The Loom

Toxoplasma: Bet On Boys

Toxoplasma, that mind-altering, cell-manipulating, all-around awesome parasite that sits in the brains of billions of us, is back in the news. Infection with the parasite raises the chances a woman will have a boy from 51% to 72%. The average ratio of boys to girls at birth is 51%. Women with high levels of antibodies to Toxoplasma, scientists found, have a 72% chance of having a boy. While many effects of Toxoplasma probably have something to do with adaptations that allow the parasite to thrive and spread successfully, this one seems more like a side-effect, albeit a dramatic one.

Source: Guardian Unlimited | Science | Pregnant women infected by cat parasite more likely to give birth to boys, say researchers

Paper abstract here.

[Thanks to BC for the fact-check.


  1. #1 gaddeswarup
    October 12, 2006

    Did you comment on before?
    I just saw it and am wondering whether it was discussed already.

  2. #2 BC
    October 12, 2006

    Something doesn’t look right about those numbers. If there are “billions” of us infected, and hey skew the male/female birthrates so much, then why aren’t the world’s total birthrates more skewed towards boys? In other words:

    If we say that 1.2 billion people are infected (20% of the world’s population), and those infections are distributed equally among men and women, then 20% of the population should have 72% boys and the other 80% should have 51% boys, leading to an overall boy birthrate of 55%. Further, countries like Brazil (which you say has an infection rate of 66.9% in your other post) should have boy birthrates of 65%. (In contrast, wikipedia says Brazil’s boy birthrate is 1.05 male(s)/female, which is 51.2% boys.)

    (Wait, I think I might’ve found the answer. You state that “Infection with the parasite raises the chances a woman will have a boy from 51% to 72%.” But, the Guardian article actually says, “They discovered that women whose antibody count was high – suggesting a substantial infection – had a much higher chance of having baby boys. In most populations the birth rate is around 51% boys, but women infected with toxoplasma had up to a 72% chance of a boy.” Note the phrases: “whose antibody count was high” rather than simply “infected” and “up to a 72% chance of a boy” rather than “Infection … raises the chances … from 51% to 72%.”)

  3. #3 Filipe
    October 13, 2006

    Strange, the effect is meaningless in terms of whole population:

    Countries with largest number of boys per 100 girls:

    China 112
    South Korea 108
    Portugal 107
    Spain 107
    Italy 107
    While in China one could argue for some political/cultural bias, that is unlikely in southern European countries. I don’t know about Korea.

    It doesn’t seem to be a mediterranean thing:
    Algeria 105
    Morocco 105
    Egipt 105
    Libya 105

    Some other countries
    Russia 106
    Greece 106
    Germany 106
    USA 105
    UK 105
    France 105
    Brazil 105
    Mexico 105
    Angola 105
    India 105

    France and Brazil are among the highest rates of infection, yet they rank with the lowest boy ratios.

    (values from the cia factbook, I’m not sure how reliable they are).

  4. #4 Steviepinhead
    October 13, 2006

    Hmmm. The France and Brazil figures suggest that perhaps some other effect of carrying a high load of the parasite serves to (at some slightly higher rate) impair (some other aspect of reproduction) for the most-infected women?

    Curiouser and curiouser.

  5. #5 Joshua
    October 13, 2006

    That is one weird bug there.

  6. #6 Seokha Kang
    October 14, 2006

    Although it is not significant recently, the male preference is widespread in South Korea.

    I think the male bias could serve adaptive value for Toxoplasma. In polygynous mammals, it is known that male offspring disperse further distance than females. Since Toxoplasma can be transmitted vertically, the male biased sex ratio of host’s offspring may help its dispersion. In addition, dispersing individuals are more often exposed to predators. Toxoplasma alters the sex of intermediate host’s offspring for finding new habitats and new hosts?

  7. #7 wholesomedick
    October 16, 2006

    … and the study is all ready on CiteULike tagged with toxoplasmosis.

  8. #8 Pierce R. Butler
    October 20, 2006

    How long until we hear of, ahem, entrepeneurs offering toxoplasma injections for women in India & China (where sons are still at a premium compared to daughters)?

    And is there any chance that such concoctions might yield the promised results?

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