Which human instincts do gamification systems appeal to? As a Stack Exchange user, I spent quite a while thinking about how they managed to trick me into putting so much effort into it.  Reputation, badges, etc., are all virtually useless, yet I put great effort into gaining them. 

I've come to the conclusion that the system at least partially exploits that I am not an entirely rational being but instead rely on instincts, for example by exploiting the urge to collect badges. What other instincts do gamification systems appealed to?
In general, I believe gamification taps into 2 main human behaviours. Competition &amp; Pride (in that order)

Competition should drive more engagement and Pride should help breed loyalty (b/c their stature is linked on your projects stature)

1. Competition


contests (top editor of week)
leaderboards
featured users


2. Pride


badges
achievements
unlocked features


Various systems balance these differently, and of course it's also very subjective but I believe that when people first sign up, it's about competition, and after they compete (and win trophies) it's about Pride.

Update: OCT-25-2013

The above 2 drivers really refer to those users who indeed buy-in and engage with your gamifications - But if you run a public website where buy-in rate could be considerably low you may have many users simply exploring. Below are some considerations to keep these users involved in your gamifications and hopefully convert them to competitive patrons.

3. Exploration (Look Before Leap Users)


public profiles (customizations, badges)
public leaderboards
faq / knowledge-base for details on rewards