“I’m sorry about what happened yesterday. I…I wasn’t myself.” Those words, or something like them, have been offered countless times as an explanation for (mis)behaviour, but what do they actually mean? People seem to have a sense of an intrinsic self, a personality they identify with, and many people are concerned about how outside factors might (like birth control pills) influence this personality or change the way they behave. In this post, I’d like to explore how we construct and define our sense of self, but rather than writing an essay on the subject, I’m hoping to start a discussion about it.

To help focus the discussion (and because I can’t resist throwing in some exciting biology), let’s talk about parasites.  Many parasites can change the behaviour of their host, sometimes quite dramatically. They’re an amazing class of creatures and would easily merit a whole series of blog posts (or a whole book, for that matter). From viruses that invade cells to wasps that lay their eggs in other animals, parasites are ubiquitous and, from our point of view, often rather unusual.   I’ll briefly discuss a few examples and then we’ll move on to the main question: How would you feel if you discovered that parasites were part of who you are?

The emerald cockroach wasp is a parasitic wasp that lays its eggs on cockroaches, which serve as a food source for the young. To capture a cockroach, a female wasp stings it twice; the first sting, near the head, stuns the roach and temporarily paralyzes it; this allows the wasp to place the second sting more precisely. By targeting specific nerves with her venom, the wasp blocks the cockroach’s escape reflex, turning it into a docile, sluggish creature.  The wasp guides her captive by tugging on its antennae, leading it back to her burrow where she lays an egg on its abdomen. A few days later, the egg hatches into a larva which feasts on the meal left by its mother. Over the next two weeks, it will eat through the insides of the still-living cockroach before making a cocoon inside its victim’s body.

Another example is the famous fungus that turns ants into zombies, using them to spread itself.  The fungus takes over the ant’s brain, making it leave the colony and climb up to the underside of a leaf.  The ant clamps its jaws down on the leaf, gripping it tightly even as the fungus kills the ant and emerges from its corpse.  Gruesome as it may sound, this puts the fungus in an ideal spot to spread its spores — growing in a rich nutrient source high above the forest floor.

A candidate parasite which might dramatically affect the behaviour of humans is the protozoan Toxoplasma gondii. This parasite, which causes toxoplasmosis, divides its life-cycle between two different hosts.  T. gondii reproduces sexually in cats, producing spores that are taken up by other warm-blooded animals, mainly rodents such as mice and rats.  The spores reproduce asexually in these animals, infecting their tissue; when an infected mouse or rat is eaten by a cat, the cycle starts again.  Rats and mice usually try to avoid getting eaten by cats, but T. gondii changes their behaviour to tip the odds in its favour.  Infected rodents become less afraid of cats and even seek out places that smell of cat urine.  It’s unclear whether T. gondii has similar effects in humans, but Professor Jaroslav Flegr thinks so.  In 2001, his research group showed that people infected with T. gondii were more likely to be involved in traffic accidents, probably due to slower reactions.  Though he hasn’t been able to prove it yet, Professor Flegr thinks T. gondii might also cause bigger changes in personality, like making people more daring.

So, the question: if you found out that you’ve had a parasite that’s been affecting your behaviour or personality for some time, would you want it removed? Let’s not worry about any health effects; we’ll just assume they’re negligible and focus on the question of identity.  To help get you going, here are some specific questions you could think about:

•  Does it matter what kind of effect the parasite has?  What if it made you daring, like T. gondii? How about more energetic? more lethargic?
• What about parasites (perhaps an STD?) that made you more sociable, flirtatious or extroverted?
• What if it was something minor, like making you favour the warmest spot in the room or prefer not to go outdoors?
• Would it make a difference if these changes were because of “your” gut bacteria instead of a parasite?
• Does it matter how long you’ve been affected? Would you want to get rid of a parasite that’s been influencing your personality for 5-6 years? 5-6 months? most of your life?