Game designer David Sirlin once wrote that World of Warcraft keeps people playing by tapping into their latent OCD tendencies.

OCD is short for Obsessive Compulsive Disorder, a psychiatric condition characterized by recurring anxieties which can only be resolved by performing meaningless rituals. Some OCD patients have to put their shoes on several times in the morning. Others walk a certain number of paces around their bedrooms before sleep. WoW players search for new gear.

At first, I agreed with Sirlin -- that's how WoW made so much money! Those devious Blizzard guys were preying on people's compulsions. The game just baits people along with a condensed stream of awards and an artificial, easy-to-climb social ladder. Damn those evil Blizzard devs!

Then I started thinking about just why people play other games, and I realized I was being hypocritical, because all game-playing is compulsive on some level. To understand this, we need to look at the concept of motivation itself, starting right at the bottom: just why does anyone do anything?

Contrary to popular belief, humans do not act in pursuit of physical sensations. The taste of great food, or the physical sensation of orgasm, are not our primary motivations. These things do matter, but they do not drive major changes in our behavior. What is really important to us is satisfying compulsive urges and maintaining good emotional states.

The only thing all of our ancestors have in common, right back to the first primordial replicator particle in a puddle on a hell-like early Earth, is that they reproduced successfully. Most of those ancestors were too stupid to think abstractly through a problem and weigh possible outcomes.

The solution that evolved was to simply react in certain genetically preprogrammed ways to certain predefined stimuli. Most creatures have rules like, "If you see a predator, move away fast" or, "If you see food, smell it. If it smells right and you aren't full, eat it." These rules are still in us today, and manifest themselves as compulsions, urges and cravings.

Not all compulsions are about direct physical results. Wolf pups, for example, play-fight with each other regularly. Play-fighting produces no immediate benefit for the wolf pups. It does not feed them or heal them or keep them warm. They do it because wolf pups who compulsively play-fought learned to fight and hunt better than those that did not, and thus reproduced more, passing more of their genes onto the next generation. The play fighting compulsion is a genetically selected trait because it provides an advantage by making the wolf learn combat and hunting skills.

Similarly, humans are also endowed with a whole smorgasbord of compulsions, most of which are only indirectly related to survival or reproduction. These compulsions and emotional triggers are what drive virtually all of our observed behaviors and habits. They are also the reason why human behavior is so irrational.

For example, I bite my nails. I don't do this because I think it makes them look good, or because I enjoy the taste. Ragged nails certainly aren't getting me any girls, or making me richer or healthier. There is no rational reason for me to do this. Even so, I and millions like me compulsively bite our nails simply because it is in our nature to do so. This is mildly obsessive-compulsive behavior, probably the expression of a genetically-coded compulsion towards self-grooming and removal of foreign objects from the body.