Stability of Modus Operandi in Sexual Offending

Abstract

Little is known about the stability of modus operandi (MO) in sexual offending. The authors studied a cohort of all sexual offenders released from prison into the Swedish community during the years from 1993 to 1997 (N = 1,303) and analyzed sexual reoffenders’ MO in terms of victim choice, offense nature, and severity, comparing prior offenses with those registered during an average 6-year follow-up. Stability in MO, explored with Cohen’s Kappa and Odds Ratios (ORs) as measures of agreement across registered sexual offenses, was high, specifically with respect to victim choice. Results are discussed in relation to sexual deviance and opportunity structure. The authors argue that assessment and management of sexual recidivism risk might benefit from information on offense MO. Furthermore, the results could inform police investigative strategies, such as linking multiple offenses committed by an unidentified offender.

Full text

10.1177/0093854804267094 ARTICLE
CRIMINAL JUSTICE AND BEHAVIOR
Sjöstedt et al. / MODUS OPERANDI STABILITY
STABILITY OF MODUS OPERANDI
IN SEXUAL OFFENDING
GABRIELLE SJÖSTEDT
NIKLAS LÅNGSTRÖM
KNUT STURIDSSON
MARTIN GRANN
Karolinska Institutet
Little is known about the stability of modus operandi (MO) in sexual offending. The authors
studied a cohort of all sexual offenders released from prison into the Swedish community during
the years from 1993 to 1997 (N= 1,303) and analyzed sexual reoffenders’ MO in terms of victim
choice, offense nature, and severity, comparing prior offenses with those registered during an
average 6-year follow-up.Stability in MO, explored with Cohen’s Kappa and Odds Ratios (ORs)
as measures of agreement across registeredsexual offenses, was high, specifically with respect to
victim choice. Results are discussed in relation to sexual deviance andopportunity structure. The
authors argue that assessment and management of sexual recidivism risk might benefit from
information on offense MO. Furthermore, the results could inform police investigative strate-
gies, such as linking multiple offenses committed by an unidentified offender.
Keywords: modus operandi; stability; sexual offending; risk assessment; crime linkage
Sexual offenders are often regarded as a discrete offender subgroup
consisting of individuals exhibiting specific sexual offense behav-
609
AUTHOR NOTE: The authors gratefully acknowledge the financial support from
the Swedish Prison and Probation Services, the National Board of Forensic Medicine,
the Söderström-Königska Foundation, and the Bror Gadelius’ Memorial Foundation.
We would also like to thank three anonymous reviewers for their comments on an ear-
lier version of this article. Correspondence concerning this article should be
addressed to Gabrielle Sjöstedt, National Criminal Investigation Department, Crimi-
nal Intelligence Service, P.O. Box 122 56, S-102 26 Stockholm, Sweden. E-mail:
gabrielle.sjostedt@cns.ki.se
CRIMINAL JUSTICE AND BEHAVIOR, Vol. 31 No. 5, October 2004 609-623
DOI: 10.1177/0093854804267094
© 2004 American Association for Correctional Psychology

iors and preferences that are stable over time (e.g., Canter & Kirby,
1995; Ward, Hudson, & McCormack, 1997). Each offender’s collec-
tive pattern of crime-related behaviors is usually referred to as the
modus operandi (MO) of the offender (e.g., Kaufman, Hilliker, &
Daleiden, 1996; Warren et al., 1999). The assumption of sexual
offense exclusiveness and stability constitutes a crucial part of sexual
offender risk assessment, treatment, and commitment laws (Simon,
1997; Soothill, Francis, Sanderson, & Ackerley, 2000). This assump-
tion is also important in police investigative strategies when creating
links between different crime scenes based on the offender’s sexual
behavior (Grubin, Kelly, & Brunsdon, 2001). For instance, a number
of countries have implemented a large data-warehousing system,
known as ViC LAS (Violence Crime Linkage Analysis System; Col-
lins, Johnson, Choy, Davidson, & MacKay, 1998), into which infor-
mation on offending behaviors of both solved and unsolved cases are
entered to identify possible links between offenses with similar MOs.
However, previous studies of sexual offender specialization have
reached diverse conclusions regarding sexual offenders’ exclusive
engagement in sexual criminality as compared to other nonsexual
offenses, and also concerning the stability of specific sexual offense
MOs. Results from a meta-analysis indicated that sexual offenders
reoffend in general criminality, including property offenses, drug-
related criminality, and nonsexual violence, to a larger degree than
they commit new sexual offenses (Hanson & Bussière, 1998). These
results were supported by Soothill and colleagues (2000) with the
addition that sexual offenders who actually do recidivate in sexual
criminality tend to repeat previously exhibited offense behaviors. The
study by Soothill et al. followed over 6,000 sexual offenders convicted
in the United Kingdom in 1973 with regard to registered criminal
offenses during a 32-year period (from 1963 to 1994). The offenders
constituted four subgroups based on main offense in the sexual con-
viction: (a) indecent assault against a female, (b) indecent assault
against a male, (c) unlawful sexual intercourse with a girl younger
than 16, or (d) indecency between males. The results suggested that
60% of the total group had at least one conviction for any other offense
(excluding the 1973 index sexual offense), and 26% were reconvicted
for another sexual offense. The sexual recidivists were most likely to
be reconvicted of the same type of sexual offense as the main index
610 CRIMINAL JUSTICE AND BEHAVIOR

offense (i.e., category a,b,c,ordas referred above). Hanson, Scott,
and Steffy (1995) followed child molesters for 15 to 30 years and
compared their offending with that of nonsexual criminals. They con-
cluded that prior offense behavior predicted future offenses of the
same type. Furthermore, Guay and colleagues studied a group of 178
sexual aggressors and found that these offenders (comprising both
child molesters and rapists) remained stable in their victim
preferences across multiple offense occasions (Guay, Proulx, Cusson,
& Ouimet, 2001).
In contrast, other studies regarding temporal stability of sexual
offending MO indicate substantial offender heterogeneity and vari-
ability concerning victim preferences and offense type (Abel, Becker,
Cunningham-Rathner, Mittelman, & Rouleau, 1988; Grubin & Ken-
nedy, 1991; Simon, 1997, 2000). For example, Abel and colleagues
(1988) found considerable versatility in victim choice and amount of
physical contact when they interviewed 561 men who were referred
for clinical evaluation or treatment for possible paraphilia. During
these interviews, performed with a Certificate of Confidentiality, 20%
of the men reported offenses against both male and female victims,
more than 40% targeted victims in various age groups, 23% had both
intra- and extrafamilial victims, and 26% engaged in sexual offenses
both with and without physical contact. When specifically studying
the commission of various paraphilic acts, the authors concluded that
paraphiliacs with only one paraphilia are rare. Sexual versatility was
also described by Weinrott and Saylor (1991), who used computer-
ized self-reporting to study 99 sexual offenders included in a treat-
ment program. Among offenders classified as rapists (adult victim),
32% also reported having had sexual contact with a child. Conversely,
12% of offenders classified as child molesters revealed attempted
forcible sex with adult females. Among offenders knownto have only
molested victims outside the home, 34% had also committed offenses
within the family, and half of all offenders known to have perpetrated
incest offenses admitted to undetected abuse of a child outside the
home. Additionally, Studer, Clelland, Aylwin, Reddon, and Monro
(2000) explored the stability in offense type over time for intrafamilial
child molesters and found that more than half of all offenders with an
intrafamilial index offense also self-reported prior extrafamilial vic-
tims. Among offenders with an extrafamilial index offense, 13% had
Sjöstedt et al. / MODUS OPERANDI STABILITY 611

also sexually molested children within the family. A self-report study
of 91 child molesters found that 28% targeted both male and female
victims, and approximately 7% had also offended sexually against
adult victims (Elliott, Browne, & Kilcoyne, 1995). A substantial het-
erogeneity in offense-related behaviors has also been noted among
rapists who tend to use various amounts of instrumental, expressive,
or sadistic violence. Some rapists direct their attacks toward strangers,
whereas others target acquainted victims. Also, some offenders
committing rape engage in a wide range of sexual acts, whereas others
exclusively attempt vaginal intercourse (Langton & Marshall, 2001).
Most previous studies have explored sexual offenders’ criminal
behavior in general terms using broad concepts, such as sexual
offense type, or have explored the consistency of some single aspect
of offending behavior. Thus, there is a need to specifically focus on
multiple aspects of sexual offense MO in terms of victim choice,
nature, and severity of sexual offending, and to explore whether sex-
ual offenders tend to repeat prior offending behavior when commit-
ting new sexual offenses. Information about possible temporal stabil-
ity of sexual offense MO—that is, agreement between prior sexual
offense behavior and subsequent recidivism—could improve risk-
assessment procedures in directing postrelease crime-preventive
interventions for offenders who tend to repeat prior sexual offending
behavior. Knowledge about the nature and severity of potential sexual
reoffending could also be useful for decision making in clinical set-
tings. Furthermore, stability in sexual offense MO across separate
offense occasions could have implications for police work strategies,
such as linking crimes with an unknown perpetrator. This study set out
to specifically explore temporal stability of sexual offense MO in a
representative national cohort of sexual offenders convicted of a new
sexual offense during an average 6-year follow-up.
METHOD
PARTICIPANTS
All adult (≥18 years) males convicted of a sexual offense defined
by chapter 6 of the Swedish Penal Code (representing both contact
612 CRIMINAL JUSTICE AND BEHAVIOR

and noncontact sexual offenses including rape, child molestation,
indecent exposure, etc.), sentenced to prison in Sweden, and released
during the years between 1993 and 1997 (N= 1,400), were included in
a retrospective follow-up study. This study cohort of sexual offenders
was highly representative of identified offenders convicted for contact
sexual offenses in Sweden. To make sure that all participants were
available for follow-up, we excluded individuals (n= 97) who had
been ordered out of the country on completion of the prison sentence.
Thus, the remaining study cohort consisted of 1,303 participants with
a mean age at release from prison of 40.95 years (SD = 11.99, range
18-77). The most severe sexual offense in the index conviction
involved child molestation in 46% (n= 597) and rape in 42% (n= 548)
of the cases, and the total group had an average sentence length of
11.85 months (SD = 8.10, range 0-83). Eighty-seven percent (n=
1134) were citizens of European countries, 4% (n= 55) had African
citizenship, 6% (n= 73) Asian citizenship, and 3% (n= 33) American
citizenship.
PROCEDURE
Retrospective follow-up started at release from prison and ended
on December 31, 2000, rendering a mean postrelease follow-up time
of 5.68 years (SD = 1.39, range 3-8). Information on registered
reconvictions concerning sexual (i.e., chapter 6 of the Swedish Penal
Code) and violent nonsexual offenses (i.e., homicide, assault, rob-
bery, threats [verbally or with weapon], and violence against an offi-
cer) was retrieved from the nationwide registers of the National Coun-
cil for Crime Prevention.
Modus operandi (MO) is a broad concept that includes various
aspects of an individual offender’s specific course of action prior to,
during, and following the crime in question. Modus operandi usually
involves the offender’s way of planning, getting access to and control-
ling the victim, and also specific measures taken to escape the crime
scene without being detected or recognized (Warren et al., 1999). Fur-
thermore, MO can refer to differences in victim, perpetrator, and
offense characteristics (Kaufman et al., 1996). For the purpose of this
study, we focused on certain aspects of sexual offense MO, including
specific victim characteristics as well as the nature and severity of the
Sjöstedt et al. / MODUS OPERANDI STABILITY 613

offense. Thus, further referrals to sexual offense MO should be under-
stood in terms of this subselection of offense characteristics.
Information on prior sexual offense MO was collected retrospec-
tively using individual subject files from the National Prison and Pro-
bation Administration, including written court reports from previ-
ously registered convictions (see also Sjöstedt & Långström, 2001).
Data on sexual reoffense MO were retrieved from written court
reports, including prosecutor’s plaint, parties account, and court find-
ings. Because the Swedish judicial system does not allow for plea-
bargaining, cases are seldom lost because of sexual assaults being
pled down to general assault. For practical reasons, individual offense
occasions are usually lumped together during court procedures in
Sweden. Several authors have reported on or highlighted the extensive
underreporting of sexual (re)offending that occurs when using only
data on arrests or convictions (e.g., Doren, 1998; Hanson & Bussière,
1998; Weinrott & Saylor, 1991). For example, Weinrott and Saylor
illustrated this issue with one incest offender who self-reported more
than 1,000 sexual encounters with his two daughters and another
offender who reported one sexually abusive contact with each of 200
victims. Both offenders had only one registered sexual offense. Thus,
in an attempt to get closer to the true prevalence of sexual recidivism,
we collected data from each individual event of sexual reoffending
instead of using the court conviction as the smallest unit of analysis.
This implied that if an offender was reconvicted once for child moles-
tation, this was not automatically regarded as one reoffense occasion.
Instead, we reviewed the written court report to register victim type,
nature, and severity for each individual event of offending. Offense
occasions were considered as individual events if they were separated
by time when the offender had stopped the abusive behavior
(providing an opportunity to refrain from committing another sexual
offense).
Interrater reliability estimates (Cohen’s Kappa) for data extracted
from files were calculated on two occasions—first, when prior sexual
offense MO was coded from subject files, and then again when data on
sexual reoffense MO was extracted from written court reports. Each
estimate was computed based on 20 randomly chosen cases, inde-
pendently scored by two raters, indicating excellent interrater reliabil-
614 CRIMINAL JUSTICE AND BEHAVIOR

ity (Cohen, 1960; mean κ= .90 [range .83-1.00] and .95 [range .73-
1.00], respectively).
CODING OF SEXUAL OFFENSE MO
Various features of sexual offense MO in terms of offender-victim
contact (noncontact vs. physical contact), penetration, death threat,
victim injury, and victim characteristics (male, < 15 years, family/
relative, stranger) were coded for prior and recidivistic sexual
offenses.
Prior sexual offenses. Modus operandi characteristics in prior sex-
ual offenses were defined as follows: Noncontact offense implied a
sexual offense without physical contact (e.g., indecent exposure).
Physical contact comprised sexual offenses with any physical contact
between offender and victim including touching, fondling, or genital
contact. Penetration referred to oral, genital, or anal contact between
offender and victim. Death threat constituted both verbally expressed
threats and/or the use of weapons. Victim injury was assessed from
explicit information about victim bruises, bleeding, and so forth,
together with information about the offender’s use of violence in the
offense. It was not required that the victim had been medically exam-
ined to register the presence of victim injury. Victim male was regis-
tered when the offender had targeted a victim of male gender regard-
less of age. The absence of male victim consequently meant that the
offender only had offended against female victims. Victim < 15 years
included all victims younger than the age of 15, considered as children
in the Swedish judicial system. Victim family/related was defined as
biologically related victims as well as step relations; it was not
required that the offender lived in the same household as the victim.
Victim stranger included victims that the offender was unacquainted
with 24 hours before the offense took place. Information about vic-
tim-offender relationships defined as acquainted (but not family/
related) was not available during data collection concerning prior sex-
ual offending. Specific information on physical contact, penetration,
death threat, and victim injury was only available for the index offense
(the sexual offense that led to a prison sentence with release between
Sjöstedt et al. / MODUS OPERANDI STABILITY 615

1993 and 1997 and, thus, inclusion in this study), whereas the other
characteristics were registered if they had occurred in any prior
offense (including the index offense).
Sexual reoffending. Information on sexual reoffense MO was gath-
ered for each sexual offense occasion that was registered when
reviewing written court reports concerning sexual reconvictions dur-
ing the follow-up. The offense characteristic definitions were the
same as for prior sexual offenses. To summarize sexual reoffenseMOs
across multiple reoffense occasions, MO was coded in three different
ways: (a) for the first reoffense occasion, (b) the most frequent offend-
ing behavior across all reoffense occasions (present ≥50% of
reoffense occasions), and (c) prevalence of behavior during any of the
registered reoffense occasions (see Table 1). This implied that for
offenders with multiple reoffense occasions, different MOs could be
registered for the first as compared to the most frequent or any
reoffending behavior. However, for offenders with only one sexual
reoffense occasion, the MO during this single event was simulta-
neously registered as prevalent during the first, the most frequent, and
any reoffense occasion. Furthermore, when offenders targeted multi-
ple victims during a single offense occasion, characteristics of all vic-
tims were registered. For example, if one offender had seven offense
occasions involving a total of five stranger and four related victims
(having targeted multiple victims on two occasions), this offender
would be registered as having both a stranger and a related victim as
the most frequent victim relation. In parallel, this circumstance would
result in a coding of both any stranger and any family/related victim.
STATISTICAL ANALYSIS
Temporal stability of sexual offense MO was analyzed using
Cohen’s Kappa as a measure of chance-corrected agreement between
MO of prior sexual offenses and sexual recidivism (Kappa-estimates >
.40 = fair, > .60 = good, > .75 = excellent agreement; Cicchetti & Spar-
row, 1981). Associations between prior and recidivistic sexual offense
MO were also expressed with odds ratios (ORs) with 95% confidence
intervals (95% CIs).
616 CRIMINAL JUSTICE AND BEHAVIOR

RESULTS
RECIDIVISM CHARACTERISTICS
During follow-up, 6% (n= 75) of the total offender group (N=
1,303) were reconvicted for at least one sexual offense, and 13% (n=
166) were reconvicted for a violent nonsexual offense. Thus, violent
nonsexual reconvictions were generally more common as compared
to sexual reconvictions. Among these sexual and violent nonsexual
recidivists, a subgroup of offenders (n= 21) was reconvicted of both
reoffense types. The 75 sexual recidivists were reconvicted for a sex-
ual offense on 96 distinct sentence occasions during follow-up,
including, in total, 389 individual occasions of sexual offending (M=
5.19, SD = 10.53, Mdn = 1.00, range 1-62) directed against 276 differ-
ent victims (M= 3.68, SD = 7.13, Mdn = 1.00, range 1-45). A majority
of participants (n= 60) had one sentence occasion, 11 participants had
two, 2 participants had three, and 2 participants had four sexual
reconvictions. The sexual recidivists had a mean age at release from
prison of 37.19 years (SD = 11.53, range 20-72). Frequencies of dif-
Sjöstedt et al. / MODUS OPERANDI STABILITY 617
TABLE 1: Frequencies of Sexual Offense Modus Operandi (MO) Among Offend-
ers Reconvicted During Follow-Up (
n
= 75)
Sexual offending
Prior Reconviction
First Frequent Any
Offense MO
n
%
n
%
n
%
n
%
Noncontact offense 30 40 23 31 27 36 28 37
Physical contact 68 93a54 72 53 71 62 83
Penetration 40 57b23 31 24 32 29 39
Death threat 26 35 15 20 14 19 20 27
Victim injury 18 24 10 13 11 15 14 19
Victim
Male 11 15c12 16c13 18a18 25a
< 15 years 36 49c32 43 30 40 34 46c
Family/relative 16 21 14 19a14 19c14 19a
Stranger 34 46c31 43a33 45c34 46c
a.
n
= 73.
b.
n
= 70.
c.
n
= 74.

ferent aspects of sexual offense MO can be seen in Table 1. The
average time to first sexual reoffense was 23.34 months (SD = 18.16,
Mdn = 21.00, range 0-75).
STABILITY OF SEXUAL OFFENSE MO
Stability of MO across prior sexual offenses and sexual
reconvictions is presented in Table 2. The results indicate that victim
choice or preference is an offense characteristic that is highly stable
over time with offending behavior directed against males, children,
family/relative, and stranger victims exhibiting Kappa-values > .40
(cf. Cicchetti & Sparrow, 1981). The likelihood that a sexual offender
who had a prior sexual offense involving a male victim would, if
reconvicted, again assault a male victim during the first sexual
reoffense occasion was 180 times higher compared to offenders who
had no prior registration of a male victim (i.e., only female victims).
The elevated risk of repeating a previous victim preference was 17
times higher for child victims, 27 times higher for family/related vic-
tims, and almost 9 times higher for stranger victims, compared to
offenders who did not exhibit these MOs in prior sexual offending.
Furthermore, it is interesting to note that none of the offenders with a
noncontact index offense were registered with a sexual offense
involving physical contact as their first or most frequent recidivistic
sexual behavior. Noncontact offenses, penetration, death threat, and
victim injury were moderately stable over time (see Table 2).
DISCUSSION
Stability over time for various aspects of sexual offense MO was
studied among all offenders reconvicted for a sexual offense in a rep-
resentative nationwide 5-year cohort of sexual offenders released
from prison. Our results suggest that the examined aspects of sexual
offense MO were quite stable over time, specifically regarding victim
preferences. These results corroborate earlier findings by Guay and
colleagues (2001). No major differences were observed when stability
was analyzed in relation to the first, the most frequent, or any expres-
sion of MO in sexual reoffending. Furthermore, sexual offenders sen-
618 CRIMINAL JUSTICE AND BEHAVIOR

TABLE 2: Stability of Modus Operandi (MO) in Sexual Offending Among Offenders Reconvicted During Follow-Up (
n
= 75)
Similar MO in Sexual Reoffending
First Frequent Any
Prior Sexual Offending Kappa OR (95% CI) Kappa OR (95% CI) Kappa OR (95% CI)
Noncontact offense .34 4.63 (1.62-13.18) .35 4.58 (1.67-12.54) .38 5.25 (1.91-14.47)
Physical contact .33 a.31 a.28 9.83 (1.44-67.19)
Penetration .32 7.36 (1.92-28.28) .30 5.32 (1.57-18.07) .31 4.42 (1.49-13.13)
Death threat .31 5.50 (1.63-18.56) .34 7.03 (1.93-25.60) .38 6.00 (1.98-18.20)
Victim injury .22 4.00 (1.01-15.91) .28 5.20 (1.36-19.91) .29 4.55 (1.32-15.62)
Victim
Male .79 180.00 (16.84-1923.87) .74 130.50 (13.07-1303.01) .60 60.75 (6.76-545.73)
< 15 years .59 17.16 (5.22-56.42) .54 13.20 (4.10-42.46) .64 22.40 (6.57-76.41)
Family/relative .61 27.00 (6.16-118.35) .61 27.50 (6.28-120.49) .61 27.00 (6.16-118.35)
Stranger .49 8.91 (3.04-26.11) .53 10.76 (3.63-31.96) .51 9.26 (3.19-26.87)
Note
. Stability is expressed as agreement between prior offenses and reoffenses using Cohen’s Kappa and Odds Ratios (OR) with 95% confi-
dence intervals (95% CI).
a. No offenders with a nonphysical index offense had a physical offense as their first or most frequent sexual reoffense.
619

tenced to prison were more than twice as likely to be reconvicted for
violent nonsexual offenses compared to sexual offenses (13% vs. 6%,
respectively).
The observed results could have both individual and contextual
explanations. Earlier studies have pointed out sexual deviance as a
quite stable individual trait (e.g., Hanson, 1998), which serves as an
important motivator in sexual offending (Hanson & Bussière, 1998).
Thus, because offenders’sexual deviancies are likely to be reflected in
the MO of sexual offending (e.g., the choice of male victims), an
offender with a pervasive sexual deviance would be more likely to
present a stable MO across offense occasions. A possible contextual
explanation is that sexual offending MO reflects an opportunity struc-
ture that is part of the offender’s everyday environment. One example
could be the presence of potential victims; if an offender frequently
moves in bar quarters and another perpetrator often passes by school
or playground areas, they are likely to encounter different types of
potential victims. However, it could be argued that offenders actively
seek out certain situations, which, in turn, influence the opportunity
structure. Also, offenders characterized by a generally antisocial life-
style and/or a more nonspecific sexual deviance might be less prefer-
ential when targeting victims and rely more on contextual opportu-
nity, which would be in accord with an “opportunistic” subgroup of
sexual offenders (cf. Hazelwood, 2001; Prentky & Knight, 1991).
Soothill and colleagues argued that sexual offenders could be both
“generalists” and “specialists” simultaneously (Soothill et al., 2000).
Thus, individual offenders could engage in repeated sexual offending
as part of a generally criminal lifestyle. Hypothetically, these individ-
uals may exhibit a pattern of general criminality that is more extensive
than their sexual offending, leading to the conclusion that sexual
offenders are generalists. However, when specifically studying stabil-
ity of sexual offending MO they might still exhibit a pattern that is sta-
ble and indicative of specialist behavior. This view of sexual offenders
as both generalists and specialists was supported by our results.
STRENGTHS AND LIMITATIONS OF THE PRESENT STUDY
This study included a nationwide cohort of all sexual offenders
released from Swedish prisons during a 5-year period, a study group
620 CRIMINAL JUSTICE AND BEHAVIOR

highly representative for contact sexual offenders within the Swedish
judicial system. We collected information regarding each reoffense
occasion instead of relying on the judicial grouping into offense regis-
trations; this resulted in a much larger number of individual offense
occasions available for analysis.
Two possible explanations for stability in sexual offense MO have
been discussed thus far: (a) offender sexual deviance and (b) opportu-
nity structure. However, there is also the possibility that the observed
stability is an expression of procedures within the judicial system with
selection processes influencing which offenses lead to successful
identification, apprehension, and conviction of a perpetrator (e.g.,
Bachman, 1998; Guay et al., 2001). These problems are salient fea-
tures of most studies involving sexual offenders. At the same time,
sexual offenses that go unreported and offenders who never are con-
victed will seldom come to the attention of authorities, thus leaving
professionals without the possibility to intervene and prevent new
offenses by these “dark figure offenders.” Still, it is important to keep
in mind that the stability of registered sexual offense MO maynot gen-
eralize to self-reported sexual offense behavior (e.g., Abel et al. 1987;
Abel et al., 1988; Aylwin et al., 2000; Weinrott & Saylor, 1991; see
also Doren, 1998, for a thorough discussion on dark figures). Fur-
thermore, the relatively low recidivism rate yielded a limited study
group for the analyses of stability in sexual offending MO.
CLINICAL AND OPERATIVE IMPLICATIONS
Our data indicate a fairly robust stability in sexual offense MO that
could inform professionals in their risk-management strategies during
conditional release or parole. Dvoskin and Heilbrun (2001) proposed
that adequate risk assessment should consider “for what, over what
period of time, under what circumstances, and in light of what inter-
ventions?” (p. 6). The results from this study support further elabora-
tion of sexual offender recidivism risk in clinical practice through
specification of potential sexual reoffense characteristics and impor-
tant risk situations. When evaluating the postrelease social context, it
could be valuable to particularly address the offender’s prior victim
preferences. Probation and parole officers, as well as other profes-
sionals working with the offender in the community, could be recom-
Sjöstedt et al. / MODUS OPERANDI STABILITY 621

mended to pay attention to changes in the offender’s social situation
that influences the criminal opportunity structure.
In addition to providing potential guidance to risk assessment and
management decisions, temporal stability in offense MO is a vital part
of investigative police work, where stability in offending behavior is a
fundamental assumption when linking multiple offenses purportedly
committed by the same unknown offender. The present results con-
cerning high stability in sexual offense characteristics could possibly
inform police practices and improve criminal investigations. Because
victim choice seemed particularly consistent across repeated offense
occasions, multiple offenses directed against the same victim type
might serve as a crude first grouping strategy in this linkage process.
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