Jaroslav Flegr

• Prof. PhD.
• Professor (Full) at Charles University in Prague

The fraternal birth order effect (FBOE) is a well documented preponderance of older brothers among gay men. Until now, it has lacked a proper formal treatment in light of evolutionary theory. In this article, we propose two related mathematical models based on the rigorous application of the inclusive fitness concept. The models examine sibling riv...

In this article, I recount the journey of discovering the effects of latent toxoplasmosis on human psychology, behaviour, morphology, and health as I observed it from the closest perspective over the past 30+ years, during which our laboratory has been intensely focused on this research. I trace how we moved from the initial observations of differe...

Background: The long-term consequences of COVID-19 infection are becoming increasingly evident in recent studies. This repeated cross-sectional study aimed to explore the long-term health and cognitive effects of COVID-19, focusing on how virus variants, vaccination, illness severity, and time since infection impact post-COVID-19 outcomes. Methods:...

The long-term consequences of COVID-19 infection are becoming increasingly evident in recent studies. This repeated cross-sectional study aimed to explore the long-term health and cognitive effects of COVID-19, focusing on how virus variants, vaccination, illness severity, and time since infection impact post-COVID-19 outcomes. We examined three co...

Introduction: Latent toxoplasmosis, affecting approximately one-third of people worldwide, was once thought to be asymptomatic. However, studies in the last three decades have revealed that it can cause significant psychological and behavioral changes in humans. The observation that the behavioral impacts of toxoplasmosis manifest in opposite direc...

The gradual decrease in the prevalence of serious infectious diseases over the last century has been followed by increase in so called “modern” diseases, including allergies, chronic inflammatory conditions, psychiatric, and metabolic disorders. Between 2019 and 2022, public awareness of the threat of infectious diseases in humans was renewed by th...

One-third of humanity harbors a lifelong infection with Toxoplasma gondii, and probably about 80% are infected with human cytomegalovirus (CMV). This study aims to delineate the associations between toxoplasmosis and cognitive abilities and compare these to the associations with CMV. We evaluated the cognitive performance of 557 students, who had b...

Toxoplasma gondii is one of the most widespread parasites in the human population globally. Several modes of its transmission have been proposed: some are well-researched and confirmed, others remain unconfirmed. One unconfirmed hypothesis pertains to potential transmission of Toxoplasma gondii via oral sex (fellatio) in humans. A recent study foun...

Birth order is known to influence various personality dimensions, from intelligence to personality traits; however, its relationship with human sexuality remains substantially underexplored. This study aimed to investigate the relationship between birth order and aspects of human sexuality, specifically: onset of sexual activity, sociosexual orient...

One proposal for the persistence of homosexuality in the human population is the sexually antagonistic gene hypothesis, which suggests that the lower fertility of homosexual individuals, especially men, may be compensated by higher fertility of their relatives of the opposite sex. To test this hypothesis, we have collected data from 7,312 heterosex...

The gradual decrease in the prevalence of serious infectious diseases over the last century has been followed by increase in so called “modern” diseases, including allergies, chronic inflammatory conditions, psychiatric, and metabolic disorders. Between 2019 and 2022, public awareness of the threat of infectious diseases in humans was renewed by th...

One-third of humanity harbors a lifelong infection with Toxoplasma gondii . This parasite undergoes sexual reproduction in cats and asexual reproduction in any warm-blooded intermediate hosts. The cycle progresses as cats ingest these hosts, containing the parasite’s tissue cysts. Such infections can alter behaviors in both animals and humans, pote...

Human populations, especially European, are polymorphic in the RHD gene. A significant fraction of their members carries two copies of a mutated (deleted) allele, which results in their Rh-negative blood type. Theoretically, this polymorphism should be unstable. Carriers of the less frequent allele are penalized by reduced fertility because of the...

Human populations, especially European, are polymorphic in the RHD gene. A significant fraction of their members carries two copies of a mutated (deleted) allele, which results in their Rh-negative blood type. Theoretically, this polymorphism should be unstable. Carriers of the less frequent allele are penalized by reduced fertility because of the...

COVID-19 affects a variety of organs and systems of the body including the central nervous system. Recent research has shown that COVID-19 survivors often experience neurological and psychiatric complications that can last for months after infection. We conducted a large internet study using online tests to analyze the effects of SARS-CoV-2 infecti...

Many individuals experience persistent symptoms such as deteriorated physical and mental health, increased fatigue, and reduced cognitive performance months after recovering from coronavirus disease 2019 (COVID-19). There is limited data on the long-term trajectory and prevalence of these symptoms, especially in milder cases. Our study aimed to ass...

Bacteria Borrelia burgdorferi s. l. and even more the protist Toxoplasma gondii Nicolle et Manceaux, 1908, are known to affect the behaviour and mental health of their animal and human hosts. Both pathogens infect a significant fraction of human population, both are neurotropic and survive in the host's body for a long time. While latent infections...

Many individuals experience persistent symptoms such as deteriorated physical and mental health, increased fatigue, and reduced cognitive performance months after recovering from COVID-19. Current data are limited on the long-term trajectory of these symptoms and their prevalence in milder cases. Our study aimed to assess the persistent effects of...

Computational prediction of T cell epitopes is a crucial component in the development of novel vaccines. T cells in a healthy vertebrate host can recognize as non-self only those peptides that are present in the parasite's proteins but absent in the host's proteins. This principle enables us to determine the current and past host specificity of a p...

COVID-19 affects a variety of organs and systems of the body including the central nervous system. Recent research has shown that COVID-19 survivors often experience neurological and psychological complications that can last for months after infection. We conducted a large internet study using online tests to analyze the effects of SARS-CoV-2 infec...

Bacteria Borrelia burgdorferi s. l. and even more the protozoan Toxoplasma gondii are known to affect the behavior of their animal and human hosts. Both pathogens infect a significant fraction of human population, and both survive in the host's body for a long time. The resulting latent infections used to be considered clinically asymptomatic. In t...

SARS-CoV-2 is suspected to be the product of a natural or artificial recombination of two viruses – one adapted to the horseshoe bat and the other, donor of the spike protein gene, adapted to an unknown species. Here we used a new method to search for the original host of the ancestor of the SARS-CoV-2 virus and for the donor of its gene for the sp...

The confirmed number of SARS-CoV-2 infections up to 18 October 2022 is 626 million worldwide, but information about factors affecting the probability of infection or a severe course of COVID-19 remains insufficient and often speculative. Only a small number of factors have been rigorously examined, mostly by retrospective or cross-sectional studies...

Women with red hair color, i.e., 1–9% of female Europeans, tend to be the subject of various stereotypes about their sexually liberated behavior. The aim of the present case–control study was to explore whether a connection between red hair color and sexual behavior really exists using data from 110 women (34% redheaded) and 93 men (22% redheaded)....

Objectives Toxoplasma gondii is a widely prevalent protozoan parasite in human populations. This parasite is thought to be primarily transmitted through undercooked meat and contamination by cat feces. Here, we seek to determine if Toxoplasma gondii cysts can be found within human semen. Methods We used a mixture of histological and immunofluoresc...

Computational T-cell epitope prediction is essential in many immunological projects, including the development of vaccines. T-cells of immunocompetent vertebrate hosts can recognize as non-self only peptides which are present in the parasite's proteins and absent in the host's proteins. This basic principle allows us to predict which peptides can e...

Humans infected by Toxoplasma gondii express no specific symptoms but manifest higher incidence of many diseases, disorders and differences in personality and behavior. The aim of this study was to compare the political beliefs and values of Toxoplasma-infected and Toxoplasma-free participants. We measured beliefs and values of 2315 responders via...

One of the moral questions concerning autonomous vehicles (henceforth AVs) is the choice between types that differ in their built-in algorithms for dealing with rare situations of unavoidable lethal collision. It does not appear to be possible to avoid questions about how these algorithms should be designed. We present the results of our study of m...

Cytomegalovirus (CMV) is the herpetic virus, which infects 45–100% of people worldwide. Many reports suggest that CMV could impair the cognitive functions of infected subjects. Here we searched for indices of effects of CMV on infected subjects’ intelligence and knowledge. The Intelligence Structure Test I S T 2000 R was used to compare the cogniti...

Life-long infection with Toxoplasma , which affects 30% of the human population, has specific behavioral effects. The stress-coping hypothesis explains why the toxoplasmosis-associated behavioral changes go in opposite directions in men and women. It suggests that toxoplasmosis impairs the health of humans, which results in chronic stress. Men and...

Background Although tiny in size and mostly harmless, spiders evoke exceptional fear in a significant part of the population and arachnophobia is one of the most common anxiety disorders with prevalence 2.7–6.1%. Two standard measures have been widely used to reliably assess the emotional and cognitive component of spider fear, the Spider Questionn...

Human populations, especially European, are polymorphic in the RHD gene. A significant fraction of their members carry no copy of the coding section of RHD gene, which results in their Rh-negative blood type. Theoretically, this polymorphism should be unstable. Carriers of the less frequent allele are penalized by reduced fertility because of the i...

Toxoplasma gondii is a widely prevalent protozoan parasite in human populations. This parasite is thought to be primarily transmitted through undercooked meat and contamination by cat feces. Here, we demonstrate that Toxoplasma gondii cysts can be found within human semen, thus suggesting a potential for sexual transmission. We visualized Toxoplasm...

Women with red hair colour, i.e., 1–9% of female Europeans, tend to be the subject of various stereotypes about their sexually liberated behaviour. The aim of the present case-control study was to explore whether a connection between red hair colour and sexual behaviour really exists using data from 110 women (34% redheaded) and 93 men (22% redhead...

Humans infected by Toxoplasma gondii express no specific symptoms but manifest higher incidence of many diseases, disorders and differences in personality and behaviour. The aim of this study was to compare the political beliefs and values of Toxoplasma-infected and Toxoplasma-free participants. We measured beliefs and values of 2315 responders via...

Background Latent toxoplasmosis, i.e. a lifelong infection with the protozoan parasite Toxoplasma gondii , affects about a third of the human population worldwide. In the past 10 years, numerous studies have shown that infected individuals have a significantly higher incidence of mental and physical health problems and are more prone to exhibiting...

The confirmed number of SARS-CoV-2 infections up to 30 August 2021 is 217 mil. worldwide but information about factors affecting the probability of infection or of a severe course of Covid-19 remains insufficient and often speculative. Only a small number of factors have been rigorously examined, mostly by retrospective or cross-sectional studies....

Latent infection of the globally spread parasite Toxoplasma gondii in humans has been associated with changes in personality and behavior. Numerous studies have investigated the effect of toxoplasmosis on depression, but their results are inconsistent. Our study focused on the effect of latent toxoplasmosis on depression in men and women in associa...

Background: Although tiny in size and mostly harmless, spiders evoke exceptional fear in a significant part of the population and arachnophobia belongs to one of the most common anxiety disorders with prevalence 2.7-6.1%. Two standardized psychometrics have been widely used to reliably assess the emotional and cognitive component of spider fear, th...

Background Latent toxoplasmosis, i.e. a lifelong infection with the protozoan parasite Toxoplasma gondii, affects about a third of human population worldwide. In the past ten years, numerous studies had shown that infected subjects have a significantly higher incidence of mental and physical health problems and are more prone to exhibiting the adve...

The emergence of altruistic behavior constitutes one of the most widely studied problems in evolutionary biology and behavioral science. Multiple explanations have been proposed, most importantly including kin selection, reciprocity, and costly signaling in sexual selection. In order to test the latter, this study investigated whether people behave...

The current research aimed to examine the reasons people are single, that is, not in an intimate relationship, across eight different countries-Brazil, China, Czech Republic, Greece, Hungary, India, Japan, and the UK. We asked a large cross-cultural sample of single participants (N = 6,822) to rate 92 different possible reasons for being single. Th...

Factors which indicate lower life expectancy also induce switching to a faster life strategy, that is, a higher investment in current reproduction at the expense of future reproduction and body maintenance. We tested a hypothesis according to which impairment of individual health serves as a signal for switching to a faster life strategy using onli...

Background Latent toxoplasmosis, i.e. a lifelong infection with the protozoan parasite Toxoplasma gondii, affects about a third of human population worldwide. In the past ten years, numerous studies had shown that infected subjects have a significantly higher incidence of mental and physical health problems and are more prone to exhibiting the adve...

Human populations, especially European, are polymorphic in the RHD gene. A significant fraction of their members carries two copies of a mutated (deleted) allele, which results in their Rh-negative blood type. Theoretically, this polymorphism should be unstable. Carriers of the less frequent allele are penalized by reduced fertility because of the...

Background Latent toxoplasmosis, i.e. a lifelong infection with the protozoan parasite Toxoplasma gondii, affects about a third of human population worldwide. In the past ten years, numerous studies had shown that infected subjects have a significantly higher incidence of mental and physical health problems and are more prone to exhibiting the adve...

Our study explores the folk concept of personal identity in the developmental context. Two hundred and seventeen Czech children participated in an interview study based on a hypothetical scenario about a sudden change in their friend, someone they know, or some other unspecified person. The children were asked to judge to what extent particular cha...

Numerous recent studies show that vitamin D deficiency potentiates various chronic physical and psychiatric disorders and diseases. It has been shown that a similar range of disorders is also associated with latent infection with Toxoplasma gondii (Nicolle et Manceaux, 1908). For instance, among cancer, diabetes and schizophrenia patients, we find...

Although the aesthetic appreciation of flowers is a well-known aspect of human behaviour, theories explaining its origin are missing. The only exception is the evolutionary theory of Heerwagen and Orians. Surprisingly, it has not yet been empirically tested. The authors suggest that humans aesthetically respond to flowers because they signal food a...

Background About a third of people in the world are infected with Toxoplasma gondii. This parasite has been found in the reproductive organs and semen of males of many animal species as well as humans. The effects of toxoplasmosis on sperm count, motility, and morphology were confirmed in rats. A higher prevalence of toxoplasmosis has been observed...

Maintenance of genetic polymorphism remains one of the big questions of evolutionary biology, which for a long time tended to be explained by balancing selection. This explanation was later criticized, but now is again accepted as an important mechanism in evolution. Human blood group systems seem affected by balancing selection especially strongly...

Toxoplasmosis is one of the most widespread human parasitoses in developed countries. Sexual transmission has been confirmed in several animal species, and indirect evidence suggests that it may occur in humans. We compared the seropositivity to Toxoplasma gondii in couples who visited the Center for Assisted Reproduction in Prague from June 2016 t...

Introduction: A limited number of scientific studies explore the frequency with which various sexual positions are used in human populations and the potential of particular sexual position to facilitate female coital orgasm. Aim: The aim of this study was to provide data about the prevalence and frequency of various sexual positions, their rated...

Background Since its discovery in the 1930s, the effects of Rh phenotype on human health and wellbeing, with the exception of the effects of Rh-negativity of a mother on the risk of hemolytic anemia of Rh-positive children, has only rarely been studied. In the last few years, however, several studies have shown that Rh-negative subjects have worse...

About 1–2% of European population are redheaded, meaning they synthesize more pheomelanin than eumelanin, the main melanin pigment in humans. Several mutations could be responsible for this phenotype. It has been suggested that corresponding mutations spread in Europe due to a founder effect shaped either by a relaxation of selection for dark, UV‐p...

Factors which indicate lower life expectancy also induce switching to a faster life strategy, that is, higher investment in current reproduction at the expense of future reproduction and body maintenance. We tested a hypothesis according to which impairment of individual health serves as a signal for switching to a faster life strategy using online...

Animal phobias are one of the most prevalent mental disorders. We analysed how fear and disgust, two emotions involved in their onset and maintenance, are elicited by common phobic animals. In an online survey, the subjects rated 25 animal images according to elicited fear and disgust. Additionally, they completed four psychometrics, the Fear Surve...

Toxoplasmosis is a parasitic disease widespread in the temperate zone. The definitive hosts of Toxoplasma gondii, which causes the disease, are cats. All warm-blooded vertebrates, including humans, can be intermediate hosts. A person is usually infected by ingesting oocysts, e.g. by consuming along with vegetables some contaminated soil, by drinkin...

Although the aesthetic appreciation of flowers is a well-known aspect of human behavior, theories explaining its origin are missing. An exception is the evolutionary theory of Heerwagen and Orians, who suggest that humans emotionally respond to flowers because they signal food availability. However, fruits are stronger and more direct signals of fo...

Infection by the parasite Toxoplasma, which affects about 33% of world population, is associated with an increased risk of several mental health disorders, the most strongly with schizophrenia. It is unknown whether schizophrenia is associated with this infection the most strongly, or whether this association has just been the most intensively stud...

Statistically, the concentration of antibodies against parasites decreases with the duration of infection. This can result in false negative outcomes of diagnostic tests for subjects with old infections. When a property of seronegative and seropositive subjects is compared under these circumstances, the statistical tests can detect no difference be...

About 1-2% of European population are redhaired, meaning they synthesize more pheomelanin than eumelanin, the main melanin pigment. Several mutations could be responsible for this phenotype. It has been suggested that corresponding mutations spread in Europe due to a founder effect shaped either by a relaxation of selection for dark, UV-protective...

About 1–2% of people of European origin have red hair. Especially female redheads are known to suffer higher pain sensitivity and higher incidence of some disorders, including skin cancer, Parkinson’s disease and endometriosis. Recently, an explorative study performed on 7,000 subjects showed that both male and female redheads score worse on many h...

Many studies show that keeping cats and dogs has a positive impact on humans’ physical and mental health and quality of life. The existence of this “pet phenomenon” is now widely discussed because other studies performed recently have demonstrated a negative impact of owning pets or no impact at all. The main problem of many studies was the autosel...

Autonomous vehicles (henceforth AVs) are about to be introduced to the mass market. However, many moral questions concerning AVs still remain. One of the issues has to do with the choice between AV types that differ in their built-in algorithms for dealing with situations of unavoidable lethal collision. These situations will probably be rare, yet...

Toxoplasmosis affects about one third of human population worldwide. It has a wide range of effects on the health, immunity, behaviour, and both prenatal and postnatal outcomes of infected hosts, including humans. Among these effects, stage of infection-specific shifts in secondary sex ratio were described about ten years ago both in humans and in...

Many studies show that keeping cats and dogs has a positive impact on humans’ physical and mental health and quality of life. The existence of this “pet phenomenon” is now widely discussed because other studies performed recently have demonstrated a negative impact of owning pets or no impact at all. The main problem of many studies was the autosel...

Background No serological assay has 100% sensitivity. Statistically, the concentration of specific antibodies against antigens of parasites decreases with the duration of infection. This can result in false negative outputs of diagnostic tests for the subjects with old infectiong, e.g., for individuals infected in childhood. When a property of sero...

Despite a large volume of research on the impact of religion on different aspects of life, there is still a lack of studies from post-communist countries. In the current study, we aimed to fill this gap by investigating the relationship between religion and wellbeing, physical and mental health, education, sexual behavior and biological fitness amo...

Our study explores the folk concept of personal identity in the developmental context. Over two hundred Czech children participated in an interview study based on a hypothetical scenario about a sudden change in their friend, someone they know, or some other unspecified person. The children were supposed to judge to what extent particular changes (...

The results of previous studies overwhelmingly suggest that RhD positive heterozygotes express better health status than Rh positive homozygous, especially in RhD negative subjects. This also applies to pregnant women. According to the Trivers-Willard hypothesis, women in better physical condition should have a male-skewed sex ratio. The aim of the...

The issue with most studies concerned with mate selection preferences in humans is that they rely on declarations and rational actions of experimental subjects, which are affected by their pre-conceived opinions and prejudices. Moreover, current research suggests that subcortical structures and processes, rather than the neocortex, play the princip...

Backgrounds The prevalence of toxoplasmosis is higher in schizophrenics than in the general population. It has been suggested that certain symptoms of schizophrenia, including changes in olfactory functions, are in fact symptoms of toxoplasmosis that can be easily detected in schizophrenics only due to the increased prevalence of toxoplasmosis in t...

The increasing maximal hierarchical complexity of organisms is one of the best-supported macroevolutionary trends. The nature and causes of this trend, as well as several accompanying macroevolutionary phenomena are, however, still unclear. In this theoretical article, we propose that the cause of this trend could be the increasing pressure of spec...

Our study explores the folk concept of personal identity in the developmental context. Over two hundred Czech children participated in an interview study based on a hypothetical scenario about a sudden change in their friend, someone they know, or some other unspecified person. The children were supposed to judge to what extent particular changes (...

The beauty of flowers has long been appreciated by humans. Surprisingly, we have very limited knowledge of preferred floral traits and almost no empirical evidence for theories explaining the preference of flowers. The aim of this study was to identify the relative importance of floral color and shape properties on the rating of beauty of flowers a...

The ability of organisms to respond adaptively to environmental changes (evolvability) is usually considered to be an important advantage in interspecific competition. It has been suggested, however, that evolvability could be a double-edged sword that could present a handicap in fluctuating environments. The authors of this counterintuitive idea h...

In a macroevolutionary timescale, evolvability itself evolves. Lineages are sorted based on their ability to generate adaptive novelties, which leads to the optimization of their genotype-phenotype map. The system of translation of genetic or epigenetic changes to the phenotype may reach significant horizontal and vertical complexity, and may even...

Several recent studies have demonstrated the association of cat-related injuries with major depression and with depressiveness in the general population. It was suggested that cat-scratch disease, the infection with the bacterium Bartonella henselae, can be responsible for the observed association. However, no direct evidence for the role of the Ba...