Content uploaded by Katarina Alanko
Author content
All content in this area was uploaded by Katarina Alanko on Jan 17, 2019
Content may be subject to copyright.
Evidence for Heritability of Adult Men’s Sexual Interest in Youth
under Age 16 from a Population-Based Extended Twin Design
Katarina Alanko, PhD,* Benny Salo, MA,* Andreas Mokros, PhD,†and Pekka Santtila, PhD*
*Department of Psychology and Logopedics, Abo Akademi University, Turku, Finland; †Psychiatric University Hospital
Zurich, Center for Forensic Psychiatry, Zurich, Switzerland
DOI: 10.1111/jsm.12067
ABSTRACT
Introduction. Sexual interest in children resembles sexual gender orientation in terms of early onset and stability
across the life span. Although a genetic component to sexual interest in children seems possible, no research has
addressed this question to date. Prior research showing familial transmission of pedophilia remains inconclusive
about shared environmental or genetic factors. Studies from the domains of sexual orientation and sexually prob-
lematic behavior among children pointed toward genetic components. Adult men’s sexual interest in youthfulness-
related cues may be genetically influenced.
Aim. The aim of the present study was to test whether male sexual interest in children and youth under age 16
involves a heritable component.
Main Outcome Measures. The main outcome measure was responses in a confidential survey concerning sexual
interest, fantasies, or activity pertaining to children under the age of 16 years during the previous 12 months.
Methods. The present study used an extended family design within behavioral genetic modeling to estimate the
contributions of genetic and environmental factors in the occurrence of adult men’s sexual interest in children and
youth under age 16. Participants were male twins and their male siblings from a population-based Finnish cohort
sample aged 21–43 years (N =3,967).
Results. The incidence of sexual interest in children under age was 3%. Twin correlations were higher for monozy-
gotic than for dizygotic twins. Behavioral genetic model fitting indicated that a model including genetic effects as
well as nonshared environmental influences (including measurement error), but not common environmental influ-
ences, fits the data best. The amount of variance attributable to nonadditive genetic influences (heritability) was
estimated at 14.6%.
Conclusions. The present study provides the first indication that genetic influences may play a role in shaping sexual
interest toward children and adolescents among adult men. Compared with the variance attributable to nonshared
environmental effects (plus measurement error), the contribution of any genetic factors seems comparatively weak.
Future research should address the possible interplay of genetic with environmental risk factors, such as own sexual
victimization in childhood. Alanko K, Salo B, Mokros A, and Santtila P. Evidence for heritability of adult
men’s sexual interest in youth under age 16 from a population-based extended twin design. J Sex Med
**;**:**–**.
Key Words. Behavior Genetic; Twins; Pedophilia; Hebephilia; Sexual Interest
Introduction
There is a growing body of research concern-
ing sexual interest in both prepubescent (i.e.,
pedophilia) and pubescent (i.e., hebephilia) chil-
dren (for a review, see Seto [1]). Men are more
prone to sexual interest in children than women
[2]. The prevalence of sexual interest in children is
difficult to estimate due to the risk of underreport-
ing given the sensitivity of the topic. Santtila et al.
[3] reported 12-month incidence rates of adult
men’s sexual interest in prepubescent (less than 12
years of age) and pubescent girls and boys (12–15
years of age) for a population-based sample of male
1
© 2013 International Society for Sexual Medicine J Sex Med **;**:**–**
twins. Of the 33- to 43-year-old participants, 0.2%
reported sexual interest in children aged 12 or
younger and 3.3% in children under age 16. In a
community sample of 367 men aged 40–79 years
from a metropolitan area in Germany, 3.8%
(N =14) admitted to ever having had sexual
contact with a child [4]. In the same sample, 9.5%
of the respondents reported ever having mastur-
bated to sexual fantasies involving children. Like-
wise, in a convenience sample of 193 male
university students, 9% admitted to ever having
experienced sexual fantasies concerning children
[5]. Seto [1] concluded that pedophilia was rare,
occurring with a frequency of less than 5% in the
male population. Furthermore, the extant data,
albeit scarce, seem to indicate that sexual fantasies
involving children are more common than overt
sexual abuse of children. It should be noted,
however, that sexual abuse of children is not nec-
essarily indicative of pedophilia. Some men may
become sexually interested in or sexually abuse
children due to situational factors or antisociality,
for instance.
The etiology of sexual interest in children
remains largely unknown. Despite speculation to
this effect [6,7], no study to date has directly
addressed the possibility of sexual interest in chil-
dren being heritable. A small scale study by
Gaffney et al. [8] found sexual paraphilias to be
familial, also suggesting specificity for pedophilia
in the familial transmission. The researchers
compared the inpatient records of 21 nonpedo-
philic paraphiliacs, 33 pedophilic patients, and 33
depressed controls, studying whether other
members of the families of the participants were
also diagnosed with a paraphilia. The researchers
used the family history research method (i.e., study
of patient charts) to identify paraphiliacs among
the relatives of the participants, a method that
probably was sensitive to underreporting. Among
male first-degree relatives, the morbidity risk
for pedophilia was 10.3% within the families of
pedophilic patients compared with 3.7% within
the families of patients suffering from another
paraphilia. The morbidity risk differed signifi-
cantly (at P<0.01) between the relatives of pedo-
philic and nonpedophilic patients. Recently, a pilot
study involving five families in several genera-
tions reported familial aggregation for paraphilias
[9]. The results support heterogeneity of paraphil-
ias; in this case, the documented paraphilias
were homosexual and heterosexual pedophilia and
sexual sadism. However, the reported familial
transmission could be due to either shared envi-
ronmental or genetic influences. Therefore, a
genetically sensitive design, either a twin or an
adoption study, would be required to resolve
whether familial transmission was due to genetic
or environmental effects.
The heritability of pedophilia has most likely
not been studied because of sampling problems: a
comparatively low prevalence in the population
as well as likely resistance or denial on behalf of
participants due to the possible legal conse-
quences and social stigma related to the pheno-
type. It is also not clear why heritability should
be expected. Heritable sexual interest in prepu-
bescent children presents a Darwinian paradox
considering that alleles contributing to such
interest should be detrimental to biological
fitness and should, therefore, be selected against.
However, sexual interest in individuals of the
same sex also shows heritability [10–13], present-
ing an equal paradox. It has been suggested that
alleles contributing to sexual interest in individu-
als of the same sex could simultaneously convey
other advantages that outweigh the negative con-
sequences of homosexual interest with regard to
producing offspring. For instance, there is some
empirical evidence that genes coding for homo-
sexuality could increase fecundity in female car-
riers of the alleles [14,15]. Sexual orientation in
terms of gender can, to some extent, be com-
pared with pedophilia (as a sexual preference in
terms of age). Seto [16] concluded that in many
ways, such as age of onset and stability over time,
pedophilia and sexual orientation appear similar.
Another aspect in which similarities between
sexual gender orientation and paraphilias have
been found is concerning neurodevelopmental
factors. Rahman and Symeonides [17] found
some support for neurodevelopmental influences
on paraphilias, as has been found previously for
sexual orientation (for a review, see Rahman
[18]). The researchers found that scoring high on
the paraphilic measure was related to a signifi-
cantly greater number of older brothers, higher
right-hand 2D:4D (ratio between the second and
the fourth digit), and non-right-handedness com-
pared with low-paraphilic scorers.
In any case, it could be that sexual interest in
children could be a by-product of beneficial effects
on another phenotype. A large body of research
indicates that men are sexually attracted to indica-
tors of youthfulness in potential sexual partners
(e.g., Silverthorne and Quinsey [19]). Evolu-
tionary theory researchers have proposed that
men prefer youthfulness of the sexual object, as
2Alanko et al.
J Sex Med **;**:**–**
youthfulness can be seen as a sign of fertility (lit-
erature reviewed by Seto [1]). Pedophilia might
represent an extreme form of this youthfulness
attraction, in combination with a failure to regu-
late the mechanisms controlling for behavior in
cases when offending occurs [20]. A sizable pro-
portion of men from community samples reacted
with sexual arousal toward visual or auditive erotic
stimuli displaying or describing prepubescent
children [21–25].
Assuming that attraction to indicators of youth-
fulness is affected by multiple alleles and environ-
mental factors, a rare combination of all alleles and
environmental factors moving the person toward
extreme interest in youthfulness indicators could
result in sexual interest in children showing
heritability.
Alternatively, heritable personality or cognitive
characteristics could be the phenotypes that pre-
dispose individuals to sexual interest in children.
Pedophilic child molesters seem to differ from
nonpedophilic child molesters, sexual offenders
against adults, and normal controls in several
respects. There are indications of lower intelli-
gence [26] and altered hormone levels [27] and
hormonal responsivity [28] among pedophilic
sexual offenders, for instance. Furthermore, both
brain structure [29–32] and functioning [33–37]
seem to differ based on magnetic resonance
imaging studies. Pedophilic sexual offenders tend
to perform worse in executive function tasks
involving response inhibition and verbal fluency
[30,38,39], as well as processing speed [40],
although these findings are not unequivocal
[41,42]. In addition, pedophilic individuals seem to
differ in their electrophysiological brain responses
as two studies indicated [43,44]. Finally, child
molesters had somewhat higher average scores on
scales measuring introversion compared with
rapists with adult victims according to a meta-
analysis based on 26 studies [45] (see also Becerra-
García et al. [46]). More specifically, higher
introversion was noted for 77 noninstitutionalized
pedophiles from the community [47].
Both the peculiarities in neurocognitive abilities
and in the personality traits of pedophilic men
could be partially influenced by genetic effects. As
most of the research on pedophilia relies on
samples of incarcerated child molesters, however,
it could also be the case that the observed pecu-
liarities reflect the situation of pedophilic offenders
rather than of pedophilic individuals in general
[48]. In other words, deficits in response inhibi-
tion, brain abnormality, or particular personality
traits may not be representative of all pedophilic
men but only reflect the problems of those who
commit sexual offenses against children and get
institutionalized subsequently.
Alternatively, heritability could be due to dif-
ferences in susceptibility to environmental factors
during development. Blanchard et al. [49] noted a
cumulation of head injuries during childhood
involving unconsciousness in the biographies of
pedophilic men, for instance. Environmental
theories about sexual offending etiology stress the
role of adverse family environments (reviewed
by Hanson and Morton-Bourgon [50]). For
example, childhood sexual abuse has been found
to be more frequently reported by child sexual
offenders than non-offenders and nonsexual
offenders. These robust findings were reported in
meta-analyses both among adolescents [51] and
adults [52]. However, experiencing childhood
sexual abuse cannot be seen as a causal factor to
later child sexual offending, and generally speak-
ing, childhood sexual abuse has been associated
with an array of detrimental consequences. Sant-
tila et al. [53] reported that childhood sexual
interactions with other children increased the
likelihood of adult sexual interest in children
aged 15 or younger, especially in interaction with
own experiences of childhood physical and sexual
abuse. It may be that the likelihood of such expe-
riences resulting in sexual interest in children
might vary according to genetically determined
susceptibility of the person to environmental
influences. In a sample of hospitalized pedophilic
child molesters, Mokros et al. [54] compared the
correlative links of adverse childhood experiences
(such as parental neglect), signs of cerebral per-
turbation (e.g., low intelligence), and own expe-
riences of sexual abuse with the expression of
pedophilic inclination as evident in the offense
history. The factors of own sexual victimization
and adverse childhood experiences were statisti-
cally significant predictors for the expression of
pedophilia.
A possible way to conceptualize the potential
interplay of a genetic liability with detrimental
experiences is to consider a gene–environment
interaction model. This means some individuals
would be genetically more vulnerable to respond
to adverse environmental circumstances, such as
childhood sexual abuse, by developing sexual
interest in children. In fact, a gene–environment
interaction model could integrate findings of dif-
ferent kinds, such as early neurodevelopmental
deviances, and adverse childhood experiences,
Heritability of Sexual Interest in Youth 3
J Sex Med **;**:**–**
such as childhood peer rejection, or other experi-
ences, such as childhood sexual interaction with
other children. However, even if such an
interaction-based theory could help us understand
how a sexual interest in children could develop, it
does not explain why some men would go on to
offend children whereas other men do not offend.
For other phenotypes, such genetically deter-
mined susceptibility effects have been found. For
instance, Kohen et al. [55] reported the results
from a meta-analysis: a functional polymorphism
of the enzyme monoamine oxidase A moderated
the association between childhood maltreatment
on the risk of developing antisocial behavior, con-
stituting a genetic vulnerability factor.
The aim of the present study was to test
whether male sexual interest in children involves a
heritable component. We used twin model fitting
to estimate the likely contributions from genetic
and environmental sources. As sexual interest in
children is more common in men than in women
[2], the analyses were limited to male twins and
their male siblings. Still, the low occurrence of the
phenotype in the sample did not permit looking
specifically at pedophilic sexual interest (i.e.,
sexual interest in prepubescent children). Instead,
the study refers to sexual interest in prepubescent
and pubescent children (measured as sexual inter-
est in children under the age of 16). Adolescents
aged 15 may be considered past the age of
pedohebephilic interest (the age range for
hebephilia is usually considered 11–14); the results
from our research may concern a possible genetic
component in sexual interest directed toward
youthfulness more generally. We expected a com-
ponent of heritability to be present as earlier
studies indicated a genetic influence for the
related domains of sexual orientation [10–13], as
well as for problematic sexual behavior [56] and
sexual dysfunction [57].
Methods
Sample
The sample size was 3,967 men. For the genetic
analyses, 962 monozygotic (MZ) and 2,124 dizy-
gotic (DZ) twins, as well as 881 male siblings, were
available. Out of the pairs of twins whose zygosity
could not be determined, one twin per family was
chosen for further analyses, in which his zygosity
was set to DZ. Male twins from opposite-sex twin
pairs were also grouped together with same-sex
DZ twins. Zygosity was determined using ques-
tionnaire items completed by the twins [58]. Pre-
vious studies have shown that this method of
zygosity determination is 95% accurate when
compared with blood typing analyses [59].
All participants were 21 years of age or above.
The legal age of consent in Finland is 16 (or 18
in case the offender holds a trusted position),
with the perpetrator being at least 5 years older
than the child/youth. Thus, participants at least 5
years older than the object maximum age were
chosen. Some participants younger than 21
reported sexual interest and activity with children
below age 16; however, these participants were
excluded.
The sample came from the “Genetics of Sexu-
ality and Aggression Study” (GSA), a large
project conducted at the Center of Excellence in
Behavior Genetics at Abo Akademi University.
The main GSA sample consists of two separate
data collections. The first data collection was
carried out in 2005 and targeted 33- to 43-year-
old twins. The second data collection was carried
out in 2006 and targeted 18- to 33-year-old twins
and their siblings over 18 years of age. The par-
ticipants were identified from the Central Popu-
lation Registry of Finland and were native
Finnish speakers, born and currently residing in
Finland. There was no overlap between the
samples. In the first data collection, question-
naires, followed by a reminder letter and later a
new questionnaire, were sent to 5,000 twin pairs
(2,000 male same-sex pairs, 2,000 female same-
sex pairs, and 1,000 opposite-sex pairs) and finally
returned by 3,604 participants, resulting in an
overall response rate of 36%. The response rate
was 27% for male participants. In the second data
collection, 23,577 twins and their over 18-year-
old siblings were contacted. Those who con-
sented to participate could do so via a secure web
page or through posted questionnaires. A total of
10,524 participants responded to the survey,
yielding an overall response rate of 45%. The
response rate was 34% for male participants. The
sample consisted of 4,445 male participants alto-
gether. Out of these, a subsample was selected, as
described above. The questionnaires in both data
collections were extensive and covered a wide
range of sexual behaviors and attitudes, childhood
experiences, aggression, and alcohol use. The
purpose of the study was clearly described and
the voluntary and anonymous nature of the par-
ticipation emphasized.
The research plan was approved by the Ethics
Committee of the Department of Psychology at
Abo Akademi University.
4Alanko et al.
J Sex Med **;**:**–**
Measures
A measure consisting of three dimensions, espe-
cially designed for the GSA data collection, was
used to measure sexual interest, masturbatory fan-
tasies, and actual sexual behavior with children and
adolescents. The questions were the following:
“To which age group did the person belong that
you during the last 12 months (i) felt sexually
interested in and had sexual fantasies about, (ii)
thought of while masturbating, and (iii) engaged in
sexual activity with?” Responses were indicated as
age category options: 0–6, 7–12, 13–15, 16–19,
20–25, 26–30, 31–35, 36–40, 41–50, 51–60, and 61
and up. For the present analyses, dichotomous cat-
egories were created, consisting of affirmative
replies to any of the first three age categories (ages
0–15). Hereafter, variables were age regressed for
use in genetic analyses.
Statistical Analyses
All phenotypical statistical analyses were con-
ducted using SPSS 19 (SPSS Inc., Chicago, IL,
USA). The genetic analyses were conducted using
the program Mx [60] (Neale, Boker, Xie, & Maes,
2002), a software using structural equation model-
ing specialized for twin data. The standard quan-
titative genetic model for twin data rests on the
assumption that the observed (phenotypical) vari-
ance (Vp) in a trait is a linear function of additive
genetic influences (A), nonadditive genetic influ-
ences (D), common environmental influences (C),
and nonshared environmental influences (E) (i.e.,
Vp =A+D+C+E). Additive genetic influence
refers to the total effects of multiple alleles on the
phenotype. Nonadditive genetic influence refers
to the interactive effect among multiple alleles that
occupy the same loci on different chromosomes
(i.e., dominance) and multiple genes (i.e., gene–
gene interaction) on the phenotype. Shared and
nonshared environmental influences refer to non-
genetic influences that contribute to familial
resemblance among relatives and nongenetic
influences that uniquely influence individuals,
respectively [61]. When estimating these compo-
nents, measurement error is subsumed under the
nonshared environmental source of variance. A
twin model that includes additive genetic influ-
ences, nonadditive genetic influences, and shared
and nonshared environmental influences simulta-
neously would not be statistically identified [62].
Therefore, either nonadditive or common genetic
effects are measured together with additive genetic
and unique environmental effects. In the present
work, both ACE and ADE models were estimated
for comparative purposes.
The relative contributions of A, C/D, and E
effects for each measure were estimated using a
series of structural equation model-fitting analyses.
Models were estimated by full-information
maximum likelihood estimation, using the program
Mx [60]. The goal of this process was to minimize
twice the negative log likelihood (-2LL), which is
essentially an index of the discrepancy between the
data and the model. A -2LL estimate is estimated
for each individual, and the individual -2LL esti-
mates are summed over the entire sample to esti-
mate the overall -2LL. Comparisons between
models were made using the likelihood comparison
of the -2LL estimate for the models, which is
distributed as a chi-square statistic. A nonsignifi-
cant decrease in the -2LL indicates that the model
with fewer parameters provides a reliable and more
parsimonious fit to the data compared with the full
model and should therefore be preferred. We also
compared models using Akaike Information Crite-
rion (AIC) [63] and sample size adjusted Bayesian
Information Criterion (SABIC) [64]. Models
having lower AIC and SABIC values are preferred.
Raw data were used in the analyses with Mx.
Results
Prevalence of Sexual Interest in and Sexual Behavior
with Children under the Age of 15
Of all participants, 148 individuals reported sex-
ual interest in, masturbatory fantasies about, or
sexual activity with children under the age of 16.
Sexual interest in children was reported by 123
men, whereas masturbation to fantasies of children
under age 16 was reported by 104 individuals.
Sexual behavior together with a child under age 16
was reported by 10 individuals. Table 1 shows that
some participants had chosen several categories,
Table 1 Number of participants aged 21 or above
reporting sexual interest in and behavior with children
below 16 years of age
Age categories
0–15 (%)0–6 7–12 13–15
Interest 4 19 121 123 (3.0)
Masturbation 3 13 102 104 (2.7)
Sexual behavior 2 — 8 10 (0.3)
Note. N varied between 3,730 and 3,909 due to missing values. The partici-
pants responded to the following question: “To which age group did the person
belong that you during the last 12 months (i) felt sexually interested in and had
sexual fantasies about, (ii) thought of while masturbating, and (iii) engaged in
sexual activity with?”
Heritability of Sexual Interest in Youth 5
J Sex Med **;**:**–**
as the total number of participants who reported
sexual interest in children is lower than the sum of
participants for each variable over the three age
categories.
Correlations with Age of Participants
Small negative associations were found between
participant age and sexual interest in and mastur-
bation while thinking about children, indicating
that younger respondents (albeit above 21 years of
age) tended to report higher sexual interest
(rp=-0.08, P<0.01, Wald c2=8.61, P<0.01) and
masturbation (rp=-0.09, P<0.01, Wald c2=4.32,
P<0.05) involving children from these age
groups. A nearly significant negative association
was found also between participant age and sexual
behavior with children (rp=-0.08, P<0.01, Wald
c2=3.4, P=0.06).
Twin Intraclass Correlations (ICCs)
ICCs were computed separately for the two vari-
ables of sexual interest toward children. There
were, however, too few cases to calculate twin ICC
reliably for the sexual behavior with children vari-
able. For the other two variables, the correlations
indicated that genetic influences contributed to
the variance in these measures, as MZ correlations
were consistently higher than DZ correlations
(Table 2). There was no suggestion of the involve-
ment of shared environmental influences as the
DZ correlations were not higher than half the MZ
correlations. Genetic factors did not account com-
pletely for the observed variance because the MZ
twin correlations were not united. This denotes
that nonshared environmental influences and/or
measurement error also contribute to variance in
the variables of interest.
Genetic Model Fitting
Results from model-fitting analyses are presented
in Table 3. Analyses were not conducted for sexual
behavior with children because there were too few
participants reporting such activity.
Small genetic influences were found for both
the sexual interest and masturbatory fantasies. No
common environmental influences were found,
and an ADE model better fitted the data. When
estimating whether a significant decrease in model
fit occurred when a parameter (A,D) was left out
of the model, leaving both genetic parameters out
of the model resulted in a significant decrease in
model fit. However, it was not possible to identify
what kind of genetic influences were more influ-
ential. That is, if genetic influences were entirely
left out of the model, then the model did not
describe the observed data as well as a model
including genetic influences. The nonshared envi-
ronmental component explained the largest pro-
portion of the variation in the individual responses
for both the sexual interest in and masturbatory
fantasies about children variables.
Discussion
The present study was the first behavior genetic
study on male sexual interest in and masturbatory
fantasies about children and adolescents under the
age of 16. This is the first study that has been able
to empirically demonstrate a heritable component
to sexual interest in children. The study was based
on a population-based large sample, avoiding
many of the sampling problems of prior research.
A heritable component was found for both
sexual interest in and masturbatory fantasies about
children, as expressed by the items “I have felt
sexual interest toward and had sexual fantasies
about persons below age 16” and “I have mastur-
bated to fantasies of persons below age 16.” The
genetic component was small and the remaining
variation between participants was explained by a
large nonshared environmental component. The
present study was, thereby, able to resolve
why Gaffney et al. [8] as well as Labelle et al. [9]
found pedophilia to be a familial paraphilia. The
researchers in the first study found that pedophilia
was specifically transmitted in the family, in
Table 2 Twin intraclass correlations (95% confidence intervals) for sexual interest in, masturbation with, and sexual
activity with children
MZM DZM Sibling–sibling Twin–sibling
Sexual interest 0.20 (0.07, 0.31) -0.04 (-0.16, 0.08) -0.03 (-0.32, 0.27) -0.03 (-0.25, 0.18)
Masturbation 0.19 (0.06, 0.31) -0.05 (-0.17, 0.08) -0.03 (-0.36, 0.31) -0.03 (-0.26, 0.19)
Sexual activity — — — —
Note. There were too few cases per cell to estimate correlations for sexual activity. N varied between 3,730 and 3,909 due to missing values. The participants
responded to the following question: “To which age group did the person belong that you during the last 12 months (i) felt sexually interested in and had sexual
fantasies about, (ii) thought of while masturbating, and (iii) engaged in sexual activity with?” Variables were age-regressed prior to calculating the correlations.
MZM =monozygotic male pairs; DZM =dizygotic male pairs
6Alanko et al.
J Sex Med **;**:**–**
contrast to other paraphilias, which were nonspe-
cifically transmitted. The findings of the latter
study supported the heterogeneity and familiality
of paraphilias. The question raised by the studies
was whether the transmission was caused by
genetic influences or family environment. It
seems, in light of the present results, that at least
part of the transmission could be due to genetic
influences.
The fact that a genetic component to sexual
interest in children was found would seem at a first
glance to suggest a Darwinian paradox. Why
would genes that do not promote reproductive
success remain in the gene pool? As presented in
the introduction, there are several alternative
theoretical explanations according to which sexual
interest in children could be genetically influ-
enced. Considering that also adolescents (those
aged 15) were included in the object age group, it
is possible that a more general youthfulness attrac-
tion might account for (part of) the genetic influ-
ences. However, the present study was not able to
resolve which of these possibilities is most likely.
The results about the prevalence rates of pedo-
philic and hebephilic sexual interest and behavior
are consistent with the reported results using a
subgroup from the same sample [3]. The present
study reported on an additional item: masturba-
tion to thoughts about children of specific age
groups. The prevalence of male participants
reporting masturbating to thoughts about children
below age 16 was 2.7, which is in line with the
sexual interest prevalence estimate. The assump-
tion made during data collection was that mastur-
bation to thoughts about children would be less
frequent than sexual interest to children. Mecha-
nisms controlling for acting out fantasies behav-
iorally, even in private, were thought to be stricter
than those for sexual thoughts and interest. This
finding is also in accordance with the data reported
by Ahlers et al. [4], as well as by Brière and Runtz
[5] who noted lower frequencies for overt behavior
than for ideation. Furthermore, the prevalence
data showed that sexual interest in pubescent chil-
dren is more common than sexual interest in pre-
pubescent children—an outcome that is plausible
if pedophilic sexual interest is considered a more
extreme (and therefore less common) variant of
sexual age preference than hebephilic sexual inter-
est [3]. A small negative correlation was found
between sexual interest and age, that is, younger
men reported slightly more sexual interest in chil-
dren below age 16. The correlation might be
explained by a trend by younger participants to
Table 3 Estimates and 95% confidence intervals from full ACE and ADE models for male sexual interest and masturbation to thoughts of children below age 15
AC/DE -2LL* AIC
BIC adjusted
for sample size
Dc2
ACE vs. CE ACE vs. AE ACE vs. E
ADE vs. DE ADE vs. AE ADE vs. E
ACE
Sexual interest 0.106 (0.000; 0.197) 0.000 (-0.000; 0.072) 0.894 (0.803; 0.985) -2,764.933 -10,700.933 -11,098.546 3.334 0.00 5.223
Masturbation 0.131 (0.000; 0.256) 0.000 (0.000; 0.092) 0.869 (0.744; 0.996) -3,190.416 -10,792.416 -10,842.909 2.811 0.00 4.117
ADE
Sexual interest 0.000 (0.000; 0.158) 0.146 (0.000; 0.245) 0.854 (0.756; 0.956) -2,767.562 -10,703.562 -11,099.861 0.000 2.629 7.852*
Masturbation 0.000 (0.000; 0.200) 0.198 (0.000; 0.332) 0.802 (0.668; 0.950) -3,193.126 -10,795.126 -10,844.264 0.000 2.710 6.827*
*P<0.05; **P<0.01; ***P<0.001
The best fitting models are bolded.
A=additive genetic influences; C=common environmental influences; D=nonadditive genetic effects; E=nonshared environmental influences; AIC =Akaike Information Criterion; -2LL =twice the negative log likelihood;
BIC =Bayesian Information Criterion
Heritability of Sexual Interest in Youth 7
J Sex Med **;**:**–**
report on sexual interest in adolescents (or chil-
dren), for example, there might be fewer restric-
tions for a 22-year-old than for a 45-year-old to
report that he can find a 15-year-old attractive. If
considering the youthfulness attraction hypoth-
esis, it could be that men in their most fertile years
express a stronger attraction to youthfulness than
slightly older men.
Limitations and Future Research
It was not possible to differentiate between pedo-
philic and hebephilic interest, or interest in ado-
lescents aged 15 (i.e., aged above what has been
considered age limits of hebephilia), due to the low
frequency of both types of sexual interest. More-
over, there were too few informative pairs of twins
reporting sexual activity with children to enable
statistical analyses. Therefore, the results and the
subsequent discussion mainly concern sexual inter-
est in and masturbatory fantasies about children.
There are some limitations that need to be con-
sidered. First, it is likely that the extent to which
participants have reported sexual interest, mastur-
batory fantasies, and, especially, sexual activity
with children is an underestimate of true preva-
lence rates. Very likely, social desirability and fear
of consequences of being detected might have led
some participants not to report truthfully. Further-
more, it is possible that self-selection bias and
attrition bias were influencing individuals who
decided to fill in the questionnaire and to complete
it. Individuals with socially undesirable sexual
interests might have not wanted to fill in the ques-
tionnaire in the first place or then decided not to
complete it. However, it is also possible that indi-
viduals who were concerned about their sexual
interests actually were more eager to participate
and complete the questionnaire, as they might
have felt that somebody at last openly asked about
the topic.
These first results suggest that future research
should aim at studying genetic influences on sexual
interest in children and pedophilia more in depth.
A pertinent question to study would be gene–
environment interactions involving childhood
maltreatment and sexual abuse experiences. Fur-
thermore, the relevance of genetic correlations
should be explored. Given the low prevalence of
the phenotype of sexual interest in children, it
seems challenging, however, to combine it with
another relatively rare condition. Between 1% and
3% of boys, for instance, were estimated to
become victims of sexual abuse in Finland until
the age of 15 years [65]. Consequently, an even
much larger sample than the present one or other
research designs would be necessary to study both
phenotypes in conjunction.
Acknowledgment
The work has been partly funded by the MiKADO
project Missbrauch von Kindern: Aetiologie, Dunkelfeld,
Opfer.
Corresponding Author: Katarina Alanko, PhD,
Department of Psychology and Logopedics, Abo
Akademi University, Turku 20500, Finland. Tel:
+358-2-2154946; Fax: +358 2 2154833; E-mail:
katarina.alanko@abo.fi
Conflict of Interest: None.
Statement of Authorship
Category 1
(a) Conception and Design
Katarina Alanko; Pekka Santtila; Benny Salo
(b) Acquisition of Data
Pekka Santtila
(c) Analysis and Interpretation of Data
Katarina Alanko; Pekka Santtila; Benny Salo
Category 2
(a) Drafting the Article
Katarina Alanko; Pekka Santtila; Andreas Mokros
(b) Revising It for Intellectual Content
Katarina Alanko; Pekka Santtila; Andreas Mokros
Category 3
(a) Final Approval of the Completed Article
Katarina Alanko; Pekka Santtila; Benny Salo;
Andreas Mokros
References
1 Seto MC. Pedophilia and sexual offending against children:
Theory, assessment, and intervention. Washington, DC:
American Psychological Association; 2008.
2 Smiljanich K, Brière J. Self-reported sexual interest in chil-
dren: Sex differences and psychosocial correlates in a university
sample. Violence Vict 1996;11:39–50.
3 Santtila P, Antfolk J, Hartwig M, Sariola H, Sandnabba
K, Mokros A. Men’s sexual interest in children: One-year
incidence and correlates in a population-based sample of
Finnish male twins. 2012 (2012 Manuscript submitted for
publication).
4 Ahlers CJ, Schaefer GA, Mundt IA, Roll S, Englert H, Willich
SN, Beier KM. How unusual are the contents of paraphilias?
Paraphilia-associated sexual arousal patterns in a community-
based sample of men. J Sex Med 2011;8:1362–70.
5 Brière J, Runtz M. University males’ sexual interest in
children: Predicting potential indices of “pedophilia” in a
non-forensic sample. Child Abuse Negl 1989;13:65–75.
8Alanko et al.
J Sex Med **;**:**–**
6 Langevin R. A comparison of neuroendocrine and genetic
factors in homosexuality and in pedophilia. Ann Sex Res
1993;6:67–76.
7 Tost H, Vollmert C, Brassen S, Schmitt A, Dressing H, Braus
DF. Pedophilia: Neuropsychological evidence encouraging a
brain network perspective. Med Hypotheses 2004;63:528–31.
8 Gaffney GR, Lurie SF, Berlin FS. Is there familial transmission
of pedophilia? J Nerv Ment Dis 1984;172:546–8.
9 Labelle A, Bourget D, Bradford JMW, Alda M, Tessier P.
Familial paraphilia: A pilot study with the construction of
genograms. ISRN Psychiatry 2012 [Epub ahead of print]
doi:10.5402/2012/692813.
10 Alanko K, Santtila P, Harlaar N, Witting K, Varjonen M, Jern
P, Johansson A, von den Pahlen B, Sandnabba NK. Common
genetic effects on gender atypical behavior in childhood and
sexual orientation in adulthood: A study of Finnish twins. Arch
Sex Behav 2010;39:81–92.
11 Bailey JM, Dunne MP, Martin NG. Genetic and environmen-
tal influences on sexual orientation and its correlates in an
Australian twin sample. J Pers Soc Psychol 2000;78:524–36.
12 Långström N, Rahman Q, Carlström E, Lichtenstein P.
Genetic and environmental effects on same-sex sexual behav-
ior: A population study of twins in Sweden. Arch Sex Behav
2010;39:75–80.
13 Santtila P, Sandnabba NK, Harlaar N, Varjonen M, Alanko K,
von der Pahlen B. Potential for homosexual response is preva-
lent and genetic. Biol Psychol 2008;77:102–5.
14 Camperio Ciani A, Cermelli P, Zanzotto G. Sexually antago-
nistic selection in human male homosexuality. PLoS One
2008;3:e2282.
15 Camperio Ciani AS, Fontanesi L, Iemmola F, Giannella E,
Ferron C, Lombardi L. Factors associated with higher fecun-
dity in female maternal relatives of homosexual men. J Sex
Med 2012;9:2878–87.
16 Seto M. Is pedophilia a sexual orientation? Arch Sex Behav
2012;41:231–6.
17 Rahman Q, Symeonides DJ. Neurodevelopmental correlates
of paraphilic sexual interests in men. Arch Sex Behav
2008;37:166–72.
18 Rahman Q. The neurodevelopment of human sexual orienta-
tion. Neurosci Biobehav Rev 2005;29:1057–66.
19 Silverthorne ZA, Quinsey VL. Sexual partner age preferences
of homosexual and heterosexual men and women. Arch Sex
Behav 2000;29:67–76.
20 Quinsey VL, Lalumière ML. Evolutionary perspectives on
sexual offending. Sex Abuse 1995;7:301–15.
21 Fedora O, Reddon JR, Morrison JW, Fedora SK, Pascoe H,
Yeudall LT. Sadism and other paraphilias in normal controls
and aggressive and nonaggressive sex offenders. Arch Sex
Behav 1992;21:1–15.
22 Freund K, Langevin R, Cibiri S, Zajac Y. Heterosexual
aversion in homosexual males. Br J Psychiatry 1973;122:
163–9.
23 Freund K, McKnight CK, Langevin R, Cibiri S. The
female child as a surrogate object. Arch Sex Behav 1972;2:119–
33.
24 Hall GCN, Hirschman R, Oliver LL. Sexual arousal and
arousability to pedophilic stimuli in a community sample of
normal men. Behav Ther 1995;26:681–94.
25 Quinsey VL, Steinman CM, Bergersen SG, Holmes TF.
Penile circumference, skin conductance, and ranking responses
of child molesters and “normals” to sexual and nonsexual visual
stimuli. Behav Ther 1975;6:213–9.
26 Cantor JM, Blanchard R, Robichaud LK, Christensen BK.
Quantitative reanalysis of aggregate data on IQ in sexual
offenders. Psychol Bull 2005;131:555–68.
27 Bain J, Langevin R, Hucker S, Dickey R, Wright P, Schonberg
C. Sex hormones in pedophiles: I. Baseline values of six
hormones; II. The gonadotrophin releasing hormone test. Ann
Sex Res 1988;1:443–54.
28 Gaffney GR, Berlin FS. Is there hypothalamic-pituitary-
gonadal dysfunction in paedophilia? A pilot study. Br J Psy-
chiatry 1984;145:657–60.
29 Cantor J, Blanchard R. White matter volumes in pedophiles,
hebephiles, and teleiophiles. Arch Sex Behav 2012;41:749–52.
30 Hucker S, Langevin R, Wortzman G, Bain J, Handy L, Cham-
bers J, Wright S. Neuropsychological impairment in pedo-
philes. Can J Behav Sci 1986;18:440–8.
31 Schiffer B, Peschel T, Paul T, Gizewski E, Forsting M,
Leygraf N, Schedlowski M, Krueger THC. Structural brain
abnormalities in the frontostriatal system and cerebellum in
pedophilia. J Psychiatr Res 2007;41:753–62.
32 Schiltz K, Witzel J, Northoff G, Zierhut K, Gubka U, Fell-
mann H, Kaufmann J, Tempelmann C, Bogerts B. Brain
pathology in pedophilic offenders—Evidence of volume
reduction in the right amygdala and related diencephalic struc-
tures. Arch Gen Psychiatry 2007;64:737–46.
33 Sartorius A, Ruf M, Kief C, Demirakca T, Bailer J, Ende G,
Henn FA, Meyer-Lindenberg A, Dressing H. Abnormal
amygdala activation profile in pedophilia. Eur Arch Psychiatry
Clin Neurosci 2008;258:271–7.
34 Schiffer B, Krueger T, Paul T, de Greiff A, Forsting M,
Leygraf N, Schedlowski M, Gizewski E. Brain response to
visual sexual stimuli in homosexual pedophiles. J Psychiatry
Neurosci 2008;33:23–33.
35 Schiffer B, Paul T, Gizewski E, Forsting M, Leygraf N, Sched-
lowski M, Krueger THC. Functional brain correlates of het-
erosexual paedophilia. Neuroimage 2008;41:80–91.
36 Poeppl TB, Nitschke J, Dombert B, Santtila P, Greenlee MW,
Osterheider M, Mokros A. Functional cortical and subcortical
abnormalities in pedophilia: A combined study using a choice
reaction time task and fMRI. J Sex Med 2011;8:1660–74.
37 Walter M, Witzel J, Wiebking C, Gubka U, Rotte M, Schiltz
K, Bogerts B, Heinze HJ, Northoff G. Pedophilia is linked to
reduced activation in hypothalamus and lateral prefrontal
cortex during visual erotic stimulation. Biol Psychiatry
2007;62:698–701.
38 Eastvold A, Suchy Y, Strassberg D. Executive function profiles
of pedophilic and nonpedophilic child molesters. J Int Neu-
ropsychol Soc 2011;17:295–307.
39 Joyal CC, Black DN, Dassylva B. The neuropsychology and
neurology of sexual deviance: A review and pilot study. Sex
Abuse 2007;19:155–73.
40 Suchy Y, Whittaker JW, Strassberg DS, Eastvold A. Neu-
rocognitive differences between pedophilic and nonpedophilic
child molesters. J Int Neuropsychol Soc 2009;15:248–57.
41 Cohen LJ, Nikiforov K, Gans S, Poznansky O, McGeoch P,
Weaver C, King EG, Cullen K, Galynker I. Heterosexual male
perpetrators of childhood sexual abuse: A preliminary neurop-
sychiatric model. Psychiatr Q 2002;73:313–36.
42 Schiffer B, Vonlaufen C. Executive dysfunctions in pedophilic
and nonpedophilic child molesters. J Sex Med 2011;8:1975–
84.
43 Flor-Henry P, Lang RA, Koles ZJ, Frenzel RR. Quantitative
EEG studies of pedophilia. Int J Psychophysiol 1991;10:253–8.
44 Howard RC, Longmore FJ, Mason PA, Martin JL. Contingent
negative variation (CNV) and erotic preference in self-
declared homosexuals and in child sex offenders. Biol Psychol
1994;38:169–81.
45 Levin SM, Stava L. Personality characteristics of sex offenders:
A review. Arch Sex Behav 1987;16:57–79.
46 Becerra-García JA, García-León A, Muela-Martínez JA, Egan
V. A controlled study of the Big Five personality factors in sex
offenders, non-sex offenders and non-offenders: Influence
of group and childhood type. J Forens Psychiatry Psychol
(in press).
Heritability of Sexual Interest in Youth 9
J Sex Med **;**:**–**
47 Wilson GD, Cox DN. The child-lovers: A study of pedophiles
in society. London: Peter Owen; 1983.
48 Green R. Is pedophilia a mental disorder? Arch Sex Behav
2002;31:467–71.
49 Blanchard R, Christensen BK, Strong SM, Cantor JM, Kuban
ME, Klassen P, Dickey R, Blak T. Retrospective self-reports of
childhood accidents causing unconsciousness in phallometri-
cally diagnosed pedophiles. Arch Sex Behav 2002;31:511–26.
50 Hanson RK, Morton-Bourgon KE. (2004). Predictors of
sexual recidivism: An updated meta-analysis. (Research Rep.
No. 2004–02). Ottawa, Canada: Public Safety and Emergency
Preparedness Canada.
51 Seto MC, Lalumière ML. What is so special about male ado-
lescent sexual offending? A review and test of explanations
through meta-analysis. Psychol Bull 2010;136:526–75.
52 Jespersen AF, Lalumière ML, Seto MC. Sexual abuse history
among adult sex offenders and non-sex offenders: A meta-
analysis. Child Abuse Negl 2009;33:179–92.
53 Santtila P, Mokros A, Hartwig M, Varjonen M, Jern P, Witting
K, von der Pahlen B, Sandnabba NK. Childhood sexual inter-
actions with other children are associated with lower preferred
age of sexual partners including sexual interest in children in
adulthood. Psychiatry Res 2010;175:154–9.
54 Mokros A, Wagner A, Osterheider M, Seto MC, Nitschke J.
Risk factors in the expression of pedophilia. (Manuscript sub-
mitted for publication).
55 Kohen JK, Caspi A, Taylor A, Williams B, Newcombe R,
Craig IW, Moffitt TE. MAOA, maltreatment, and gene–
environment interaction predicting children’s mental health:
New evidence and a meta-analysis. Mol Psychiatry 2006;11:
903–13.
56 Långström N, Grann M, Lichtenstein P. Genetic and envi-
ronmental influences on problematic masturbatory behavior
in children: A study of same-sex twins. Arch Sex Behav
2002;31:343–50.
57 Jern P, Santtila P, Witting K, Alanko K, Harlaar N, Johansson
A, von der Pahlen B, Varjonen M, Vikström N, Ålgars M,
Sandnabba NK. Premature and delayed ejaculation: Genetic
and environmental effects in a population-based sample of
Finnish twins. J Sex Med 2007;4:1739–49.
58 Sarna S, Kaprio J, Sistonen P, Koskenvuo M. Diagnosis of twin
zygosity by mailed questionnaire. Hum Hered 1978;28:241–
54.
59 Eisen S, Neuman R, Goldberg J, Rice J, True W. Determining
zygosity in the Vietnam Era Twin Registry: An approach using
questionnaires. Clin Genet 1989;35:423–32.
60 Neale MC, Boker SM, Xie G, Maes HM. Mx: Statistical mod-
eling. 6th edition. Richmond, VA: Department of Psychiatry;
2002.
61 Plomin R, DeFries JC, McClearn GE, McGuffin P. Beha-
vioral genetics. 4th edition. New York: Worth Publishers;
2001.
62 Posthuma D, Beem AL, de Geus EJC, van Baal GCM, von
Hjelmborg JB, Iachine I, Boomsma DI. Theory and practice in
quantitative genetics. Twin Res 2003;6:361–76.
63 Akaike H. Factor analysis and AIC. Psychometrika
1987;52:317–32.
64 Sclove SL. Application of model-selection criteria to some
problems in multivariate analysis. Psychometrika 1987;52:
333–43.
65 Sariola H, Uutela A. The prevalence of child sexual abuse in
Finland. Child Abuse Negl 1994;18:827–35.
10 Alanko et al.
J Sex Med **;**:**–**
