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ORIGINAL PAPER
Individuals with Mild Intellectual Disability or Borderline
Intellectual Functioning in a Forensic Addiction Treatment
Center: Prevalence and Clinical Characteristics
Ilse Luteijn
1,2
&Robert Didden
3
&Joanneke Van der Nagel
4
Published online: 7 July 2017
#The Author(s) 2017. This article is an open access publication
Abstract Knowledge regarding substance-related problems
and offending behavior in individuals with mild intellectual
disability or borderline intellectual functioning (MBID; IQ
50-85) has increased over the last years, but is still limited.
The present study examined differences in prevalence and
clinical characteristics of individuals with and without
MBID in a forensic addiction treatment center. Participants
were 190 court mandated male clients of a low to high security
forensic addiction treatment center in the Netherlands (aged
between 21 and 59 years old, 82% of Dutch origin). Of the
total sample 39% could be identified with MBID which is
much higher than the estimated 12% to 15% of the general
population. Results showed that clients with MBID reported
significantly lower scores on desire for help, compared to
clients without MBID (F (1, 73) = 5.12, p = .027). Against
expectations, no significant group differences were found for
aggression during treatment while controlling for impulsivity,
treatment duration and type of substance use and offense. As
results of the present study showed that clients with MBID are
overrepresented in the forensic addiction treatment center,
future research should further explore characteristics and re-
sponses to treatment of these clients. More knowledge about
the characteristics of clients with MBID will not only help to
better identify these clients, but will also to improve treatment
for this group
Keywords Mild intellectual disability .Substance use .
Offending behavior
Introduction
It is generally acknowledged that substance use and offending
behavior are closely intertwined (Kopak et al. 2014;Vaughn
et al. 2016). An estimated 50 to 66% of offenders meet DSM-
IV criteria (American Psychiatric Association 2000) for sub-
stance abuse or dependence, whereas only 9% of the general
population meet these criteria (Mumola and Karberg 2006). In a
sample of clients in forensic mental health services, 78% met
the criteria for at least one lifetime substance use disorder diag-
nosis (Ogloff et al. 2015). The association between substance
use and offending behavior can be explained by several reasons
including that substance use may exacerbate psychological
symptoms (Swanson et al. 2008), thereby increasing the likeli-
hood of offending behavior (Giancola 2004). Furthermore, of-
fenders were arrested for drug trafficking and/or committed
their offense to support their substance use (Kopak et al.
2014). As a result, addiction treatment centers and criminal
justice systems often deal with the same individuals and higher
percentages of substance abuse and dependency are found in
forensic samples (Chandler et al. 2009; Lindsay et al. 2013).
When it comes to substance-related problems and
offending behavior, individuals with mild intellectual disabil-
ity or borderline intellectual functioning (MBID; intelligence
quotient (IQ) 50–85) (American Psychiatric Association
Electronic supplementary material The online version of this article
(doi:10.1007/s41252-017-0031-7) contains supplementary material,
which is available to authorized users.
*Ilse Luteijn
i.luteijn@Tactus.nl
1
Tactus Verslavingszorg, P.O. Box 154, 7400 AD Deventer, the
Netherlands
2
Behavioral Science Institute, Radboud University Nijmegen,
P.O. Box 9104, 6500 HE Nijmegen, the Netherlands
3
Trajectum, Hanzeallee 2, 8017 KZ Zwolle, the Netherlands
4
Radboud Universiteit Nijmegen / ACSW, NISPA, Postbus 6909,
6503 GK Nijmegen, the Netherlands
Adv Neurodev Disord (2017) 1:240–251
DOI 10.1007/s41252-017-0031-7
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2013) are perceived as a risk group (see Carroll Chapman and
Wu 2012; Duijvenbode et al. 2015). Individuals with MBID
comprise a substantial part of the population (12–15%)
(Emerson et al. 2010) and experience serious challenges in
society (Emerson 2011; Turygin et al. 2014). Besides a greater
risk of exposure to social disadvantage, these individuals are
1.2 times more likely to use drugs (Gigi et al. 2014). Several
factors like inadequate coping skills (Hartley and MacLean
2008), impaired inhibition (Bexkens et al. 2014), and limited
social skills (Carroll Chapman and Wu 2012) increase the risk
of substance-related problems in individuals with MBID
(Duijvenbode et al. 2015). Other factors associated with
MBID like susceptibility to social pressure (Taggart et al.
2006) and a lack of inability to understand the potentially
severe consequences of substance abuse (Chaplin et al. 2011)
also increase the risks in these individuals (Duijvenbode et al.
2015; Slayter and Steenrod 2009).
Although substance-related problems in individuals with
MBID have gained more attention in the academic and clinical
field (see Carroll Chapman and Wu 2012; Duijvenbode et al.
2015), these problems are still understudied and reliable data on
prevalence and risk factors are lacking (VanDerNagel et al.
2013). Researchers who studied substance use in individuals
with MBID found that substance use by these individuals was
related to various problems including offending behavior
(Chapman 2012;Gigietal.2014). This might also explain
why individuals with MBID constitute a growing percentage
of offenders within the justice system and are overrepresented
in forensic institutions (Hellenbach et al. 2016; Herrington
2009). In a study by Plant et al. (2011), it was found that half
of the clients in a forensic institution could be identified with
MBID and substance-related problems. Of these clients, 35%
hadusedsubstancespriortotheoffenseleadingtotheiradmis-
sion. Individuals with MBID and substance-related problems
seem to be less likely to receive treatment or to remain in ad-
diction treatment (Carroll Chapman and Wu 2012). This might
explain why these individuals often receive treatment when
they are already involved in the justice system due to the con-
sequences of their substance use (Chaplin et al. 2011;Cocco
and Harper 2002).
Although it is known that individuals with MBID are over-
represented in both addiction treatment centers and forensic
institutions (Bhandari et al. 2014), it remains to be determined
(1) what the prevalence of MBID in a forensic addiction treat-
ment center is, (2) what characteristics of this subpopulation
are, and (3) if these clients differ in response to treatment, com-
pared to clients without MBID. Considering the risk factors of
MBID in both areas, it is important to investigate the prevalence
and characteristics of clients in forensic addiction treatment
centers. In addition, studying differences in behavior that are
evident for the success of treatment provides more insight in if
and how clients with MBID differ from clients without MBID.
As motivation for treatment is one of the most important factors
contributing to the success of treatment (Kopak et al. 2016;
Stevens et al. 2015), it is important to study whether motivation
differs for clients with MBID compared to those without
MBID. This also holds true for aggression during treatment,
as this behavior is associated with more dropouts from addic-
tion programs with more risk at relapse in substance use and
offending behavior as a result (Bali et al. 2006; Liu et al. 2013).
Studying motivation and aggression during treatment thus pro-
vides information about differences between clients with and
without MBID. Such knowledge could help to increase our
knowledge about the clinical characteristics and responses to
treatment of clients with MBID.
While legal pressures may be sufficient to enroll a client into
treatment, personal motivation for treatment is evident when it
comes to commitment to change and to maintain recovery
(Groshkova 2010). Increasing motivation thus is an important
aspect for all clients; however, it is specifically challenging for
clients with MBID (Slayter 2010; Carroll Chapman and Wu
2012). That is, these clients are more likely to externalize the
cause of their problems and they also might experience their
environment as more negative when compared to clients with-
out MBID (Nader-Grosbois and Vieillevoye 2012).
Consequently, they might be in denial or are not aware of the
severe consequences of their substance use, which makes them
not motivated to make changes or to engage with treatment
programs (Taggart et al. 2006). To explore this hypothesis, it
should be investigated whether clients with MBID in a forensic
addiction treatment center are less motivated for their treatment
than clients without MBID.
Just like motivation, aggression during treatment has a large
impact on the success of treatment (Liu et al. 2013). Research
has shown that aggression is associated with both substance-
related problems and offending behavior (Cuomo et al. 2008)
and therefore a common problem in both addiction treatment
centers and forensic institutions (Jacob and Holmes 2011;
Mericle and Havassy 2008). Aggression is defined in different
ways and there are different definitions of aggression (Archer
and Coyne 2005). The most widely accepted definition is the
one proposed by Berkowitz (1993), a goal-directed behavior
that has a deliberate intent to harm or injure another object or
person. A number of distinctions can be made, for example, the
distinction between direct/overt aggression versus indirect/
covert aggression (Brugman et al. 2015;Richardsonand
Green 2006). Overt or direct aggression is marked with behav-
iors that are openly hostile such as hitting, pushing, kicking,
biting, and hair pulling. Covert or indirect aggression is marked
by hostility, irritability, suspicion, lying, and covert feelings of
anger (Olson et al. 2013). Several factors are known to increase
the likelihood of aggressive behavior. For example, impulsivity
is often related to the emotional and instrumental aspects of
aggression (Bevilacqua and Goldman 2013; Hatfield and
Dula 2014). Impulsivity can be described as actions which are
poorly conceived, prematurely expressed, unduly risky, or
Adv Neurodev Disord (2017) 1:240–251 241
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inappropriate to the situation and that often result in undesirable
consequences (Dally et al. 2011). Impulsive individuals are
more likely to show difficulties in delay to rewards and often
make quick decisions due to a lack of self-control and reflection
(Winstanley et al. 2006).
Factors like presence of an intellectual disability seem to
increase the likelihood of impulsive behavior (Crocker et al.
2007). Due to impulse control difficulties, individuals with
MBID may be less likely to control their behavior, which
can lead to the expression of irritability and aggression
(Dodge and Pettit 2003; Van Nieuwenhuijzen et al. 2006). In
addition, they may not have learned alternative coping skills
to deal with stressful situations, which can result in impulsive
reactions (Hartley and MacLean 2008). Individuals with good
communication skills can often describe their feelings of irri-
tability and frustration, but individuals with lower cognitive
and social skills may not be able to identify the reason for their
reaction (Hurley 2008). Based on the literature, it can be hy-
pothesized that clients with MBID are more likely to show
aggression during treatment in a forensic addiction treatment
center compared to clients without MBID. As impulsivity is
strongly related to aggression, this should be included when
studying aggression during treatment in a forensic addiction
treatment center. In addition, as frequency of aggression inci-
dents depends on treatment duration and substance use and
offending behavior are associated with aggression (Cuomo
et al. 2008; Lynne-Landsman et al. 2011), these factors should
be controlled for.
The present study examined if individuals with MBID are
overrepresented in forensic addiction treatment centers and
whether they differ in their motivation and aggressive behavior
during treatment from clients without MBID. To investigate
this, data was collected from clients in a forensic addiction
treatment center located in the Netherlands. At first, the preva-
lence of clients with MBID was examined. As an estimated 12
to 15% of the general population have a MBID and previous
research shows that these clients are overrepresented in both
addiction and forensic treatment centers, it was expected that
more than 15% of the clients in a forensic addiction treatment
center can be identified with MBID (IQ 50–85). Second, as
previous research shows that individuals with MBID are less
likely to show motivation, it was expected that clients with
MBID are less likely to report motivation for treatment than
clients without MBID. Finally, as research has shown that
MBID is related to higher expressions of overt and covert ag-
gression, it was hypothesized that clients with MBID are more
likely to report covert and overt aggression than clients without
MBID, while controlling for impulsivity, treatment duration,
and type of substance use and index offense. In addition, it
was expected that for clients with MBID, more aggression in-
cidents are reported by the clinical staff during treatment than
for clients without MBID, while controlling for impulsivity,
treatment duration, and type of substance use and index offense.
Method
Participants
The present study was conducted in a forensic addiction treat-
ment center with low- to high-security wards in the
Netherlands. This clinic offers an integrated approach for
substance-related problems, offending behavior, and related
problems. The start of the treatment is especially focused on
the motivation and examining the psychological functioning
of the clients, and the end of the treatment is focused on
rehabilitation.
Participants were 190 male inclients between 21 and
59 years old (M=38.7years,SD =7.96years).Themajority
of participants were from Dutch origin (n= 157, 82%), while
the others originated from Surinam/Dutch Antilles (n=15,
8%), Turkey/Morocco (n= 11, 6%), or other Western (n=5,
4%) or non-Western (n= 4, 2%) countries. Participants were
all diagnosed with substance dependence, substance abuse, or/
and pathological gambling according to DSM-IV-TR criteria
(American Psychiatric Association 2013). In addition, they
were all mandated by court to engage in treatment for
substance-related problems and offending behavior. The total
average duration of treatment was 7.12 months (range 0–25),
and of all participants, 44% completed their treatment.
Participants were divided into two groups (MBID versus
no MBID) based on IQ. IQ was assessed using the most recent
scores on the Dutch version of the Wechsler Adult Intelligence
Scale third (WAIS-III-NL) (Uterwijk 2000)orfourthedition
(WAIS-IV-NL) (Benson et al. 2010) in the participants’file.
Substance use-related problems were assessed using DSM-IV-
TR criteria and diagnosed by an independent professional.
Participants were supposed to be abstinent from substances
and were monitored during treatment using urine controls.
All participants were diagnosed with a substance dependency
for one or more substances, and offenses were committed
under the influence of substances or were related to substance
abuse/dependency. The various types of index offenses were
based on the categorization used by the Dutch court system
(see Appendix).
Procedure
In the present study, existing data were used which were col-
lected by the clinical staff in the period of January 2011 to
January 2015. There were no pre-defined inclusion criteria to
participate in the study, except that clients were admitted to the
forensic addiction treatment. When a client started treatment,
total IQ was determined by an independent professional. For
the present study, all the data of the questionnaires on motiva-
tion, impulsivity, and aggression were derived from the cli-
ent’s casefile. The self-report questionnaires were completed
in a quiet room by clients with a clinical staff member who
242 Adv Neurodev Disord (2017) 1:240–251
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helped them if necessary. The proxy reports were completed
by the clinical staff. All questionnaires were completed after a
period of abstinence and all participants finished treatment
when data were retrieved from the casefiles when participants
finished their treatment. This study was approved by the in-
stitute’s scientific committee. At the start of their treatment, all
clients consented to the use of their casefile data for future
scientific studies. As this was a retrospective study using
casefile data only, no additional consent was sought.
Measurements
Intelligence IQ score was assessed with the WAIS-III-NL or
WAIS-IV-NL (Wechsler 2008).TheWAIS-IIIandtheWAIS-
IV are scales to assess the cognitive ability of adolescents and
adults in the age of 16 to 90. The WAIS-III provides scores for
Verbal Intelligence Quotient (VIQ) and Performance
Intelligence Quotient (PIQ) along with four secondary indices
(verbal comprehension, working memory, perceptual organiza-
tion, and processing speed). The WAIS-IV has ten subtests
which make up four index scores: the Verbal Comprehension
Index (VCI), the Perceptual Reasoning Index (PRI), the
Working Memory Index (WMI), and the Processing Speed
Index (PSI). In previous studies, the test-retest reliabilities
ranged from 0.88 to 0.89 and the inter-rater coefficients for
the WAIS-III were all above 0.80 (Wechsler 1997). For the
WAIS-IV, test-retest reliabilities ranged from 0.70 to 0.90 and
the inter-rater coefficients were all above 0.90 (Benson et al.
2010;Wechsler2008). The Total Intelligence Quotient (TIQ) is
based on the scores of the four index scores. The WAIS is the
most commonly used test to assess intelligence in adults, also in
adults with intellectual disability (MacLean et al. 2011). Results
of the WAIS collected in the treatment center were no older than
4 years.
Motivation for Treatment The Dutch version of the
Motivation for Treatment (MfT) assesses the person’s motiva-
tion for treatment. The instrument measures if respondents
recognize their problems and whether they would like to have
treatment (Weert-Van Oene et al. 2002). The MfT consists of a
self-report scale for the client and an observation scale (MfT-
o) (also indicated as proxy report) for the clinical staff. The
questionnaire is divided into four subscales: treatment readi-
ness, desire for help, problem recognition in general, and
problem recognition specific (for example substance use and
offending behavior). Respondents were given 29 statements
such as BGoing into treatment may be your last chance to
solve your drug/alcohol problems^or BThis treatment pro-
gram can really help you,^which they could answer on a
five-point Likert scale (1 = disagree strongly, 5 = agree strong-
ly). A higher score on a subscale reflects a greater motivation
for treatment. The observation scale for the clinical staff con-
sists of the same statements. Research has shown that the MfT
is a reliable and valid instrument; however, this was only
tested in a sample of individuals with average intellectual
functioning (Weert-Van Oene et al. 2002). In the present study,
Cronbach’s alpha for the entire sample for the self-report was
0.88, and for the proxy report, Cronbach’s alpha was 0.90. For
the self-reports completed by the clients with MBID,
Cronbach’s alpha was 0.76. A Cronbach’s alpha of 0.80 was
found for the proxy report of the clients with MBID. For the
clients without MBID, a Cronbach’s alpha of 0.81 was found
for the self-report scores and a Cronbach’s alpha of 0.87 for
the proxy report.
Aggression The Buss-Durkee Hostility Inventory–Dutch ver-
sion (BDHI-D; Lange et al. 2005) was used to measure aggres-
sion at the beginning of treatment. The BDHI-D measures both
the experience and the expression of aggression in adolescents
and adults from 15 years old. Lange et al. (2005) translated the
BDHI into Dutch and validated it for use in the Netherlands.
The BDHI-D consists of 40 dichotomized (true of false) items
and three scales (i.e., overt aggression, covert aggression, and
socially desirability). A high score on the overt aggression scale
indicates physical and verbal aggression. An example of such
an item is BIf somebody hits me first, I let him have it.^Ahigh
score on the covert aggression scale indicates that an individual
experiences aggression and can be an indication for psychopa-
thology. An example of a covert aggression item is BIknowthat
people tend to talk about me behind my back.^The internal
consistency in previous research on the scale for overt aggres-
sion was α= 0.79 and on the scale for covert aggression
α=0.82(Langeetal.2005). Research has shown that the
BDHI-D is a reliable and valid instrument (Lange et al.
2005). The present study shows a reliability for overt aggres-
sion of α= 0.69 and for covert aggression of α=0.77.For
clients with MBID, a Cronbach’s alpha 0.72 was found for the
overt aggression and 0.75 for the covert aggression. For clients
without MBID, a Cronbach’s alpha 0.79 was found for the
overt aggression and 0.77 for the covert aggression.
In addition to self-report questionnaires, incidents of ag-
gression were recorded by the clinical staff. When such an
incident occurred, the clinical staff reported this in the clients’
file. Only the frequency of incidents of aggression was record-
ed, not the severity or the type of aggressive behavior. In this
way, the reports only showed that an aggression incident has
occurred, not if this aggression was physical or verbal.
Impulsivity The Dutch version of the Barrat Impulsiveness
Scale-11 (BIS-11; Patton et al. 1995) is a well-validated self-
report questionnaire to assess impulsiveness (Stanford et al.
2009). The BIS-11 includes 30 items scored on a four-point
Likert scale (1 = never, 4 = always) describing common impul-
sive or non-impulsive behaviors and preferences. Example
questions include BI say things without thinking,^BI get easily
bored when solving thought problems,^and BI change jobs.^
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The degree of impulsivity is measured by adding up the scores
of each item. Cronbach’s alpha was 0.89 in the present study,
which is a slightly higher alpha compared to previous studies in
which Cronbach’s alpha ranged from 0.79 to 0.83 (Patton et al.
1995). Research has shown that the BIS-11 is a reliable and
valid instrument (Stanford et al. 2009).
Data Analyses
At first, it was assessed for how many clients questionnaires on
aggression, impulsivity, and motivation for treatment had been
collected. Then, data sets were merged using the ID code of the
casefiles and divided into two groups based on the total intelli-
gence score (i.e., IQ 50–85 versus IQ > 85). For the same clients,
the frequency of aggression incidents was matched using the ID
code of the casefiles. To minimize loss of power, the different
types of substances were reduced to the substances that were
most often used in both groups (alcohol, cannabis, cocaine/speed,
and heroine). Cocaine and speed were combined into one cate-
gory as these substances are both stimulants (Fletcher et al.
2011), and 85% of the participants in the study were diagnosed
with abuse or dependence on both substances. For type of index
offenses, only violent offenses were included as covariate as this
type of offense is related to aggression (Vassos et al. 2014).
IBM SPSS Statistics (version 21) was used to conduct the
statistical analyses. As for only 75 participants data for motiva-
tion for treatment were found, an analysis of variance
(ANOVA) was performed to explore group differences in the
self-reports and proxy reports for motivation. To minimize loss
of power, no covariates were included in this analysis. After
conducting the ANOVA, correlations between the self-reports
and proxy reports were calculated for the two groups.
A multivariate analysis of covariance (MANCOVA) was
assessed to explore group differences on aggressive behavior
(i.e., overt aggression, covert aggression, and frequency of
aggression incidents) while controlling for impulsivity, treat-
ment duration, and type of substance use and offense. After
the assumptions for MANCOVA were checked in SPSS,
means and standard deviations were calculated. Then, it was
determined whether the MANCOVA was statistically signifi-
cant by looking at the Wilks’lambda. If this result was statis-
tically significant, post hoc tests were used to explore group
differences on aggressive behavior.
With G*Power version 3.1.92, a post hoc power analysis
was performed for the MANCOVA with two groups (MBID
and no MBID), seven predictors (impulsivity, treatment dura-
tion, violent index offense, alcohol, cannabis, cocaine/speed,
and heroine), and three response variables (overt aggression,
covert aggression, and aggression incidents). Based on the sam-
plesizeof190,anαof 0.05, and a small effect size of 0.1, a
power of 0.80 was found. This means that for the MANCOVA
performed in this study, the sample size should be large enough
to detect group differences.
Results
Clients with and without MBID did not differ in age (t
(188) = 1.58, p= 0.115) and ethnic background (F(1,
188) = 0.31, p= 0.578). In addition, no group differences were
found for treatment duration (t(188) = 1.33, p= 0.184) and
successful completion of treatment (t(190) = 0.39, p=0.699).
Participants were sentenced for various offenses (see Table 1),
but mostly for offenses against property. The results in Tables 1
and 2show that groups did not significantly differ in type of
index offense and substance/addiction. Table 3shows that
groups did not significantly differ in the reasons for ending
the treatment (e.g., successful completion, relapse in substance
use, aggression incident).
Prevalence MBID
The average total IQ of the entire sample was 87.9 (SD = 11.3,
range = 61–125), which is significantly lower than the average
intelligence score of 100 of the general population (t
(189) = −14.8, p=0.000)(Kaufman2012). Figure 1shows
the normal distribution of IQ scores of the present sample
compared to the normal distribution of the general population.
For the total sample, a total IQ score of 87.9 (SD = 11.3) was
found. For clients with MBID, a total IQ score of 78.0
(SD = 5.8), and for clients without MBID, a total IQ score
of 94.5 (SD = 9.0), was found. Seventy-five participants
(39%) were identified with MBID, defined as having an IQ
between 50 and 85 (American Psychiatric Association 2013;
Schalock et al. 2010). This percentage is significantly higher
than 12–15%, which would be expected based on the distri-
bution of IQ scores of the general population (z=9.45,
p=0.000).
Motivation for Treatment
Results of Mft self-reports and proxy reports were found for 30
clients with MBID and 45 clients without MBID. The results of
the one-way ANOVA are presented in Table 4. No significant
group differences were found regarding self-reports of readi-
ness for treatment, problem recognition general, and problem
recognition specific. However, group differences were found
for desire for help (F(1, 73) = 2.15, p= 0.027), with clients
with MBID showing significantly lowerscoresonthisvariable
than clients without ID. This means that clients with MBID are
less likely to experience a desire for help at the beginning of
treatment compared to clients without MBID. Looking at the
proxy reports, no group differences were found on all four
motivation treatment scales. This means that the clinical staff
did not report any differences between groups on the several
measurements for motivation at the beginning of treatment.
To explore the relationships between the different vari-
ables, correlations between self-reports and proxy reports of
244 Adv Neurodev Disord (2017) 1:240–251
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
clients with MBID without MBID were calculated (see
Tab le 5and Fig. 2). For clients with MBID, significant corre-
lations were found between self-reports and proxy reports for
desire for help and problem recognition general. For clients
without MBID, significant correlations were also found for
these scales, and for readiness for treatment. Looking at the
correlations and plots, there is a large difference between the
self-reports and proxy reports on the readiness for treatment
scale. Self-reports and proxy reports were correlated for cli-
ents without MBID, but not for clients with MBID.
Aggression
For 188 participants, aggression measurements could be
assessed. The number of aggression incidents per participant
ranged from 0 to 3. Table 6shows the results of the
MANCOVA controlling for impulsivity, treatment duration,
and type of substance use and index offense. Of the 192 clients
who completed the aggression self-report questionnaires, 188
participants also completed the impulsivity questionnaires at
the beginning of their treatment. Results of Wilks’lambda
showed that groups did not significantly differ on the levels
of aggression, with F(3, 183) = 0.138, p= 0.937, Wilk’s
Λ= 0.998, and partial η
2
= 0.002. As this result is not statis-
tically significant, no further follow-up tests based on intelli-
gence were performed. Although no significant group differ-
ences were found for the aggression measurements, results did
show a significant effect of impulsivity on overt (F(1,
179) = 47.31, p= 0.000) and covert aggression (F(1,
179) = 43.01, p= 0.000), meaning that clients who did report
more impulsivity were also more likely to report more overt
(β=4.77,t=6.70,p= 0.000) and covert aggression (β=6.08,
t=6.59,p= 0.000). In addition, a significant effect of violent
index offenses was found on frequency of aggression inci-
dents (F(1, 179) = 6.79, p= 0.010), which means that clients
who committed a violent offense before treatment were more
likely to be reported for an aggression incident during treat-
ment (β=0.23,t=2.64,p=0.010).
Discussion
The present study examined the prevalence and characteristics
of clients with mild intellectual disability or borderline intel-
lectual functioning (MBID; IQ 50–85) in a forensic addiction
treatment center. In addition, it was investigated whether cli-
ents with MBID differed in motivation and aggression during
treatment compared to clients without MBID.
The first hypothesis concerned the higher prevalence rate
of clients with MBID in a forensic addiction treatment center
as 39% of the sample could be identified with MBID. This
result confirms the hypothesis that more than 15% of the cli-
ents in a forensic addiction treatment center could be identi-
fied with MBID, based on the 12 to 15% found in the general
population (Emerson et al. 2010). This result is in line with
results based on studies in forensic institutions and prisons
showing that 10 to 40% of the clients could be identified with
aMBID(Lindsayetal.2013; Ståhlberg et al. 2010) and that
Tabl e 1 Types of index offenses
(n=190) MBID (n=75) NoMBID
(n= 115)
t(188) pvalue
%%
Property offenses 96.1 89.3 1.66 0.099
Violent offenses 71.1 68.8 0.40 0.688
Vandalism and public-order offenses 25.0 25.9 −0.82 0.414
Drug-related offenses 23.7 20.5 0.41 0.682
Traffic violations 17.1 22.3 −0.83 0.410
Possession of weapons 14.5 10.7 0.23 0.543
Sex offenses 0.0 1.9 −0.16 0.247
Multiple responses were possible.
Tabl e 2 Types of substance use/addiction (n=190)
MBID (n= 75) No MBID
(n= 113)
t(188) pvalue
%%
Alcohol 61.8 58.0 0.68 0.497
Cannabis 64.5 56.3 1.16 0.246
Cocaine 78.9 81.3 −0.43 0.667
Heroine 28.9 36.6 −1.02 0.308
Speed 13.2 19.6 −1.12 0.265
GHB 6.7 7.1 −0.35 0.729
XTC 6.6 8.0 −0.55 0.580
Medication 7.9 6.3 0.22 0.828
Gambling 4.2 4.4 0.32 0.701
Other
a
3.1 1.7 0.90 0.299
Multiple responses were possible
a
The category Bother^includes substances that are not very commonly
used or new types of drugs such as ketamine, LSD, Qat, and new psy-
choactive stimulants
Adv Neurodev Disord (2017) 1:240–251 245
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
high percentages of these clients reported substance-related
problems (Bhandari et al. 2014). The results of the present
study show that clients with MBID are also overrepresented
in forensic addiction treatment centers. This result can be ex-
plained by a range of factors that make individuals MBID more
at risk for substance-related problems and offending behavior,
such as inadequate coping skills (Hartley and MacLean 2008),
susceptibility to social pressure (Taggart et al. 2006), and lack
of inability to understand the severe consequences of substance
use (Chaplin et al. 2011). In addition, previous research
showed that individuals with MBID are less likely to receive
treatment or to remain in voluntary treatment, compared to
clients without MBID (Carroll Chapman and Wu 2012). This
might also explain the higher prevalence rates of clients with
MBID in the forensic addiction treatment center.
The second hypothesis concerned differences between cli-
ents with and without MBID in motivation for treatment with
the expectation that clients with MBID were less motivated
for treatment than clients without MBID. At first, differences
in self-reports were investigated. In line with our expectation,
results showed that clients with MBID reported lower scores
on desire for help than clients without MBID. This result can
be explained by the fact that individuals with MBID are more
likely to externalize the cause of their problems and therefore
may be less likely to admit that they need help (Nader-
Grosbois and Vieillevoye 2012; Taggart et al. 2006). In
addition, they may not fully understand the severe conse-
quences of their problems and therefore have the feeling that
they can solve their problems on their own (Chaplin et al. 2011).
Although a significant difference was found for desire for
help, no group differences were found on the other self-report
scales. Also on the proxy reports, no significant group differ-
ences were found. These results are not in line with our ex-
pectations that clients with MBID are more likely to be in
denial of their substance-related problems and therefore less
motivated to make changes regarding their substance use
(Taggart et al. 2006). An explanation is that the subsample
that completed the motivation for treatment questionnaires
was too small to detect any group differences. Another expla-
nation is the interpretation of the motivation in clients with
MBID by the clinical staff. Correlations in the present study
showed that self-reports and proxy reports for readiness for
treatment were significantly correlated for clients without
MBID but not for clients with MBID. The clinical staff report-
ed high scores on readiness for treatment for clients with
MBID, which may indicate that the staff overestimates these
clients on this specific variable or underestimate problems in
these clients. This result is also interesting as it is known that
there are methodological and measurement issues while
studying behavior in clients with MBID (Carroll Chapman
and Wu 2012). The reliability of self-reports completed by
clients with MBID can be affected by their limited cognitive
Tabl e 3 Reasons for termination
of treatment (n=190) MBID (n=75) NoMBID
(n= 113)
t(188) pvalue
%%
Successful completion 46.1 43.2 0.42 0.678
Aggression 2.6 8.0 −1.52 0.129
Relapse in substance use 19.7 19.6 0.07 0.941
Relapse in delinquent behavior 0.0 0.9 −0.82 0.416
Leaving without permission 26.3 28.6 0.13 0.899
Other
a
13.2 8.9 0.96 0.377
Multiple responses were possible
a
The category Bother^includes reasons like not following the rules of the treatment center and transfer to another
center where treatment was more suitable
0
10
20
30
40
50
60
50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140
%
IQ score
Normal distibution of
theoretical sample
Present sample
Normal distribution of
present sample
Fig. 1 Histogram of the
distribution of IQ scores in the
present sample compared to the
theoretical sample (general
population)
246 Adv Neurodev Disord (2017) 1:240–251
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
capacities, verbal comprehension, and self-reflection
(Schalock et al. 2010). In addition, it is known that clients
with MBID are also more susceptible for desirable answers
(Jobson et al. 2013; Langdon et al. 2010). Furthermore, proxy
reports such as those of clinical staff members often underes-
timate clients’substance-related problems (Wilson et al.
2004). This indicates that we have to be careful with the in-
terpretation of measurements in clients with MBID in both the
academic and clinical field. More research on the application
of self-reports and proxy reports has important implications
for the treatment of clients with MBID.
The last hypothesis concerned group differences in aggres-
sion during treatment while controlling for impulsivity, treat-
ment duration, and type of substance use and index offense. In
contrast with our expectations, results showed that clients with
MBID did not report more overt and covert aggression during
treatment than clients without MBID. In addition, no group
differences were found for the frequency of aggression inci-
dents during treatment, controlled for impulsivity, treatment
duration, and type of substance use and offense. Although re-
sults could not be compared with previous research, several
studies show that clients with MBID are more likely to show
aggressive behavior due to impulse control difficulties and in-
adequate coping skills (Hartley and MacLean 2008;van
Nieuwenhuijzen et al. 2006). Just like for the motivation for
treatment questionnaires, it could be that the self-report ques-
tionnaires for aggression were not fully adapted for clients with
MBID. Especially as reliability for both overt and covert ag-
gression in the present study was under 0.80, this should be
taken in consideration when interpreting the results. In addition,
the records of aggression incidents in the present study may not
be specific enough and sensitive to the interpretation of the
clinical staff which is also indicated by other studies
(Vereenooghe and Langdon 2013). However, in contrast to pre-
vious research, it could also be that clients with MBID did not
differ in their aggression compared to those without MBID. For
these reasons, measuring and interpreting aggression behavior
in clients with MBID should be examined in future research.
Strengths, Limitations, and Future Directions
This study has several strengths. Most important, the results of
the present study provided more knowledge about the preva-
lence and clinical characteristics of clients with MBID in a
forensic addiction treatment center compared to those without
MBID. As such, it provides important insights for both the
academic and clinical field, for example, that clients with
MBID are overrepresented (i.e., 39%) in the forensic addiction
treatment center. This study can be the beginning of future
research on the characteristics and differences in response to
treatment in clients with MBID in forensic addiction treatment
centers. Finally, the present study indicated the importance of
using suitable and reliable self-reports and proxy reports in
clients and stresses the need for more research on this topic.
Especially looking at the large number of clients with MBID in
the forensic addiction treatment center, it is important to exam-
ine which instruments are used, how these are interpreted, and
what this means for the treatment of these clients.
The results of this study should be interpreted in the context
of a number of shortcomings. As this was a clinic-based study
Tabl e 4 Results of a one-way analysis of variance for group differences on motivation for treatment (n=75)
MBID (n= 30) No MBID (n=45) F(1, 73) pvalue
MSD MSD
Self-report
Readiness for treatment 3.97 0.51 3.98 0.59 0.00 0.990
Desire for help 3.48 0.65 3.82 0.65 5.12 0.027
Problem recognition general 3.61 1.24 4.00 0.93 2.36 0.129
Problem recognition specific 3.22 1.21 3.54 0.93 1.67 0.200
Proxy report
Readiness for treatment 3.75 0.53 3.57 0.49 1.19 0.171
Desire for help 3.59 0.61 3.57 0.63 0.07 0.791
Problem recognition general 3.85 0.86 3.70 0.84 0.48 0.491
Problem recognition specific 3.56 0.85 3.48 0.73 0.169 0.682
Mmean, SD standard deviation
Tabl e 5 Correlations between self-reports and proxy reports of moti-
vation for treatment scales (n=75)
MIBD No MBID
Readiness for treatment 0.16 0.52**
Desire for help 0.41* 0.37*
Problem recognition general 56** 0.39**
Problem recognition specific 0.20 0.20
*p<0.05,**p<0.001
Adv Neurodev Disord (2017) 1:240–251 247
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
using existing data, this has several consequences for the inter-
pretation of the results. At first, only first measurements of
motivation for treatment and aggression self-reports could be
included, which means that differences in the course of the
treatment process could not be examined. In addition, only a
small subsample for the motivation for treatment questionnaires
could be included, which negatively influences the power of the
analysis. Furthermore, IQ scores were derived from the results
of the WAIS-III and WAIS-IV. The question is how valid these
results are when measuring MBID in combination with a past
of substance abuse. In addition, results of the WAIS-III and
WAIS-IV are not fully comparable and some measurements
were older than 2 years. This also applies to the measurements
of motivation for treatment, impulsivity, and aggression.
1
2
3
4
5
Proxy-report
Self-report
No MBIDMBID
Readiness for treatment
Desire for help
Problem recognition general
Problem recognition specific
0
1
2
3
4
5
Proxy-report
Self-report
1
2
3
4
5
Proxy-report
Self-report
1
2
3
4
5
Proxy-report
Self-report
1
2
3
4
5
Proxy-report
Self-report
1
2
3
4
5
Proxy-report
Self-report
1
2
3
4
5
Proxy-report
Self-report
1
2
3
4
5
12345
12345 12345
12345 12345
12345
12345 12345
Proxy-report
Self-report
Fig. 2 Plots of the correlations
between self-reports and proxy
reports for the motivation for
treatment scales (n=75)
Tabl e 6 Group differences on aggression measurements (n=188)
MBID No MBID F(1, 179) pvalue
MSD MSD
Overt aggression 12.42 0.39 12.31 0.48 0.07 0.793
Covert aggression 10.75 0.05 9.97 0.62 0.80 0.371
Aggression incidents 0.27 0.05 0.29 0.06 0.06 0.814
248 Adv Neurodev Disord (2017) 1:240–251
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Finally, the study was performed in one single-treatment center
and included only male clients, which means that the results
cannot be generalized to other institutions and samples.
Several suggestions for future research can be made. As
results of the present study showed that clients with MBID
are indeed overrepresented in the forensic addiction treat-
ment center, future research should further investigate the
treatment of substance-related problems and offending be-
havior in this group. Next to that, it should be further exam-
ined how differences in motivation and aggression differ
during entire treatment using repeated measures.
Especially for differences in aggression incidents, it would
be interesting to include the time until the incident occurs
using Poisson regression survival analysis (VanDeSande
et al. 2011). Other factors should also be included like dif-
ferences in motives for substance use (Thornton et al. 2012),
coping skills (Hartley and MacLean 2008;van
Nieuwenhuijzen et al. 2006), and executive functioning
(Willner et al. 2010). Furthermore, differences in self-
reports and proxy reports should be further investigated.
Especially as results in the present study indicate that self-
reports and proxy reports for readiness for treatment were
more correlated for clients without MBID, it should also be
studied whether instruments in forensic addiction treatment
centers are appropriate for and fully adapted to clients with
MBID. Especially as underrecognition of problems in cli-
ents with MBID is a risk (Wieland and Zitman 2016), this
should be taken in consideration when interpreting the re-
sults of the proxy reports completed by the clinical staff. As
this might influence the treatment of clients with MBID, it is
important to investigate whether this is the case.
Furthermore, it would be useful to investigate whether cli-
ents with MBID are more likely to relapse in substance use
and/or offending behavior during and after treatment
performing a follow-up study. Especially as there are few
specialized treatments available for this specific group, it
should be studied how this influences the success of treat-
ment and recidivism of clients with MBID. When more
knowledge about the clinical characteristics and risk factors
in these clients is available, treatment can be improved.
The present study showed that of the clients in a forensic
addiction treatment center, 39% could be identified with
MBID. In addition, it was found that clients with MBID
are less likely to experience a desire for help compared to
clients without MBID. Future research should further ex-
plore differences in motivation between clients with and
without MBID and how these instruments are interpreted
by the clients and the clinical staff. Clients with MBID form
a specific group in the forensic addiction treatment centers
which requires specialized treatment from multidisciplinary
teams with knowledge of substance-related problems,
offending behavior, and MBID. More research is necessary
to adapt these specialized treatments to the needs and
intellectual functioning of clients with MBID and eventual-
ly prevent them for relapsing in substance use and offending
behavior.
Acknowledgements This research was financed by Tactus
Verslavingszorg. The authors are grateful to all clients and clinical staff
who participated in the study.
Author Contributions IL: designed and executed the study, analyzed
the data, and wrote concepts of the paper. RD: advised on the design,
assisted with data analyses and writing of the paper. JVDN: advised on
the design, assisted with the data analyses and writing of the paper.
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