Actions speak louder than words. A thousand "I love you"s doesn't equal one "I do". Perhaps our most important beliefs are expressed by what we do, not what we say. Daniel Dennett's Intentional Stance theory uses an action-oriented definition of belief:

Here is how it works: first you decide to treat the object whose behavior is to be predicted as a rational agent; then you figure out what beliefs that agent ought to have, given its place in the world and its purpose. Then you figure out what desires it ought to have, on the same considerations, and finally you predict that this rational agent will act to further its goals in the light of its beliefs. A little practical reasoning from the chosen set of beliefs and desires will in most instances yield a decision about what the agent ought to do; that is what you predict the agent will do.

What we say or think we believe is vulnerable to distortion due to a desire to signal, and due to the fact that our consciousness only has partial access to our brain's map of reality. By ignoring our words and looking at our actions we can bypass these shortcomings. It's probably a major reason as to why prediction markets work so well.