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Women infected with parasite Toxoplasma have more sons

Abstract

The boy-to-girl ratio at birth (secondary sex ratio) is around 0.51 in most populations. The sex ratio varies between societies and may be influenced by many factors, such as stress and immunosuppression, age, primiparity, the sex of the preceding siblings and the socioeconomic status of the parents. As parasite infection affects many immunological and physiological parameters of the host, we analyzed the effect of latent toxoplasmosis on sex ratios in humans. Clinical records of 1,803 infants born from 1996 to 2004 contained information regarding the mother’s age, concentration of anti-Toxoplasma antibodies, previous deliveries and abortions and the sex of the newborn. The results of our retrospective cohort study suggest that the presence of one of the most common parasites (with a worldwide prevalence from 20 to 80%), Toxoplasma gondii, can influence the secondary sex ratio in humans. Depending on the antibody concentration, the probability of the birth of a boy can increase up to a value of 0.72, C.I.95 = (0.636, 0.805), which means that for every 260 boys born, 100 girls are born to women with the highest concentration of anti-Toxoplasma antibodies. The toxoplasmosis associated with immunosuppression or immunomodulation might be responsible for the enhanced survival of male embryos. In light of the high prevalence of latent toxoplasmosis in most countries, the impact of toxoplasmosis on the human population might be considerable.

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Acknowledgements

The authors thank M. Maly, A. Kubena and especially S. Krackow for help with statistical analysis and P. Kodym and J. Havlíček for discussion and comments on this manuscript. This research was supported by the Grant Agency of the Czech Republic 206/05/H012 and by the Czech Ministry of Education (grant 0021620828). The study was approved by the IRB Faculty of Science, Charles University, and complied with the current laws of the Czech Republic.

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Correspondence to J. Flegr.

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Kaňková, Š., Šulc, J., Nouzová, K. et al. Women infected with parasite Toxoplasma have more sons. Naturwissenschaften 94, 122–127 (2007). https://doi.org/10.1007/s00114-006-0166-2

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Keywords

  • Human sex ratio
  • Secondary sex ratio
  • Immunosuppression
  • Manipulation hypothesis
  • Trivers–Willard hypothesis