According to a study by Janus and Janus,14 percent of male and 11 percent of female responders in the United States have had personal experience with sadomasochistic sexual practices. Although human behavior is complex, an intriguing link has been identified between sex and violence and a common parasite, Toxoplasma gondii, which is acquired from cats.
It is well-established that parasitic infections can change the behavior of animals in profound ways that promote the spread of the parasite. One of the most peculiar examples is a switch in the normal behavior of mice to fear the odor of urine of their main predator, cats, to become attracted to the smell after being infected by Toxoplasma. This parasite reproduces in the gut of cats and it can infect most mammals that come in contact with it. The infection rate in people ranges from 10 percent to 80 percent in different countries. In addition to altering fear circuitry and behavior, Toxoplasma infection increases the levels of dopamine in the brain of infected rodents by enzymes in the parasite that synthesize dopamine (tyrosine hydroxylases). Other studies have reported that males who are infected also have higher levels of testosterone, a hormone associated with aggression and sexual behavior. Could this parasite from human pets be a factor in human sexual behavior?
Extrapolating animal studies to human behavior is tenuous, because the human brain, and human behaviors, are far more complicated than the nervous systems and comparatively simple behaviors of animals, but the evidence suggests that there is a clear relation between Toxoplasma infection in people and increased sexual arousal by someone else’s fear, danger, pain, powerlessness or humiliation. To test the hypothesis that people who are infected by Toxoplasma would be more highly attracted to sexual behaviors involving fear, danger, pain, and submissiveness, researchers at the University of Prague surveyed the sexual preferences and behaviors of 5,087 volunteers who were Toxoplasma free and 741 who were infected. The analysis found that subjects who were infected expressed significantly higher attraction to bondage, violence, sex with animals, and fetishism. In addition, males who were infected reported increased attraction to masochism and rape.
Interestingly, these individuals were less likely to act upon these urges than the people who were not infected. The researchers attribute this to another consequence of Toxoplasma infection that was identified in the survey. The data showed a decrease in novelty seeking in infected individuals, which could inhibit them from performing non-conventional sexual activities, despite their heightened arousal by them.
Additional research is needed to confirm and extend these intriguing findings. A limitation of the study is that it relied on self-reported information by volunteers who completed a detailed questionnaire over the internet about their sexual preferences, behaviors and Toxoplasma infection status, which required approximately 90 minutes to complete. How representative this group of volunteers is of the general population and how reliable their responses to the questions are, remain uncertain. Secondly, while the links between Toxoplasma infection to BDSM and other sexual behaviors were strong and statistically significant, this is only one of many factors that enter into sexual preference and behavior. While it is identified as a significant factor, infection by Toxoplasma could have a relatively small influence on sexual behavior in combination with all the other factors that are involved. Nevertheless, the parallel studies in animals showing changes in neural circuitry that coactivate sex-related and fear-related circuitry in the amygdala, and the switch from fear to attraction to the odor of their predator’s urine, provide a scientific underpinning for the possibility that behaviors, including sexual behaviors, can be influenced by parasitic infections.