The celebrated Arrow's impossibility theorem demonstrates how no voting system can consistently aggregate individual preferences.
One tenet of the theorem is that irrelevant alternatives should not change the result of an election. For instance, suppose that candidate A would beat candidate B in a two-way race. But in a three-way race also involving candidate C, voters now prefer candidate B. Some people might call this the Ralph Nader effect.
I have known about the Amazon Kindle for years, and I've used the iPhone version for quite a while. Once the iPad came out, I filtered with buying one. As I mulled over the iPad's high price, the Kindle started to look better by comparison. A few weeks ago, I bought one (unfortunately, this was before the $70 price drop).
I just had lunch with someone yesterday who had the same experience. Why has the introduction of the iPad spurred us to buy Kindles?