Check out Wikipedia's List of cognitive biases -- it's awesome. Long story short, human brains are basically designed to get things wrong. We're built to survive, not to interpret, so there's a lot of ways humans think that help us consistently miss the point. One of the big ones is the Availability heuristic, which is our tendency to give priority to information that's easy to remember when evaluating how likely something is. For example, if you've seen six major news stories about children being kidnapped in the past three months, you're going to think it's a lot more likely that your kids are going to be kidnapped.
In 1973, Amos Tversky and Daniel Kahneman first studied this phenomenon [...]. The availability heuristic is an unconscious process that operates on the notion that "if you can think of it, it must be important.
This heuristic, and heuristics[1. A heuristic is a simplified method for solving a problem, which leaves out detail and accuracy for a gain in speed. The alternative is an algorithm, a method for solving a problem which guarantees a correct outcome, but can take a lot longer and requires more energy. People use heuristics because it's often less time-and-energy consuming to deal with the consequences of occasional failure than it is to spend the time getting it 100% right every time. This can go wrong when misapplied, but isn't itself inherently wrong.] in general, make it easier for us to deal with day-to-day problem solving, but in organized society, especially one as information-dense as the one we live in, the availability heuristic turns up wrong answers more often than right ones.
Media coverage can help fuel a person's example bias with widespread and extensive coverage of unusual events, such as homicide or airline accidents, and less coverage of more routine, less sensational events, such as common diseases or car accidents.
It might even be the case that we as a society tend to see things as getting worse because they're getting better -- as it becomes more unusual, say, for someone to be mugged, cases of muggings are more shocking, more widely reported, and therefore more easily brought to mind. As a consequence, people might think mugging rates have gone up. (Charts and stuff below the fold.)
A 2007 Gallup poll shows trends that might be due in part to the availability heuristic. This chart shows crime rates in the US between 1973 and 2007:
Here's 1989-2007, this time charting the perception of crime rate change: