Your Cat Could Change Your Personality

A few years ago, I read about this strange parasite that cats pass to rats, and rats pass back to cats, that caused infected rats to lose their fear of cats (the title of the original paper was, "Fatal attraction in rats infected with Toxoplasma gondii"¹. Now researchers have demonstrated that it can have more subtle effects when humans become infected. First, I learned that researchers showed that those who are infected with the parasite, whom researchers had known showed slower reaction times in laboratory experiments, were 2.65 times more likely to be in car accidents than uninfected people. Now I've learned that they've also discovered even stranger effects on people's personalities (thanks to John Hawks, who linked this article). In 1999, this paper reported the differences found in women who were infected. This is from the abstract:

The subjects with latent toxoplasmosis had higher intelligence, lower guilt proneness, and possibly also higher ergic tension. The difference in several other factors (desurgency/surgency, alaxia/protension, naivete/shrewdness, and self sentiment integration) concerned changes in the variances, rather then the mean values of the factors.

Then I found this 1996 paper which showed than men who were infected were more likely to disregard rules, were less trusting and tolerant, and more jealous, reserved, critical, detached, and guilt prone. Infected women, on the other hand, were more "warmhearted," outgoing, easygoing, trusting, accepting, tolerant, astute, worldly, and polished. In other words, the effects on men and women were in entirely opposite directions. Even more disturbing was the 2003 paper reporting research using military personell that showed infected men to be more impulsive, extravagant, and disorderly (they looked more ragged), along with having lower IQs, lower education levels, and lower levels of novelty seeking.

All this sounds pretty bad for us men, and perhaps not that great for women either. Men become dirty, dogmatic recluses, and women become naive, outgoing, and promiscuous (some of the evidence indicates that they have sex with more men). The authors of the 2003 paper (the first author is the same in each of the papers that I've linked) suggest that the specific behavioral differences in infected men may indicate that the parasite affects the dopamanergic system, which may be why there is a link between infection and schizophrenia.

It turns out, though, that the most recent research indicates that the parasite may not be the cause of some of the behavioral differences. Instead, it appears that there are physical differences between infected and uninfected males that are unrelated to infection, and which may be related to decreased immune response to the parasite. These differences may also be associated with some of the behavioral differences, indicating that the differences may have existed in the individuals prior to infection. So, the parasite may not be causing people's personalities to change afterall. Or at least, it may not be causing all of the differences (e.g., IQ differences and differences in reaction times). The jury is still out, though, so for now, I'm going to be staying away from cats and cat people.

¹Berdoy, M., Webster, J. & Macdonald, D. (2000). Fatal attractions in rats infected with Toxoplasma gondii. Proceedings of the Royal Society of London (B) 267: 1591-1594.