The Relation of Toxoplasma Infection
and Sexual Attraction to Fear, Danger,
Pain, and Submissiveness
and Radim Kuba
Behavioral patterns, including sexual behavioral patterns, are usually understood as biological adaptations increasing the fitness of
their carriers. Many parasites, so-called manipulators, are known to induce changes in the behavior of their hosts to increase their
own fitness. Such changes are also induced by a parasite of cats, Toxoplasma gondii. The most remarkable change is the fatal
attraction phenomenon, the switch of infected mice’s and rat’s native fear of the smell of cats toward an attraction to this smell.
The stimuli that activate fear-related circuits in healthy rodents start to also activate sex-related circuits in the infected animals.
An analogy of the fatal attraction phenomenon has also been observed in infected humans. Therefore, we tried to test a
hypothesis that sexual arousal by fear-, violence-, and danger-related stimuli occurs more frequently in Toxoplasma-infected
subjects. A cross-sectional cohort study performed on 36,564 subjects (5,087 Toxoplasma free and 741 Toxoplasma infected)
showed that infected and noninfected subjects differ in their sexual behavior, fantasies, and preferences when age, health, and the
size of the place where they spent childhood were controlled (F(24, 3719) ¼2.800, p< .0001). In agreement with our a priori
hypothesis, infected subjects are more often aroused by their own fear, danger, and sexual submission although they practice
more conventional sexual activities than Toxoplasma-free subjects. We suggest that the later changes can be related to a decrease
in the personality trait of novelty seeking in infected subjects, which is potentially a side effect of increased concentration of
dopamine in their brain.
Toxoplasma gondii, sexual behavior, sadism, sexual domination, sexuality
Date received: September 22, 2015; Accepted: June 24, 2016
Biologists are usually inclined to explain sexual preferences
and behavior as biological adaptations covering individual
reproductive success (Barbaro, Pham, & Shackelford, 2015;
Barthes, Crochet, & Raymond, 2015; Hellstrand & Chryso-
choou, 2015; Russell, DelPriore, Butterfield, & Hill, 2013).
Such an interpretation concerns not only the ‘‘normal’’ sexual
behavior and preferences expressed by a majority of people but
also less frequent behavioral patterns, such as homosexuality,
sadism, masochism, fetishism, and so on. Alternative sexual
patterns are present in all populations in relatively high and
constant frequencies, suggesting that these are sustained there
by some form of selection, for example, frequency dependent
selection rather than represent a part of sexual pathology. It has
been, for example, suggested that male homosexuality can
increase either the direct fitness of its carrier (Dewar, 2003;
Macintyre & Estep, 1993) or it can increase his inclusive
fitness by channeling resources toward his female relatives
(Kirby, 2003; Rahman & Hull, 2005). Similarly, sexual dom-
inance and sexual submissiveness have been suggested to
increase the biological fitness of individuals by increasing the
likelihood of women expressing sexual submissiveness, for
example, masochism, to acquire ‘‘good genes’’ or ‘‘good
resources’’ from dominant men (Jozifkova, Bartos, & Flegr,
Department of Philosophy and History of Science, Faculty of Science, Charles
University, Prague, Czech Republic
National Institute of Mental Health, Klecany, Czech Republic
Jaroslav Flegr, Department of Philosophy and History of Science, Faculty
of Science, Charles University, Vinic
´7, 128 44 Prague 2, Czech Republic.
July-September 2016: 1–10
ªThe Author(s) 2016
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2012). It has also been suggested that sexual dominance and
submissiveness could increase in-pair cohesion and coopera-
tion which could consequently result in an increased number of
offspring in hierarchically disparate couples even within a
modern community (Jozifkova, Konvicka, & Flegr, 2014).
An alternative evolutionary explanation of less frequent sex-
ual behavior traits and sexual preferences is suggested by the
general conception of extended phenotype (Dawkins, 1983)
and its special application, the parasite manipulation theory
(Moore, 2002). Certain categories of phenotypic traits of an
organism could be the products of genes sitting within a body
of other biological species and increasing their own between-
generation transmission potential often at the expense of the
fitness of carriers of these traits. Many parasites can change the
morphology or behavior of their host in such a way that
increases the efficiency of their transmission from an infected
to noninfected host. Such manipulative activity was observed
in many parasites transmitted from intermediate hosts to defi-
nitive hosts by predation. In human parasites, manipulative
activity, both the induction of morphological and behavioral
changes, was observed in the protozoan parasite Toxoplasma
gondii (Flegr & Hrdy
´, 1994; Webster, 1994). This parasite can
infect any warm-blooded animal, including about one third of
humans worldwide (Pappas, Roussos, & Falagas, 2009; Tenter,
Heckeroth, & Weiss, 2000). Toxoplasma can reproduce sexu-
ally in an intestine of its definitive host, a cat. It has been
known for a long time that the parasite may live dormant for
years in tissue cysts in immunoprivileged organs, inducing
many behavioral changes in its intermediate host. For example,
it prolongs the reaction times of its host, increases its vigility,
and alters the tendency to seek new stimuli (Flegr et al., 2003;
Hodkova´, Kodym, & Flegr, 2007; Hrda´, Voty
´pka, Kodym, &
Flegr, 2000). The most remarkable toxoplasmosis-associated
behavioral change is the so-called fatal attraction phenom-
enon—the switch from the mice’s and rats’ native fear of the
smell of cats toward an attraction to this smell (Berdoy, Web-
ster, & Macdonald, 2000). The existence of this phenomenon
has been observed in mice and rats in about 20 studies per-
formed in several laboratories worldwide and its analogy,
higher ratings of attractiveness of smell of highly diluted cat
(but not other species) urine was observed even in humans
(Flegr, Lenochova´, Hodny
´, & Vondrova´, 2011). The neurolo-
gical mechanism of this change in rats was revealed recently. It
was shown that in the infected rats expressing fatal attraction,
Toxoplasma is able to reprogram the brain’s genetic machinery
by specific demethylation of certain regulatory elements of
genes (Dass & Vyas, 2014). The cat odor activates fear-
associated medial amygdala circuits in normal rodents. In the
Toxoplasma-infected rodents, the same odor also activates the
amygdala circuits responsible for sexual behavior. It is possible
that Toxoplasma could increase its chances for alimentary
transmission from infected rodents to cat intestine by making
any perceived danger (not just the odor of cats) ‘‘smell sexy’’ to
the infected host. Currently, between 10%and 80%of people
are infected in most countries (Pappas et al., 2009; Tenter et al.,
2000). However, less than 100 years ago, most of the
population was probably infected in all countries where feline
hosts of Toxoplasma lived, that is, nearly everywhere. It is there-
fore possible that sexual preferences and behaviors are at least
partly influenced by the Toxoplasma infection. It is even possi-
ble that the frequent association of sexual arousal with violence,
danger, fear, and exchange of power could be a product of
manipulation activity of Toxoplasma, which was adaptive in its
natural hosts—the rodents (Flegr & Markos, 2014).
In the present study, we tested a hypothesis that Toxo-
plasma-infected and Toxoplasma-free subjects differ in their
sexual behavior, desires, and preferences. Specifically, we
tested our animal studies-based hypothesis that the infected
subjects were more often sexually aroused by their own fear,
danger, and sexual submission. For this purpose, we ran a
large-scale cross-sectional questionnaire study on the Internet
population with a precise number of 36,564 Czech and Slovak
people who completed the survey.
Material and Method
The Internet questionnaire was distributed as a Qualtrics
survey. The subjects were invited to participate in the study
using a Facebook-based snowball method (Kankova, Flegr, &
Calda, 2015) by posting an invitation to participate in a ‘‘study
testing certain evolutionary psychological and parasitological
hypotheses, containing many questions related to sexual life’’
on the wall of the Facebook page ‘‘guinea pigs’’ (‘‘Pokusnı´
kra´lı´ci’’ in Czech) for Czech and Slovak nationals willing to
take part in diverse evolutionary psychological experiments
(www.facebook.com/pokusnikralici). The participants were
informed about the aims of the study on the first page of the
electronic questionnaire. They were also provided with the
following information: ‘‘The questionnaire is anonymous and
obtained data will be used exclusively for scientific purposes.
Your cooperation in the project is voluntary and you can ter-
minate it at any time by closing this web page. You can also
skip any uncomfortable questions; however, the most valuable
are the complete data. Only subjects above 15 years old are
allowed to take the questionnaire. If you agree to participate in
the research and are above 15, press the ‘Next’ button.’’ Some
pages of the questionnaire contained the Facebook share but-
ton. These buttons were pressed by 1,163 participants, which
resulted in obtaining data from 36,564 responders in total
between January 22, 2015, and February 11, 2016. The project,
including the method of obtaining an electronic consent with a
participation in the study, was approved by the institutional
review board of the Faculty of Science, Charles University
(Komise pro pra´ci s lidmi a lidsky
´m materia´lem Prˇı´rodoveˇdeck´e
Fakulty Univerzity Karlovy), No. 2015/01.
Theelectronicsurvey¼Sexual Preferences and Behaviors
Inventory 2015 (SPBI-2015) consisted of five already published
questionnaires studying various facets of human sexuality, such
as the Hurlbert Index of Sexual Narcissism (Hurlbert, Apt,
Gasar, & Wilson, 1994), the Revised Sociosexual Orientation
Inventory (SOI–R) (Penke & Asendorpf, 2008), the Three-
Domain Disgust Scale (Tybur, Lieberman, & Griskevicius,
2009), the International Personality Item Pool (IPIP)–Domi-
nance Scale (Goldberg, 1999; Goldberg et al., 2006), and the
Attraction to Sexual Aggression Scale (Malamuth, 1989) mod-
ified and supplemented with questions to cover a broader spec-
trum of sexual preferences and sexual behaviors. The survey
also contained a questionnaire collecting various socioeco-
nomic, demographic, health-related, epidemiologic and psycho-
logical data, and three projective psychological tests.
Altogether, the survey consisted of 701 questions (a few more
in female version) and the mean time necessary to complete it
was about 89 min (the mode was 72 min). In the present study,
we used only the information about gender, age, the size of the
population of the town where the responders spent most of their
childhood rated on 6-point scale, sexuality-related questions,
and three health-related questions (physical and psychological
conditions rated on 6-point scales and number of specialized
medical doctors the subject had to regularly attend [not for pre-
vention] at least once in the past 5 years). The subject also had to
answer a question ‘‘Are you infected with Toxoplasma, the cat
parasite that is dangerous especially to pregnant women?’’ by
ticking one of the three suggested answers: ‘‘I do not know or am
not sure, I was not laboratory tested,’’ ‘‘No (I was tested by a
medical doctor and the blood test gave a negative result,’’ and
‘‘Yes (I was tested by a medical doctor and the blood test gave a
positive result.’’ Implicitly, the first answer (‘‘I do not know
...’’) was ticked.
At the end of the questionnaire, the participants could pro-
vide their unique ‘‘guinea pig code.’’ Because of the rather
sensitive sex- and health-related contents of this questionnaire,
it was ‘‘signed’’ only by 8%of the responders. However, other
questionnaires distributed in this community were signed by
most of responders (Flegr & Hodny, 2016). These results
showed that most of participants who were aware of their tox-
oplasmosis status (and nearly all male responders) had been
tested for toxoplasmosis in our lab. We always used comple-
ment fixation test, IgG enzyme-linked immunosorbent assay
(ELISA), and IgM ELISA tests to reveal the Toxoplasma infec-
tion and to discriminate between acute and latent form of the
infection (Flegr, Kodym, & Tolarova´, 2000).
Before statistical analysis, we filtered out less than 1%of our
data because it appeared suspicious (too high or too short body
height, too low or too high body mass or age, too short duration
of the test, etc.).
IBM SPSS Statistics 21.0 was used for all statistical tests
including factor analysis (principal axis factoring [PAF], mean
substitution of missing data, and direct oblimin normalized
rotation). We computed optimal number of independent factors
using parallel analysis (O’Connor, 2000).
Associations between ordinal and binary data were analyzed
by partial Kendall’s correlation test (Kanˇ kova´, Kodym, &
Flegr, 2011; Siegel & Castellan, 1988). This test measures the
strength and significance of association between binary, ordi-
nal, and continuous data regardless of their distributions. The
partial Kendall’s treflects the probability that the value of a
particular dependent variable for the Subject A is higher than
for Subject B when the value of an independent variable for
Subject A is higher than for the Subject B. This technique
enabled us to control for one confounding variable, for exam-
ple, the age of a responder. The Excel sheet for computing
partial Kendall’s tand the significance between Variables A
and B after the Variable C is controlled based on Kendall t’s
AB, AC, and BC. It is available at http://web.natur.cuni.cz/
flegr/programy.php. Toxoplasmosis is known to have different,
often opposite impacts on the behavior and personality men
and women. Therefore, we performed all analyses for all
responders and also separately for the male and female
The final data set of 36,564 responders contained records of
2,470 Toxoplasma-free men (age 37.86, SD ¼13.99), 212
Toxoplasma-infected men (age 36.15, SD ¼13.97), 2,617
Toxoplasma-free women (age 33.73, SD ¼11.91), and 529
Toxoplasma-infected women (age 34.83, SD ¼11.28). Infected
women were older than noninfected women (t(3146) ¼
2.034, p¼.042); the difference in age between infected and
noninfected men was not significant (t(2682) ¼1.711, p¼
.088). The prevalence of toxoplasmosis in women (16.8%) was
higher than in men (7.9%), w
¼103.575, p< .00001.
The sexual behavior, desires, and preference of responders
were characterized by 350 variables, including their responses
to abovementioned standard questionnaires (Sexual Narcis-
sism, Sexual Attitude Scale SOI-R, Disgust Scale, Dominance
Scale, and modified Attraction to Sexual Aggression Scale) and
questions of our SPBI-2015 (see Online Supplement 1) and the
questionnaire concerning sexual preferences and types of por-
We computed the optimal number of 24 independent factors
using parallel analysis. The actual eigenvalues from PAF and
the random order eigenvalues from parallel analysis are illu-
strated in Figure 1.
Therefore, the factor analysis reduced variables to 24 inde-
pendent factors with eigenvalues higher than 2.361 that
together explained 53.034%of the between-subject’s variance
of sexual behavior and preferences. The nature of most of these
factors was able to be identified on the basis of variables that
positively or negatively loaded particular factors—factor
names as well as example items defining the factors and the
eigenvalue for each factor are represented in Table 1 (for full
detail, see Online Supplement 2).
A multivariate analysis of variance with all 24 factors
as dependent variables showed a significant effect of toxo-
plasmosis for all subjects (F(24, 5828) ¼3.506, p< .0001),
Flegr and Kuba 3
men (F(24, 2682) ¼2.358, p< .0001), and women (F(24,
3146) ¼1.798, p¼.001). Regardless of the fact that age
correlated with most of sexual life–related factors, the multi-
variate analyses of covariance with toxoplasmosis status and
the age of subjects as independent variables gave nearly iden-
tical results (F(24, 5828) ¼3.484, p< .0001), men (F(24,
2682) ¼2.290, p< .0001), and women (F(24, 3146) ¼
1.782, p¼.011). Very similar results were also obtained
when a larger set of potential confounding variables, namely,
age, population size of the place of residence, self-rated phys-
ical conditions, psychical conditions, and the number of med-
ical specialists visited more than once during the past 5 years
were controlled: all (F(24, 3719) ¼2.800, p< .0001), men (F(24,
1542) ¼1.899, p¼.005), and women (F(24, 2177) ¼1.732, p¼
.015). Here, the number of subjects was lower, and therefore p
values were higher as not all subjects responded to all questions.
Most sexual life–related factors had nonnormal distribution.
To avoid performing a different type of transformation for each
of 24 different factors, we used the nonparametric Kendall
method for searching for association between toxoplasmosis
and particular factors. We used nonparametric partial Kendall
correlation test that enabled us to control for one confounding
variable, here the age of responders. The results presented in
Table 2 show that 18 of the 24 sex life–related factors, included
the factor F3 (violence-associated arousal), expressed signifi-
cant association with Toxoplasma infection for all subject, 11
for women, and 12 for men. Most of the observed associations
(32) were significant after backward sequential correction for
multiple tests (Table 2) and many of them (21), including that
for sadism, remained significant even after the most conserva-
tive simple Bonferroni’s correction for multiple tests. More-
over, the number of observed significant associations was 41,
that is, 10
times higher than was the theoretical number of
false negative results for 72 tests.
Results of partial Kendall’s tcorrelations between 24 sex-
related factors obtained by factor analysis and Toxoplasma
infection. The effect of age of subject was controlled. The
absence and presence of Toxoplasma infection was coded with
1 and 2, respectively. Therefore, positive ts corresponded to
positive association between particular factor and toxoplasmo-
sis. Significant results are marked bold and .000 denote p<
.0005. The asterisks denote that the associations are significant
after backward sequential correction for multiple tests.
Latent toxoplasmosis had specific effects on human sexual
life–related traits. The infected subjects expressed a lower ten-
dency toward sexual dominance, tattoo and piercing, watching
pornography, group sex, and they are less often engaged in
activities that include Bondage, Discipline and Sado-Maso-
chism (BDSM). However, they expressed higher attraction to
bondage, violence, zoophilia, fetishism, and, in men, also to
masochism, and raping and being raped. Generally, infected
subjects expressed higher attraction to nonconventional sexual
practices, especially the BDSM-related practices, but they also
reported to perform such activities less often than the Toxo-
plasma-free subjects. These observations agreed with our ani-
mal studies-based hypothesis of coactivation of danger- and
sex-related hypothalamic circuits in Toxoplasma-infected sub-
jects (Dass & Vyas, 2014; Flegr & Markos, 2014).
The relation between sex and danger, fear, power exchange,
and pain is rather close in human populations. Kinsey’s data
showed that about 12%of women and 22%of men reported
having an erotic response to a sadomasochistic story and 55%
of women and 50%of men reported having responded eroti-
cally to being bitten. Janus and Janus (1993) showed that about
14%of male and 11%of female responders in the United States
had personal experience with sadomasochistic (SM) sexual
practices and about 8–10%of responders had some SM ‘‘toys’’
at their homes. An Australian study showed that 2%men and
1.4%women had during last 6 months participated in SM
activities (Richters, Grulich, de Visser, Smith, & Rissel,
2003). Preferences for BDSM activities in the Czech Republic
were studied by an Internet trap method (Jozı´fkova´ & Flegr,
2006). The subjects who clicked the banner displayed in the
web interface of e-mail boxes of main Czech free e-mail pro-
vider were offered to choose graphic gates labeled with icons of
homosexual or heterosexual partner of different hierarchical
position (submissive, equal, and dominant). About 13.5%of
men and 20.5%of women had chosen dominant partners and
36.6%of men and 19.8%women had chosen submissive part-
ners. In the current questionnaire study, the men and woman
were asked to rate how much they were sexually aroused by
their own or somebody else’s powerlessness using 6-point scale
anchored with 0 (not at all)5(very strongly). Being aroused by
one’s own powerlessness (ratings 3–5) was reported by 20.04%
of men and 18.45%women, while being aroused by somebody
else’s powerlessness was reported by 33.55%of men and
11.34%of women. Toxoplasma probably only slightly
increases the tendencies of infected subjects to be sexually
aroused by BDSM stimuli and especially by sexual
Figure 1. Eigenvalues for parallel analysis and principal axis factoring.
submissiveness, one’s own fear, and one’s own pain in male
subjects. It could hardly by fully responsible for humans’ sex-
ual arousal by BDSM stimuli, as the Toxoplasma infection
explained only relatively small part of the between-subjects
variability in BDSM-related traits. In its natural hosts, the
rodents, even a small effect of infection on attractiveness of a
smell of dangerous feline predators could result in the fatal
attraction phenomenon, and by this, it can increase the chance
of the parasite’s transmission from intermediate to definitive
host by predation.
Lower tendencies for performing nonconventional sexual
practices in the infected subjects could be related to their
Table 1. The Listing of Factor Names as Well as Example Items Defining the Factors and Eigenvalues for Each Factor.
Factor Number, Name, and Example of Items Defining the Factor Eigenvalues
F01 sadism/sexual dominance: Sexually aroused when someone else is feeling fear, danger, pain, powerlessness, or humiliation. Not
sexually aroused when feeling his own fear, danger, pain, powerlessness, or humiliation.
F02 homosexuality (french kiss and oral sex): Sexually aroused by people of the same sex and performing homosexual activities
(e.g., french kissing, oral sex, and anal sex). Not sexually aroused by people of the opposite sex and doing homosexual activities
(e.g., french kissing, oral sex, and anal sex).
F03 violence (aroused by): Sexually aroused by feeling fear, danger, pain, powerless, or humiliated in general (own and someone
F04 french kiss (attractiveness): The respondent stated higher affinity toward heterosexual french kissing (thinking of trying,
attractiveness in general, quantity of engaging, and attractiveness in virtual reality)
F05 piercing and tattoo (having): The respondent stated more body piercings and tatoos (on face and on intimate body parts). 9.59
F06 tattoo and piercing (intimate attractiveness): The respondent stated higher attractiveness to body piercing and tatoos (on face
and on intimate body parts).
F07 sexual submissiveness/masochism: Sexually aroused when feeling his own fear, danger, pain, powerlessness, or humiliation. 7.12
F08 SOI-R (promiscuity and no attachment): The respondent stated higher numbers of sexual partners, doesn’t have problem
having sex with person without a long-term, serious relationship (SOI-R questionaire).
F09 raping: The respondent stated higher affinity to rape–role-play (thinking of trying, attractiveness in general, and attractiveness
in virtual reality).
F10 bondage (attraction): The respondent stated higher affinity to bondage sex (thinking of trying, attractiveness in general, and
attractiveness in virtual reality).
F11 SM porn watching: The respondent stated higher affinity to consumation of erotic materials (pictures, videos, etc.) covering
especially sadomasochistic sexual themes (torture, tying, spanking, whipping, etc.).
F12 pathogens-related disgust: The respondent stated higher pathogens-related disgust (dog’s excrement, red sores, sweaty hands,
mildew-in-fridge, smelly person, etc.; Three-Domain Disgust Scale).
F13 attractiveness in virtual reality: The respondent stated higher affinity toward several sexual activities but only in virtual reality
(heterosexual french kissing, group sex, oral sex, anal sex, bondage-discipline sex, and sex with sadomasochistic themes).
F14 non-BDSM porn watching: The respondent stated higher affinity toward consumation of erotic materials (pictures, videos, etc.)
covering non-BDSM themes (e.g., general nudity, genitals, group sex, homosexual sex, oral sex, and gentle porn).
F15 missing some sex activity: The respondent stated an affinity toward some sexual activity not listed in our questionaire—they
could add it and rate the frequency of thinking of trying, attractiveness in general, quantity of engaging, and attractiveness in
F16 group sex, more same-sex participants (attractiveness): The respondent stated higher affinity toward group sex activities with
one opposite sex and several same-sex partners (thinking of trying, attractiveness in general, and attractiveness in virtual reality).
F17 sadomasochism (doing): The respondent stated higher quantity of engagement in several sadomasochistic activities (e.g.,
bondage-discipline sex, sex covering powerlessness, pain, threat, humilliation, and role-plays).
F18 homosexual sex (doing): The respondent stated higher quantity of engagement in several homosexual activities (e.g., french
kissing, homosexual group sex, oral sex, and anal sex).
F19 morality-related disgust: The respondent stated higher morality-related disgust (theft of candy bar, robbing a neighbor, student
cheating, faking signature, and jumping the queue stated as disgusting; Three-Domain Disgust Scale).
F20 sexual narcissism (no emotion): The respondent obtained a higher score in the questions of the ‘‘Hurlbert Index of Sexual
Narcissism’’ covering emotional dimension (e.g., he or she stated that their relationship can keep them from engaging in a lot of
fulfilling sexual activities, not enough people have sex for fun anymore, that he or she has no sexual inhibitions, and that too much
‘‘relationship closeness’’ can interfere with sexual pleasure).
F21 tattoo—face þblood (attractiveness): The respondent stated higher attractiveness to face tatoos (active and also passive) and
sexual arousal by manipulation with blood.
F22 zoophilia: The respondent stated higher affinity toward sex with animals (thinking of trying, attractiveness in general, quantity of
engaging, and attractiveness in virtual reality).
F23 anal sex and nude photos: The respondent stated higher affinity toward heterosexual anal sex and taking nude pictures
(thinking of trying, attractiveness in general, quantity of engaging, and attractiveness in virtual reality).
F24 fetishism: The respondent stated higher affinity toward sexual arousal by activities covering fetish (objects such as velvet, latex,
leather, underwear, banknotes, etc.) and also consumation of erotic materials (pictures, videos, etc.) covering especially
Flegr and Kuba 5
impaired health status (Flegr, Prandota, Sovickova, & Israili,
2014). However, these negative associations were observed
(especially in men) even when the health-related covariates
were controlled. We suppose that a decrease of personality
factor novelty seeking, which was observed in infected men
and women in several studies (Flegr et al., 2003; Novotna´ et al.,
2005; Skallova´ et al., 2005), could be responsible for the
observed negative associations between toxoplasmosis and
performing nonconventional sexual activities. This personality
trait was measured, for example, by Cloninger’s seven-factors
Temperament and Character Inventory (TCI) questionnaire
(Cloninger, Przybeck, Svrakic, & Wetzel, 1994) and is
expected to negatively correlate with the concentration of
dopamine in the ventral midbrain (Cloninger, Svrakic, & Przy-
beck, 1993). It was suggested that an observed decrease of
novelty seeking is caused by increased concentration of this
neurotransmitter in the brain of infected hosts, and this
increased concentration could be responsible for higher inci-
dence of schizophrenia of infected subjects (Flegr et al., 2003).
It was originally supposed that the dopamine was produced by
stimulated lymphocytes in the locally damaged brain tissue
(Novotna´ et al., 2005). Later, it was observed that the Toxo-
plasma genome contains two genes for tyrosine hydroxylases,
the enzymes catalyzing the key step in biochemical pathway of
dopamine synthesis (Gaskell, Smith, Pinney, Westhead, &
McConkey, 2009). Following studies showed that a large
amount of this neurotransmitter is indeed synthetized in tissue
cysts of Toxoplasma in the brain of infected rodents
(Prandovszky et al., 2011). Novelty seeking (NS) has four sub-
scales: Exploration Excitability, Impulsiveness, Extravagance,
and Disorderliness. With the exception of the Exploration
Excitability, the other three components of NS are lower in
Toxoplasma-infected subjects. This means that infected people
are on average more reflective, tend to require more detailed
information when making an opinion, and are not easily dis-
tracted. They are also more reserved, slow, and controlled; they
do not waste their energy and feelings. They tend to be orga-
nized, methodical, and prefer activities with strict rules and
regulations. We suppose that these personality traits could
result in observed differences in reported sexual preferences
It was observed that Toxoplasma-infected male subjects and
male rats had higher level of testosterone (Flegr, Lindova´, &
Kodym, 2008; Vyas, 2013). Vyas and his group have shown
that Toxoplasma upregulates the synthesis of this hormone by
increasing the number of luteinizing hormone receptors on
Leydig cells, that is, the receptors that regulate the synthesis
of testosterone in testes (Lim,Kumar,HariDass,&Vyas,
2013). They suggested that this could be a part of the manip-
ulation activity of the parasite aimed toward an increase sexual
activity, which could enhance sexual transmission of Toxo-
plasma from infected males to noninfected females. Our cur-
rent results provided no evidence of increased sexual activity of
infected subjects. It is possible that the increase of testosterone
observed in relatively young students and recently infected rats
is just a transient effect of postacute toxoplasmosis, as many
Table 2. Association Between Sex-Related Factors and Toxoplasma Infection.
Factor Number and Name All, tpnWomen, tpNMen, tpn
F01: Sadism/sexual dominance .04 .000* 5,828 .01 .255 3,146 .08 .000* 2,682
F02: Homosexuality (french kiss and oral sex) .01 .103 5,828 .03 .005* 3,146 .02 .129 2,682
F03: Violence (aroused by) .02 .046 5,828 .03 .014* 3,146 .01 .427 2,682
F04: French kiss (attractiveness) .00 .676 5,828 .01 .611 3,146 .01 .396 2,682
F05: Piercing and tattoo (having) .04 .000* 5,828 .02 .084 3,146 .03 .008* 2,682
F06: Tattoo and piercing (intimate attractiveness) .02 .014* 5,828 .01 .565 3,146 .03 .036 2,682
F07: Sexual submissiveness/masochism .00 .903 5,828 .00 .961 3,146 .04 .004* 2,682
F08: SOI-R (promiscuity and no attachment) .04 .000* 5,828 .03 .030 3,146 .01 .529 2,682
F09: Raping .02 .050 5,828 .01 .389 3,146 .04 .001* 2,682
F10: Bondage (attraction) .03 .001* 5,828 .02 .053 3,146 .06 .000* 2,682
F11: SM porn watching .03 .002* 5,828 .02 .111 3,146 .01 .253 2,682
F12: Pathogens-related disgust .02 .079 5,828 .03 .005* 3,146 .01 .572 2,682
F13: Attractiveness in virtual reality .01 .093 5,828 .03 .032 3,146 .01 .283 2,682
F14: Non-BDSM porn watching .04 .000* 5,828 .03 .026 3,146 .01 .263 2,682
F15: Missing some sex activity .04 .000* 5,828 .02 .102 3,146 .03 .035 2,682
F16: Group sex, more same-sex participants (attractiveness) .03 .000* 5,828 .01 .364 3,146 .02 .244 2,682
F17: Sadomasochism (doing) .04 .000* 5,828 .05 .000* 3,146 .06 .000* 2,682
F18: Homosexual sex (doing) .04 .000* 5,828 .02 .037 3,146 .05 .000* 2,682
F19: Morality-related disgust .02 .036 5,828 .00 .820 3,146 .01 .259 2,682
F20: Sexual narcissism (no emotion) .06 .000* 5,828 .04 .001* 3,146 .06 .000* 2,682
F21: Tattoo—face þblood (attractiveness) .03 .000* 5,828 .04 .003* 3,146 .04 .001* 2,682
F22: Zoophilia .02 .026 5,828 .03 .012* 3,146 .00 .836 2,682
F23: Anal sex and nude photos .03 .000* 5,828 .01 .286 3,146 .06 .000* 2,682
F24: Fetishism .04 .000* 5,828 .02 .057 3,146 .01 .522 2,682
Note. Significant results are marked bold and .000 denote p< .0005. The asterisks denote that the associations are significant after backward sequential correction
for multiple tests.
other toxoplasmosis-associated changes are (Flegr et al., 2014;
Hrda´ et al., 2000; Kanˇkova´, Kodym, et al., 2007; Kanˇkova´,
ˇulc, et al., 2007), and disappears in later phases of infection
or even turns into a decrease of concentration. Such a decrease
was observed in infected male and female mice and in female
students (Flegr, Lindova´, Pivonˇ kova´, & Havlı´cˇ ek, 2008; Kanˇ-
kova´ et al., 2011).
Homosexuality, that is, the sexual attractiveness of same sex
but not opposite sex subjects—the second most important
dimension obtained by the factor analysis—showed no associ-
ation with toxoplasmosis. In contrast, the practicing homosex-
ual sex, the factor F18, correlated positively with
toxoplasmosis. This provides new support for the recent claims
that toxoplasmosis could be most probably transmitted by
unprotected sex (Flegr, Klapilova´, & Kanˇkova´, 2014). Men
to men transmission by ejaculate could explain our nonpub-
lished observation that high number of sexual partners was a
highly important risk factor for acquiring toxoplasmosis not
only for women but also for men.
We have no explanation for the robust results showing a
positive association between Toxoplasma infection and zoophi-
lia. Infected female subjects reported that they had sex with
animals more often than Toxoplasma-free subjects. The most
parsimonial, but still rather improbable, explanation of the
observed pattern is that sex with some species of animal could
be a source of Toxoplasma infection for humans. In our popu-
lation, the female responders most often reported having sex
with a dog. A dog is one of few animal species for which the
experimental transmission by ejaculate has been demonstrated
(Arantes et al., 2009).
Limitations of the Present Study
The major limitation of the present study is that it is based on
self-reported information on subjects’ sexual life and self-
reported Toxoplasma infection status. It is highly probable that
some subjects intentionally or unintentionally provided false
information. Probably, only highly motivated people finished
the 90-min questionnaire. At the end of the questionnaire, the
responders were also asked to rate the percentage of truthfully
answered sexual life–related questions. The mean answer was
about 96%. We believe that most of responders really try to
provide truthful answers. It is, of course, clear that some sub-
jects that were tested negative for toxoplasmosis in the past
could have acquired the Toxoplasma infection in meantime.
Such misclassification could increase the risk of false negative
not the risk of false positive results of statistical tests. Another
limitation of the present study is a strong sieve effect that
probably influenced the composition of the study population.
People entered the study absolutely voluntarily and obtained no
reward for their participation. It is highly probable that only
certain kind of people (e.g., extreme altruists or people
extremely interested in sex) voluntarily spent about 90 min
answering the questions of the present electronic questionnaire.
Therefore, the obtained results cannot be generalized to whole
The Facebook community Guinea Pigs (Czech version of
the website) primarily consists of subjects willing to regularly
participate in various evolutionary psychology studies, and
the present test was advertised as a part of the study focused
on testing a nonspecified ‘‘evolutionary psychology and para-
sitology hypothesis.’’ The whole 36,000 sets were used for the
computation of sexuality-related factors. This Facebook com-
munity consists of people of various ages, education levels,
occupations, and places of residence. However, most of the
subjects (and nearly all the male subjects) who know their
Toxoplasma-infection status are former students of biology
who were tested for toxoplasmosis during systematic research
of behavioral effects of latent toxoplasmosis which has been
running at the Faculty of Science for the past 23 years. In last 2
years, the members of the Guinea Pigs community participated
in several Internet studies that have mostly no relation to tox-
oplasmosis. The question on toxoplasmosis status was just one
of several hundreds of questions to be responded by the parti-
cipants of the study. The participants did not know that the
present study tested the hypothesis about relation between tox-
oplasmosis and sexual life and especially the hypothesis con-
cerning sexual dominance and toxoplasmosis. However, the
personality profile of infected and noninfected subjects is
known to differ (Flegr, 2013) and therefore observed differ-
ences in sexual life–related factors could be just side effects of
these general personality differences. In the future, instrumen-
tal sexological methods should be used to confirm the existence
of differences in sexual preferences of Toxoplasma-infected
and Toxoplasma-free subjects.
Many of the observed effects are highly significant even
after the most conservative Bonferroni’s correction for multi-
ple tests partly because of the large number of participants.
However, their effect sizes are relatively low but comparable
with those of other already published behavioral effects of
latent toxoplasmosis. It has been discussed earlier (Flegr,
2013) that due to the large variability of human populations
in particular behavioral traits and also in the size and direction
of behavioral responses to particular environmental and genetic
factors, toxoplasmosis usually explains only between 1%and
5%of the variability of behavioral traits. For example, the
recently observed effect of toxoplasmosis on performance of
students in acoustic reaction time test explained between 2%
and 3%of the variability in the subjects’ performance (Pripla-
tova, Sebankova, & Flegr, 2014). Probably the strongest effect
of latent toxoplasmosis on human behavior observed in past
25 years, its effect on attraction to the smell of highly diluted
cat urine in men, explained 9.8%of the between-subjects varia-
bility in this trait (Flegr et al., 2011).
Our study confirmed the existence of specific differences in
sexual behavior, desires, and preferences between Toxoplasma-
infected and Toxoplasma-free subjects. The character of these
changes, that is, the higher attraction to bondage, violence, and,
in men, to masochism and raping supports our hypothesis about
Flegr and Kuba 7
the coactivation of sex-related and fear-related medial amyg-
dala circuits in humans. It must be stressed that the Toxoplasma
infection explains only small part of the variability in BDSM-
associated traits. It was shown that sensitivity of current ser-
ological tests is not ideal and that even in populations of young
seronegative subjects, 5–15%individuals are, in fact, Toxo-
plasma infected (Flegr & Havlı´cˇek, 1999; Flegr, Hrda´, &
Kodym, 2005). In older age strata, the seroprevalence of tox-
oplasmosis decreases (Kolbekova´ , Kourbatova, Novotna´,
Kodym, & Flegr, 2007), which suggests that the frequency of
false negatives among older people is probably much higher. It
could be speculated that the false negatives, that is, the subjects
with the oldest infections and therefore lowest concentration of
anamnestic anti-Toxoplasma antibodies, could be responsible
for certain BDSM traits in seemingly Toxoplasma-free sub-
jects. A more probable explanation, however, is that the sexual
arousal by danger, fear, and so on could have a common neu-
rophysiological mechanism (possibly the coactivation of sex-
related and fear-related circuits in amygdala), however this
coactivation could have various independent causes and Tox-
oplasma just (mis)uses this property of the mammal brain for
manipulation with behavior of their intermediate hosts.
We would like to thank Lenka Prˇı´platova´ for her help with preparing
the electronic questionnaire; Charlie Lotterman, Martin Hu
Julie Nova´kova´ for their help with English version of this article; and
ˇernı´kova´ for her help with statistical analysis.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to
the research, authorship, and/or publication of this article.
The author(s) disclosed receipt of the following financial support for
the research, authorship, and/or publication of this article: The
authors’ work was supported by the Charles University of Prague
(GAUK 269215, Grant UNCE 204004), the Grant Agency of the
Czech Republic (Grant no. 16-20958 S), and by the project ‘‘National
Institute of Mental Health (NIMH-CZ)’’ (Grant number ED2.1.00/
03.0078; and the European Regional Development Fund). The funders
had no role in study design, data collection and analysis, decision to
publish, or preparation of this article.
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