathology was increased in children of low
socioeconomic status and children in single-parent homes. Other factors
were associated with psychopathology was moderate ID (compared to mild
or profound), limitations in adaptive behavior, impairments in language
development, and poor socialization. Another family factor that can con-
tribute to mental illness (depression) is maternal depression (Esbensen,
Mailick Seltzer, & Greenberg, 2006).
5.6.2. Maladaptive behaviors
Prevalence of problem behaviors in general [Jones et al., 2008 (Study 16,
Table 9.1)] are related to out-of-home care, as is self-injury [Cooper et al.,
2009a (Study 1, Table 9.1)] and aggression [Cooper et al., 2009b (Study 3,
Table 9.1); Crocker et al., 2006 (Study 5, Table 9.1)]. Tyrer et al. (2006;
Study 23, Table 9.1) found that aggression was less prevalent in individuals
who lived independently than those who did not.
5.7. Psychological coping skills
As mentioned earlier, Harris (2006) speculated that one obvious reason
why the ID population may be especially vulnerable to psychopathology
are cognitive impairments that result in a lack of adaptive coping responses
and are likely to increase perceived stress. Coping skills is likely to be a
concept highly related to social adaptive behavior.
5.7.1. Mental illness
In the only study of its kind to date, Hartley and MacLean (2008) found a
significant negative correlation between the use of coping efforts and
psychological distress in individuals with ID (without ASD). A significant
negative correlation was discovered between active coping strategies and
psychological distress, while a trend toward a positive correlation was
observed between distress and avoidant coping. These findings support
the idea that inadequate coping by individuals with ID may be related to
increased symptoms of anxiety and depression, and that increasing active
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coping efforts in response to stress may help to decrease vulnerability to
anxiety and depression. Active coping was defined as the effort to gain
control over a stressful situation or control over one’s own emotions, and
actively using other people as resources to seek solutions or manage affect.
Avoidant coping was defined as avoiding or disengaging from a stressful
situation (behavioral avoidance coping) or wishing that the stressful situa-
tion did not happen or avoiding thinking about it (cognitive avoidance
coping). In an earlier study, adults with mild ID reportedly engaged in
more avoidant coping, which was related to depression and anxiety
(Hartley & MacLean, 2005). More research is needed in the area of coping
skills as a preventive strategy and about ways to enhance those skills.
5.7.2. Maladaptive behaviors
No study was identified that examined the relationship between behavior
problems and coping skills per se. However, coping skills are likely related
to social adaptation, communication, and intellectual functioning. The
relationship between level of functioning and behavior problems has been
discussed earlier in this chapter.
6. Conclusions
Although it had long been ignored, it is now generally accepted that
individuals with ID can and do experience mental illnesses such as anxiety,
depression, and schizophrenia. Indeed, we now know that the population
with ID is particularly vulnerable to mental illness and behavior disorders.
Having said that, however, due to a plethora of methodological difficulties
in epidemiological research, we are still far from having a firm grip on how
much more vulnerable individuals with ID are than the population at large.
At a minimum, only studies that directly compared an ID population with a
typical population using the same research methodology can make such
comparative statements. We found only three recent studies that met this
criterion [de Ruiter et al., 2007 (Study 7, Table 9.1), Emerson, 2003 (Study
9, Table 9.1), Emerson & Hatton, 2007 (Study 10, Table 9.1)]. All three of
them focused on children and adolescents (from 5 to 18 years). Emerson
(2003) and Emerson and Hatton (2007) showed that the prevalence of
significant psychopathology in children with ID was 4.8 and 4.5 times as
high as in the comparison group without ID (39 vs. 8.1% and 36 vs. 8%,
respectively). Psychopathology caseness in both studies was based on the
DAWBA (Goodman et al., 2000), which is based on DSM-IV and ICD-10
criteria. This is noteworthy in so far as studies that used multiple assessment
and screening criteria indicated that prevalence estimates of mental illness
that were based on DSM-IV and ICD-10 criteria were consistently lower
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than clinical diagnoses or screenings based on specially ID-adapted diag-
nostic criteria such as the Diagnostic Criteria for Psychiatric Disorders for use with
Adults with Learning Disabilities/Mental Retardation (Royal College of
Psychiatrists, 2001).
Prevalenceratesofglobalmentalillness across all ID levels, sex, and
age in the studies summarized in Table 9.1 ranged from approximately 16
to 54%. A coarse and very general comparison benchmark may be the
prevalence of active psychiatric disorders of any kind in the general U. S.
populationthatwasfoundtobe20%(lifetimeprevalenceof32%)
according to the Epidemiologic Catchment Area Study (Robins, Locke, &
Regier, 1991).
As with mental illness, prevalence estimates for maladaptive behavior
varied considerably as a function of assessment criteria. Prevalence rates
from studies listed in Table 9.1 for global maladaptive behavior ranged
from 0.1 to 23%. For specific types of maladaptive behavior, the range
for aggressive behavior was 6–32%, forself-injuriousbehavior4–21%,
and for destructive behavior from 2 to 19%. Obstacles for obtaining
reliable estimates of maladaptive behavior prevalence include the lack
of standardized categorical definitions and cutoff criteria in terms of
behavior problem severity. No direct comparisons of the prevalence of
maladaptive behavior in individuals with ID and in those without ID
have been found.
Given the wider ranges of psychopathology prevalence estimates, it is
not surprisingly that the information on potential risk factors and markers is
also quite erratic. In summarizing the main findings, arguably, the metho-
dologically strongest studies found no relationship between cognitive func-
tioning level and mental illness. The prevalence of maladaptive behavior,
on the other hand, was strongly negatively correlated with ID level. This
relationship may differ depending upon the type of behavior problem: for
instance, aggressive behavior may be less negatively correlated with level of
functioning than self-injury and stereotyped behavior.
Chronological age was not found to be a predictor of mental illness in
adults or in children. On the other hand, age clearly is related to the
prevalence of general maladaptive behavior. But like with most other
correlates or risk factors in this population, the available data vary widely
across studies. One constant trend is that the prevalence of challenging
behavior increases from childhood to early or mid-adulthood, and then
slowly decreases as people age.
Sex does not seem a strong predictor of mental illness in general. Some
relationship was found between sex and specific mental disorders. For
instance, personality disorders seem more common in men, while dementia
and adjustment disorders might be more prevalent in women. No sex
differences were reported in anxiety or schizophrenia and some contra-
dicting information has been found for depression. Among adolescents
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girls, internalizing behaviors are more prevalent than in males. Whether sex
is a risk marker for maladaptive behavior is not clear as of yet. Male
predominance in aggressive and destructive behavior seems likely, but the
available results are conflicting.
It appears as if ASD is a risk factor for other forms of mental illness.
Some studies suggested that individuals with ID and ASD were more prone
to mood disorder, anxiety, and other related symptoms than those without
ASD, while other studies did not. As far as maladaptive behavior is con-
cerned, one would expect ASD to be a risk factor given that stereotyped
behavior is symptomatic for ASD. Indeed, some studies suggested that the
severity of ASD was positively correlated to the number and the intensity of
such challenging behaviors as self-injury, stereotypies, aggression, and
destruction, and that individuals with ASD exhibited significantly higher
levels of problem behaviors. However, other credible studies did not find
such a relationship.
There is also growing evidence that certain forms of mental illness may
be phenotypic for certain genetic or chromosomal conditions. Fragile X
syndrome, for instance, seems related to social anxiety, while Prader–Willi
syndrome is associated with affective disorders with psychotic features.
Anxiety, hyperactivity, and inattentiveness are also common in Williams
syndrome. Research that links specific genetic markers, such as the repeti-
tion of a specific nucleotide sequence in fragile X or the microdeletion or
mutation of Smith–Magenis to certain types of mental illness will add not
only to our understanding of the incidence of psychiatric disorders in
individuals with ID, but also in the general population. Evidence also
points to the fact that there are probably interactions between chromosomal
conditions and sex, especially in X-linked chromosomal conditions. For
example, depression was often found in women with fragile X. Phenotypes
of maladaptive behavior have been known for some time such as in Lesch–
Nyhan syndrome, Cornelia de Lange syndrome, Smith–Magenis
syndrome, Rett syndrome, and Prader–Willi syndrome.
Children with ID, especially in the mild and moderate ID range, have a
greater exposure rate to the social/environmental risk factors than typically
developing children. Some evidence suggests that social disadvantage also
increased their risk of psychiatric disorders. Inadequate coping may be
related to increased risk of anxiety and depression.
Although risk factors and markers were discussed in this chapter one by
one, we do not imply that mental illness or maladaptive behavior can be
best predicted by single factors. Rather, vulnerability for psychopathology
is most likely a function of complex interactions of mostly poorly under-
stood factors. Such factors may include neurobiological substrates, beha-
vioral characteristics, and socioeconomic conditions (Harris, 2006;
Sturmey, Lindsay, & Didden, 2007). Figure 9.3 presents a hypothetical
biosocial–behavioral path model that depicts the multivariate and
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interdependent nature of prenatal and postnatal influences that contribute
to a person’s psychosocial adaptation in general and to mental health or
illness in particular (adapted from the “new morbidity”model by Baume-
ister, 1988). Genetic endowment, behavioral, and mental health character-
istics of the mother and socioeconomic status tend to determine material
resources, which in turn influence access to support services. These factors
also influence the quality of a nurturing and safe environment.
In reviewing the literature, many of the methodological difficulties
inherent in accurately determining the prevalence rate of psychopathology
in individuals with ID became apparent. Nineteen different diagnostic and
screening instruments were identified in a review of 23 research publica-
tions on prevalence of psychopathology in individuals with ID. The spread
of prevalence rates for specific syndromes even within a single study had an
odds ratio of 1/4.8 [10.9–52.2%; Cooper et al., 2007 (Study 2, Table 9.1)]
depending upon whether the diagnosis was by clinical opinion or strictly
based on ICD-10 or DSM-IV-TR criteria. Use of classification systems
developed for individuals with normal intellectual functioning, such as the
ICD-10 or the DSM-IV-TR, may lead to significant under diagnosis of
mental illness in this population. Supplemental systems such as the DM-ID
(NADD, 2007) and the DC-LD (Royal College of Psychiatrists, 2001) may
Personal and
psychosocial
resources,
family function,
supports
Material
resources
SES,
demographics,
maternal mental
health
Genetic,
chromosomal,
biological
(teratogens)
Resource variables
Catalytic variable
Distal Predisposing
Variables
Proximal Variables
Outcome
(psychosocial
adaptation, mental
health)
correlational link
possible causal link
causal link
Postnatal
Cognitive functioning,
environmental
stimulation,
life events, stress
Prenatal
Figure 9.3 Generalized model of dynamic influences on individual health,
adaptation and development (adapted from Baumeister, 1988).
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help to improve the diagnostic accuracy in individuals with below average
intellectual functioning (Mikkelsen et al., 2005). Whether an entirely new
system for identifying and labeling psychopathology in individuals with ID
is developed, or an existing system such as the DM-ID or DC-LD is used, it
is important that researchers and practitioners to reach consensus on which
instruments and which classification system identifies and best meets the
needs of this population.
Some of the problems in diagnosing psychiatric disorders in individuals
with ID are the same as diagnosing psychiatric disorders in the general
population, including the controversy around validity of psychiatric diag-
noses, usefulness of diagnostic classification systems, and where the line
between “normal”and “abnormal”functioning (Widiger & Coker, 2003).
Progress in assessing, diagnosing, understanding, and treating emotional and
behavioral disorders in individuals with ID continues to be slow but steady.
It is hoped that those of us who work with this vulnerable population will
continue to address the multiple obstacles that create barriers for
identifying, understanding, treating, and preventing the threat of mental
illness.
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