Take mind controlling parasites, for example. These are viruses and simple organisms that have evolved such that they can alter the behavior of their hosts. Essentially, they cognitively re-engineer their victims, turning them into their transmission vectors. It is not uncommon for organisms to leech off several different species in this way as part of their reproductive cycle.
For example, there is Plasmodium gallinaceum, more commonly known as malaria. It's been known for some time that this
Specifically, a mosquito will continue to search for victims until it reaches a threshold volume of blood. When it hits this threshold point, it stops host-seeking. It is thought that the stage-specific effect of the malaria parasite on host-seeking behavior is likely to be an active manipulation to increase its transmission success.
Then there's Dicrocoelium dendriticum. It's a virus that primarily infects sheep -- but it has a rather convoluted way of going about its reproductive business. First, adult worms lay eggs in the bile ducts of the sheep and are excreted. These eggs are in turn ingested by various species of land snails and the eggs hatch in their digestive tracts. This hatching releases a compound that continues to change until it is released by the snail in the form of a slimeball. This slimeball is then eaten by ants. This eventually develops into metacercariae within the abdominal cavity of the ants.
And here's where it gets interesting (not that it hasn't been a riveting tale to this point): the ant's behavior is in turn altered such that it is compelled to climb to the very top of a blade of grass where it waits to get eaten by sheep. The sheep eats the grass with the ant on it and subsequently becomes infected. The cycle is complete.
Similarly, Euhaplorchis californiensis causes fish to shimmy and jump so wading birds will grab them and eat them for the same reason.
Hairworms, which live inside grasshoppers, eventually need to leave their hosts to continue their life cycle. Rather than leave peacefully, however, they release a cocktail of chemicals that makes the grasshoppers commit suicide by leaping into water. The hairworms then swim away from their drowning hosts. Nice, eh?
Think humans are immune to mind controlling parasites? Think again. It is suspected that Toxoplasma gondii, a parasite that is often contracted by humans from their cats, affects human psychology. Normally the parasite works to manipulate rodents, but some scientists speculate that human cognition can also be altered.
Jaroslav Flegr, a parasitologist at Charles University in Prague, administered psychological questionnaires to people infected with Toxoplasma. He discovered that those who are infected show a small tendency to be more self-reproaching and insecure. Strangely, infected women tend to be more outgoing and warmhearted, while infected men tend to be more jealous and suspicious. Flegr has also shown that Toxoplasma may have an effect on human sex ratios -- to the tune of 260 boys for every 100 girls! (As an aside, it's worth noting that Flegr's research has been rejected by 8 journals, usually without formal review). Less controversial are studies that have shown links between Toxoplasma and schizophrenia.
This brings to mind a number of issues (no pun intended), including the freewill problem and the disturbing ease at which a virus can impact on something as important as an agent's behavior. The prospect exists for a deliberately engineered virus that can direct human psychology and decision making. The degree to which a virus could control behavior is an open question, but I'm inclined to think it's fairly limited. There's only so much you can do with germs, and simple organisms seem to be the most manipulable. Nano is a different story altogether, though.
Of course, there are other self-replicating entities that control human psychology much more profoundly than any mind control virus could. I'm thinking, of course, of memes. Cults, most notably Scientology, thrive on manipulating people with memetic techniques. Religions work in a similar manner but the degree to which a person is impacted varies from religion to religion. Propaganda is yet another way in which people's behavior can be modified.
Cults, religions and propaganda aside, a significant portion of human behavior is dictated by memetic influences. You can't escape the memepool; all our thinking is guided in part by the memes we carry. Our individual psychologies are molded by three basic influences: the memes we are exposed to, our genetic predispositions, and how we've been socially conditioned. This is a dynamic process that changes over time. It's my feeling that memes take up the largest chunk of this pie in terms of impact.
Oh, and apparently there's a fourth influence: Toxoplasma gondii. Just keep this in mind the next time you have to clean up your cat's poop.