The abstract is a brilliant summary of a very complex subject! It takes so much work to do all this research and it's so difficult to summarize until after it's all been done and then finally you get an abstract that explains it perfectly😂
I found the following quite intriguing: “Indeed, some diseases of old age may be due not to old age itself but to long-present parasites that have nothing to lose anymore. By that stage of life, the host has stopped having sex, at least regularly, and can be harmed without any real loss.” Could you elaborate on this? In what way would such parasites harm hosts that have reached old age? In other words, how would such a change of strategy benefit the parasite and what would it consist of?
On further thought, it’s not that the parasite gains anything by “deliberately” harming the host; it’s just that mutations that harm the non-sexually-active host are no longer selected against because they make no difference. Is that it?
Yes, that's it. The selection pressure to keep the host alive will weaken as the host becomes older. This is mainly because the host is no longer an interesting vehicle for spreading the parasite to new hosts. It's also because the host has fewer years to live.
Gregory Cochran was, I believe, the first person to make this argument, but I haven't been able to find a relevant quote from his written work. I also develop the same argument in my 2020 article:
Frost, P. (2020). Are fungal pathogens manipulating human behavior? Perspectives in Biology and Medicine, 63(4), 591–601. https://doi.org/10.1353/pbm.2020.0059
The procedure of eliminating one's pubic hair entirely is known as "Brazilian Wax". I think that's no accident; if it were known as, say, "Finnish Wax" or "Irish Wax", now that'd be an accident!
In countries that are hot/warm, like Brazil, it makes sense to eliminate one's pubic hair: due to parasites accumulating there!
I thought it out upon learning that in countries like India and Mexico, the cuisine is spicy due to evolutionary reasons: again, parasites!
Pubic hair removal may have begun in ancient times as a means to discourage pubic lice infestation. In recent times, it seems to be driven by Internet porn:
"Pubic hair removal has been common since the ancient times. Pubic hair was rarely depicted in artistic representations of the nude until the late 19th century. It is postulated that the current trend of pubic hair removal may be related to the increased accessibility of Internet-based pornography. Anecdotally, pubic hair removal may carry benefits regarding increased sexual sensation and satisfaction though there is no quantitative research in this field."
Keep it up: both, in Academia and on The Internet!
BTW,
What are your thoughts on Dr Dutton's "Gay Shaman Theory"?
Y'know... I think there's something to it: I recall reading a paper that families, in Asia, who forced their youngest sons to become monks (i.e., celibate) were HIGHER in fertility. And that's a "birth order" thing, as well: younger sons are more likely to be gay due to hormonal changes after a couple of pregnancies, no?
I like the Gay Shaman theory, but in what direction does the arrow of causality point? In the past, Catholics with large families would dedicate at least one of their offspring to the Church. This was less likely if the parents had few children (because they had married late in life or had fertility problems).
There is an association between birth order and the likelihood of being gay. As Wikipedia puts it:
"[It is] present in men with older biological brothers, and not present among men with older step-brothers. Hypothesized to be the result of a maternal immune response to male fetuses, whereby antibodies neutralize male Y-proteins thought to play a role in sexual differentiation during development, it would leave some regions of the male brain in the 'female typical' arrangement – resulting in attraction to males."
I'm not sure that a pathogenic explanation is excluded. Older brothers may bring microbes into the household during the formative life stage of a younger brother. This might be less likely with older stepbrothers, who would be older on average than full brothers and less often at home.
I recall Gregory Cochran's "gay germ" hypothesis — that homosexuality is caused by a pathogen. The appeal is an argument from elimination: exclusive homosexuality sharply cuts reproductive success and should be selected against, yet it persists with some heritability. Seeing no mechanism that could maintain such a costly trait, Cochran inferred an infectious cause. The problem is that it stays pure conjecture — no organism has ever been found, and it's produced no research in decades.
There is an alternative hypothesis: sexually antagonistic selection, a genetic factor that predisposes males to homosexuality while raising fertility in their female relatives, so the trait breaks even and persists. And this one has data — Camperio-Ciani found that the female maternal relatives of gay men tend to have more children than average, just as an X-linked factor of this kind predicts.
Yes, it was pure conjecture. But we're now starting to get evidence of a pathogenic cause, specifically the changes in gut microbiome among men who have sex with men—specifically, a reduction in the numbers of commensal bacteria that help repair and preserve the intestinal lining. This suggests that an unknown STD is destroying them in order to facilitate entry into a new host.
Exclusive male homosexuality may have several unrelated causes, some being pathogenic and others being genetic. I'm familiar with the genetic theories, notably the recent study from Iran: https://link.springer.com/article/10.1007/s12110-025-09507-8
Yes, but commensal bacteria (the normal microbiota of the gut, skin, oral cavity, urogenital tract, etc.) can be depleted or thrown into dysbiosis by a range of factors. Antibiotics — by far the most significant. Broad-spectrum agents kill commensals alongside pathogens, sharply reducing diversity and total abundance. Recovery takes weeks to months, and some species never fully return. This is the classic cause of, for example, C. difficile overgrowth after the protective flora is wiped out. Moreover, there are dozens of additional reasons for dysbiosis for people who introduce objects into the colon. It seems to me somewhat implausible that gondii et al. evolved such a complicated mechanism to parasitize humans, (1) destroy commensals, (2) infect the exact portion of the brain, (3) cause the specific behaviour.
- Toxoplasma gondii isn't a likely candidate for the pathogen that manipulates sexual orientation.
- The unknown pathogen doesn't destroy all commensal bacteria in the gut, only those that maintain the integrity of the intestinal lining.
- It's possible that exclusive male homosexuals more often use antibiotics, but I have trouble believing that this statistical difference (if it does exist) would be large enough to explain the difference in gut biodome.
- Such behavioral specificity is actually quite common among behavior-altering pathogens.
Digging deeper, I found a 2022 study published in Gut. Gay men were found to be more than twice as likely to develop inflammatory bowel disease (IBD) as heterosexual men. Complex patterns of parasitic manipulation of hosts exist but no one ever linked the two conditions.
"Parasites don't want to kill you. They want to use you."
Thanks, that is interesting.
So, these parasites use humans, so the humans will sexually use other humans. I am guessing that these parasites are against pornography, since that puts a stop to their life cycle—kind of a 'Selfish Gene' scenario.
Of course, many successful bacteria and viruses do not kill their host, so as not to interrupt their reproduction.
These parasites are not "against" pornography. Rather, they have not yet found a way to circumvent the negative influence of porn on real-life sexual encounters.
I'm using words like "want" and "found" to get my argument across. I should be saying: "Parasites have not been selected to kill us. They have been selected to use us."
But many people would have no idea what those sentences mean. I am using anthropomorphic language because we have been conditioned (selected?) to think in terms of conscious human agency.
"These parasites are not "against" pornography. Rather, they have not yet found a way to circumvent the negative influence of porn on real-life sexual encounters."
LOL. I wrote it tongue-in-cheek.
"But many people would have no idea what those sentences mean. I am using anthropomorphic language because we have been conditioned (selected?) to think in terms of conscious human agency."
How about the crazy cat lady parasite (Toxoplasma gondii)?
Not sure if that fits the topic (manipulation of human sexual behavior), but maybe it does. It manipulates women to be crazy cat ladies, which probably negates any possibility of sexual activity.
Which allows the parasite to control the cat lady.
But....by keeping crazy cat ladies isolated by their craziness, the parasite ensures that no male will be interested in sexual activity, thus keeping the infected woman focused on living among cat shit, providing a safe harbor for the parasite.
Sort of a manipulation of sexual activity by negating the possibility of it occurring.
After reading your paper, I guess that has been my (lifelong) problem wrt women and my sex life—not enough parasites. 😢
😂
The abstract is a brilliant summary of a very complex subject! It takes so much work to do all this research and it's so difficult to summarize until after it's all been done and then finally you get an abstract that explains it perfectly😂
The full text can be read at: https://www.researchgate.net/publication/408513206_Parasites_Manipulation_of_Human_Sexual_Behavior
Very interesting.
I found the following quite intriguing: “Indeed, some diseases of old age may be due not to old age itself but to long-present parasites that have nothing to lose anymore. By that stage of life, the host has stopped having sex, at least regularly, and can be harmed without any real loss.” Could you elaborate on this? In what way would such parasites harm hosts that have reached old age? In other words, how would such a change of strategy benefit the parasite and what would it consist of?
On further thought, it’s not that the parasite gains anything by “deliberately” harming the host; it’s just that mutations that harm the non-sexually-active host are no longer selected against because they make no difference. Is that it?
Yes, that's it. The selection pressure to keep the host alive will weaken as the host becomes older. This is mainly because the host is no longer an interesting vehicle for spreading the parasite to new hosts. It's also because the host has fewer years to live.
Gregory Cochran was, I believe, the first person to make this argument, but I haven't been able to find a relevant quote from his written work. I also develop the same argument in my 2020 article:
Frost, P. (2020). Are fungal pathogens manipulating human behavior? Perspectives in Biology and Medicine, 63(4), 591–601. https://doi.org/10.1353/pbm.2020.0059
Got it. Thank you.
BTW, Dr Frost...
I've a question that's related to parasites.
The procedure of eliminating one's pubic hair entirely is known as "Brazilian Wax". I think that's no accident; if it were known as, say, "Finnish Wax" or "Irish Wax", now that'd be an accident!
In countries that are hot/warm, like Brazil, it makes sense to eliminate one's pubic hair: due to parasites accumulating there!
I thought it out upon learning that in countries like India and Mexico, the cuisine is spicy due to evolutionary reasons: again, parasites!
Pubic hair removal may have begun in ancient times as a means to discourage pubic lice infestation. In recent times, it seems to be driven by Internet porn:
"Pubic hair removal has been common since the ancient times. Pubic hair was rarely depicted in artistic representations of the nude until the late 19th century. It is postulated that the current trend of pubic hair removal may be related to the increased accessibility of Internet-based pornography. Anecdotally, pubic hair removal may carry benefits regarding increased sexual sensation and satisfaction though there is no quantitative research in this field."
https://academic.oup.com/jsm/article-abstract/6/8/2102/6834607
Fascinating, Dr Frost!
Keep it up: both, in Academia and on The Internet!
BTW,
What are your thoughts on Dr Dutton's "Gay Shaman Theory"?
Y'know... I think there's something to it: I recall reading a paper that families, in Asia, who forced their youngest sons to become monks (i.e., celibate) were HIGHER in fertility. And that's a "birth order" thing, as well: younger sons are more likely to be gay due to hormonal changes after a couple of pregnancies, no?
I like the Gay Shaman theory, but in what direction does the arrow of causality point? In the past, Catholics with large families would dedicate at least one of their offspring to the Church. This was less likely if the parents had few children (because they had married late in life or had fertility problems).
There is an association between birth order and the likelihood of being gay. As Wikipedia puts it:
"[It is] present in men with older biological brothers, and not present among men with older step-brothers. Hypothesized to be the result of a maternal immune response to male fetuses, whereby antibodies neutralize male Y-proteins thought to play a role in sexual differentiation during development, it would leave some regions of the male brain in the 'female typical' arrangement – resulting in attraction to males."
I'm not sure that a pathogenic explanation is excluded. Older brothers may bring microbes into the household during the formative life stage of a younger brother. This might be less likely with older stepbrothers, who would be older on average than full brothers and less often at home.
Did 4chan get here first: https://archive.4plebs.org/pol/thread/340098219/
Actually, Greg Cochran was the first to put forward that hypothesis. If you haven't read the full text, you can find it on my ResearchGate website: https://www.researchgate.net/publication/408513206_Parasites_Manipulation_of_Human_Sexual_Behavior
I recall Gregory Cochran's "gay germ" hypothesis — that homosexuality is caused by a pathogen. The appeal is an argument from elimination: exclusive homosexuality sharply cuts reproductive success and should be selected against, yet it persists with some heritability. Seeing no mechanism that could maintain such a costly trait, Cochran inferred an infectious cause. The problem is that it stays pure conjecture — no organism has ever been found, and it's produced no research in decades.
There is an alternative hypothesis: sexually antagonistic selection, a genetic factor that predisposes males to homosexuality while raising fertility in their female relatives, so the trait breaks even and persists. And this one has data — Camperio-Ciani found that the female maternal relatives of gay men tend to have more children than average, just as an X-linked factor of this kind predicts.
Yes, it was pure conjecture. But we're now starting to get evidence of a pathogenic cause, specifically the changes in gut microbiome among men who have sex with men—specifically, a reduction in the numbers of commensal bacteria that help repair and preserve the intestinal lining. This suggests that an unknown STD is destroying them in order to facilitate entry into a new host.
Exclusive male homosexuality may have several unrelated causes, some being pathogenic and others being genetic. I'm familiar with the genetic theories, notably the recent study from Iran: https://link.springer.com/article/10.1007/s12110-025-09507-8
Yes, but commensal bacteria (the normal microbiota of the gut, skin, oral cavity, urogenital tract, etc.) can be depleted or thrown into dysbiosis by a range of factors. Antibiotics — by far the most significant. Broad-spectrum agents kill commensals alongside pathogens, sharply reducing diversity and total abundance. Recovery takes weeks to months, and some species never fully return. This is the classic cause of, for example, C. difficile overgrowth after the protective flora is wiped out. Moreover, there are dozens of additional reasons for dysbiosis for people who introduce objects into the colon. It seems to me somewhat implausible that gondii et al. evolved such a complicated mechanism to parasitize humans, (1) destroy commensals, (2) infect the exact portion of the brain, (3) cause the specific behaviour.
A few points:
- Toxoplasma gondii isn't a likely candidate for the pathogen that manipulates sexual orientation.
- The unknown pathogen doesn't destroy all commensal bacteria in the gut, only those that maintain the integrity of the intestinal lining.
- It's possible that exclusive male homosexuals more often use antibiotics, but I have trouble believing that this statistical difference (if it does exist) would be large enough to explain the difference in gut biodome.
- Such behavioral specificity is actually quite common among behavior-altering pathogens.
Digging deeper, I found a 2022 study published in Gut. Gay men were found to be more than twice as likely to develop inflammatory bowel disease (IBD) as heterosexual men. Complex patterns of parasitic manipulation of hosts exist but no one ever linked the two conditions.
I am making such a link.
"Parasites don't want to kill you. They want to use you."
Thanks, that is interesting.
So, these parasites use humans, so the humans will sexually use other humans. I am guessing that these parasites are against pornography, since that puts a stop to their life cycle—kind of a 'Selfish Gene' scenario.
Of course, many successful bacteria and viruses do not kill their host, so as not to interrupt their reproduction.
These parasites are not "against" pornography. Rather, they have not yet found a way to circumvent the negative influence of porn on real-life sexual encounters.
I'm using words like "want" and "found" to get my argument across. I should be saying: "Parasites have not been selected to kill us. They have been selected to use us."
But many people would have no idea what those sentences mean. I am using anthropomorphic language because we have been conditioned (selected?) to think in terms of conscious human agency.
"These parasites are not "against" pornography. Rather, they have not yet found a way to circumvent the negative influence of porn on real-life sexual encounters."
LOL. I wrote it tongue-in-cheek.
"But many people would have no idea what those sentences mean. I am using anthropomorphic language because we have been conditioned (selected?) to think in terms of conscious human agency."
I understand; I am just playing along for levity.
How about the crazy cat lady parasite (Toxoplasma gondii)?
Not sure if that fits the topic (manipulation of human sexual behavior), but maybe it does. It manipulates women to be crazy cat ladies, which probably negates any possibility of sexual activity.
Which allows the parasite to control the cat lady.
I'm not sure about the direction of cause and effect. I imagine that lonely women adopt cats as a substitute for human companionship.
"It manipulates women to be crazy cat ladies, which probably negates any possibility of sexual activity.
Which allows the parasite to control the cat lady."
But that is not conducive to reproduction by sexual activity.
But....by keeping crazy cat ladies isolated by their craziness, the parasite ensures that no male will be interested in sexual activity, thus keeping the infected woman focused on living among cat shit, providing a safe harbor for the parasite.
Sort of a manipulation of sexual activity by negating the possibility of it occurring.
Just spit-balling on the theme here....
Kind of far out, but who knows.
Is there a way to read this paper without an institutional login? Or have you written more on the topic elsewhere?
Sorry about that. I've uploaded the text to ResearchGate at this address:
https://www.researchgate.net/publication/408513206_Parasites_Manipulation_of_Human_Sexual_Behavior
Please let me know if you have trouble accessing it.