Aug. 9, 2006  -- Kevin Lafferty is a smart, cautious, thoughtful scientist who doesn't hate cats, but he has put forth a provocative theory that suggests that a clever cat parasite may alter human cultures on a massive scale.

His phone hasn't stopped ringing since he published one of the strangest research papers to come out of the mill in quite awhile.

The parasite, Toxoplasma gondii, has been transmitted indirectly from cats to roughly half the people on the planet, and it has been shown to affect human personalities in different ways.

Research has shown that women who are infected with the parasite tend to be warm, outgoing and attentive to others, while infected men tend to be less intelligent and probably a bit boring. But both men and women who are infected are more prone to feeling guilty and insecure.

Other researchers have linked the parasite to schizophrenia. In an adult, the symptoms are like a mild form of flu, but it can be much more serious in an infant or fetus. Oxford University researchers believe high levels of the parasite leads to hyperactivity and lower IQs in children.

Lafferty, who is a parasite ecologist with the U.S. Geological Survey at the University of California at Santa Barbara, is an expert on the role parasites play in the ecology of other animals.

Building on research by scientists in the Czech Republic, Lafferty took a long look at areas of the globe where infection levels are quite high, or quite low. In Brazil, for example, two out of three women of child-bearing age are infected, whereas in the United States the number is only one out of eight.

Lafferty argues in a research paper published Aug. 2 in the Proceedings of the Royal Society, Biology, that aggregate personality types, or what cultures tend to be like, fit neatly with the effects that the parasite produces in individuals.

So that led to a basic question:

Can a common cat parasite account for part -- even if only a very small part -- of the cultural differences seen around the world?

From Lafferty's perspective, that's quite likely, although he admits his theory is a bit off the wall.

"It's kind of way out in left field," he says. "I think it's the strangest thing I've ever worked on."

Bizarre, perhaps, but less so considering the wily parasite that lays the foundation for Lafferty's theory.

Toxoplasma, he notes, is "frighteningly amazing."

It can change the personality of a rat so much that the rat surrenders itself to a cat, just as the parasite wanted.

The parasite's eggs are shed in a cat's feces. A rat comes along, eats the feces, and becomes infected. The behavior of the rat undergoes a dramatic change, making the rat more adventuresome and more likely to hang out around cats.

The cat eats the rat, and the parasite completes its life cycle.

That manipulation of the local ecology is not unusual for a parasite, Lafferty says.

"This is something that many parasites do," he says. "Many manipulate hosts' behavior."

So it wasn't much of a jump to the next question.

"We have a parasite in our brain that is trying to get transmitted to a cat," he says. "This changes an individual's personality."

So if enough personalities are changed in a given society, will the culture of that society also be changed?

He's not suggesting that it's a big player in cultural evolution. Lots of other things are more powerful, ranging from geography to weather to the availability of natural resources.

But if enough of us are infected and undergo personality changes, will that also alter our combined personalities or our culture?

Lafferty admits anthropologists are not likely to embrace his theory. A single powerful leader can have a dramatic impact on a culture. We can all think of examples. But can the collective personality have a similar effect?

"Anthropologists are not in agreement that you can drive a culture from the bottom up," Lafferty says.

But he sees that happening throughout the parasitic world, involving many types of animals, so why is it inconceivable that it could also be happening among humans?

It will be a long time before we have the answer to that, if we ever do, but in the meantime here's a bit of good news.

Cat lovers need not get rid of their cats. The chances are not great that a modern cat, kept on a diet of safe cat food and not left to feed off rats, will transmit the parasite to humans. It's possible, but not likely, Lafferty says.

He ought to know. As a kid he had cats, so after he got into this line of research he assumed he had been infected with the parasite.

"So after I submitted the paper I put down my 30 bucks and got a blood test," he says. "It came out negative. I was so surprised."

And that leads him to this final comment:

"This isn't about trying to freak cat owners out," he says. "Simply having a cat as a pet doesn't mean you're going to get infected, for sure."

Of course, maybe some other parasite is making him say that.