Why we vote--Civic duty, peer pressure, or "funness"?

I'm often annoyed when people cite Wikipedia--hard to explain why, it's just that I often like to go to the original sources coupled with some kind of mental hangup--but I really enjoyed reading this entry entitled Voter Turnout.
The one part of the article breaks it down this way:
The basic formula for determining whether someone will vote is
Here, P is the probability that an individual's vote will affect the outcome of an election, and B is the perceived benefit of that person's favored political party or candidate being elected. D originally stood for democracy or civic duty, but today represents any social or personal gratification an individual gets from voting. C is the time, effort, and financial cost involved in voting. Since P is virtually zero in most elections, PB is also near zero, and D is thus the most important element in motivating people to vote. For a person to vote, these factors must outweigh C.











