A common parasite can increase a women's attractiveness to the opposite sex but also make men more stupid, an Australian researcher says.

About 40 per cent of the world's population is infected with Toxoplasma gondii, including about eight million Australians.

Human infection generally occurs when people eat raw or undercooked meat that has cysts containing the parasite, or accidentally ingest some of the parasite's eggs excreted by an infected cat.

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"Infected men have lower IQs, achieve a lower level of education and have shorter attention spans. They are also more likely to break rules and take risks, be more independent, more anti-social, suspicious, jealous and morose, and are deemed less attractive to women.

"On the other hand, infected women tend to be more outgoing, friendly, more promiscuous, and are considered more attractive to men compared with non-infected controls.

"In short, it can make men behave like alley cats and women behave like sex kittens".

Human prevalence

The U.S. NHANES (2004-2005) national probability sample found that 33.1% of U.S. persons above 12 years of age had Toxoplasma-specific IgG antibodies, indicating that they had been infected with the organism. This prevalence has significantly increased from the 1999-2000 data.[8]

It is estimated that up to 65% of all people worldwide are infected with Toxoplasma gondii. The incidence of infection varies greatly between countries, with ranges of 22% in the UK to over 88% (or 45% depending on the study) in France.[9], and South Korea's rate is only 4.3% where Brazil's rate is a high 66.9%.