A common cat parasite that can easily infect humans causes women to give birth to more boys than girls. Researchers at Prague's Charles University correlated data on nearly 2,000 babies with their mothers' levels of antibodies to toxoplasma, the parasite that infects humans through contact with cat shit and also some uncooked pork and beef. From The Guardian:
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. Toxoplasma causes congenital defects in newborns and can trigger miscarriages, but a link with the gender of newborns has never been identified before…
In most cases the parasite is harmless in humans but it is still unclear what the infection does to people in the long term. Researchers at Oxford University have proposed that the parasite could explain altered personality and changes in IQ levels after their research on rats showed that infection can persuade rats into a suicidal attraction to cats.
UPDATE: BB reader Simon Kornblith writes:
Carl Zimmer over at The Loom has an excellent article on another recent study involving the toxoplasma parasite that links it to cultural differences between nations, and provides further details on the personality changes it can induce. Link