What is a cognitive bias?
Cognitive biases are psychological tendencies
that cause the human brain to draw incorrect
Such biases are thought to be a form of "cognitive
shortcut", often based upon rules of thumb, and
include errors in statistical judgment, social
attribution, and memory.
These biases are a common outcome of human
thought, and often drastically skew the reliability of
anecdotal and legal evidence. The phenomenon is
studied in cognitive science and social psychology.
Quoted from: http://en.wikipedia.org/wiki/Cognitive_bias