Hitting people tends to make them mad.

If someone is looking forward to something, and it doesn't happen, that will cause them to be disappointed.

If someone is hungry, it's very likely that they will eat.

Folk psychology refers to the "common sense" theory of human psychology used in everyday life to predict and understand others' behavior. Description of it as a theory reflects analysis of it as a complex body of laws used — albeit implicitly — in describing and understanding people's actions. It involves attributing beliefs, desires, fears, goals, and so forth to those we are attempting to understand. These attributes are called propositional attitudes — considered one of the simplest components of thoughts, they reflect a person's mental posture towards a proposition. "María thinks that she'll be late," then, involves attributing a propositional attitude — in this case a belief in the proposition that she'll be late — to María, and can be understood as an application of folk psychology.

Broadly defined, folk psychology can be said to embody virtually all the actions we expect of others. Watching someone shade their eyes might lead you to conclude that the sun is shining in their face; you make this conclusion under principles of folk psychology — a certain sensory imput (bright light in your eyes) leads to a certain response (shading your eyes). What we expect from others from interactions of sensory perception, mental states, and actions can all be understood as a set of ideas constituting an overarching theory that we all implicitly share about how people's minds work.

Among cognitive scientists, views differ as to the relevance of folk psychology. Many believe that folk psychology likely encodes some degree of truth about the operations of the human mind, having arisen out of long years of human experience at interacting with others and possibly owing some of its origins to instinctual abilities stemming from our evolution as a social animal. On the other hand, adherents of eliminative materialism believe that folk psychology has no validity at all, and compare it to principles of 'folk physics' — like the notion that objects in motion tend to quickly come to rest. The eliminative materialists instead believe that neuroscience will eventually yield complete theories of the mind and that these folk theories will be forgotten.

Folk psychology as a theory

Conceptualizing what seem like obvious truths as a folk theory may seem odd; the discussion of folk psychology as a theory of mind dates to 1956, when Wilfrid Sellars attacked "the myth of the given", denying that we truly have an automatic understanding of the workings of the mind. The idea gained greater prominence with the rise of cognitive psychology, which attempts to find cognitive — that is, based upon mental knowledge and internal states — explanations for behavior. Cognitive psychology then examined folk psychology in an effort to understand how it is that humans predict and respond to others' behavior.

One purpose in treating folk psychology as a theory is to then examine the theory and to investigate whether it represents a useful tool to understand human behavior. Eliminative materialists would tend to question whether terms like 'belief' or 'desire' have any real meaning at all, or whether they're inaccurate ideas about how people's minds work that we use because we implicitly subscribe to a (fundamentally useless) theory. Another reason to treat folk psychology as a theory is to examine the origin of it. Some researchers believe that children develop the 'laws' of human behavior by observing others and learning things like "taking other people's toys makes them mad." Others suspect that we have an inborn set of laws to understand others' behavior — that is, that folk psychology is innate in human beings. The parallels to similar arguments in linguistics are striking (well, to this linguist anyhow) — one of the arguments made by those who suggest innateness is the argument from poverty of stimulus, which argues essentially that people have sophisticated understandings of the actions of others beyond what we could possibly glean from direct observation and childhood experimentation in the form of throwing blocks at other children.

A striking bit of evidence that folk psychology is not innate is that until a certain developmental age, children are not able to distinguish what they believe from what others believe: one experiment involved showing children boxes of crayons that (surprisingly, to the children) had birthday candles inside; when the children were then asked what a stuffed animal (who had not been present when the contents were revealed) thought was in the box, the children, contrary to logic, answered that the stuffed animal thought the box held birthday candles. This suggests that young children do not possess the ability to guess at others' mental states.

Of course, some have questioned the existence of folk psychology entirely. In some researchers' view, we understand others' actions and mental states because we simulate them in our own heads. Thus, a child who has learned that throwing blocks at the other kids tends to piss them off figures it out because they reason that if someone were to throw blocks at them, they would get pissed. This approach denies the existence of anything resembling an internalized body of laws for describing others' behavior.

Folk psychology as a mechanism for interaction

The utility of having a way to predict others' behavior hardly needs to be stated. But some of the evidence that we use a particular set of ideas to explain others' behavior lies in the tendency of humans to anthropomorphize inanimate objects. It's of course familiar to see people tend to assign mental states — though they probably realize in most cases that it's not literally accurate — to complex systems like computers. When someone says that the computer is "still thinking" or "just doesn't want to do it", they may be using a rich and complex system of ideas to describe other humans' behavior in their interactions with it. A more thorough understanding of the laws underlying folk psychology then promises to make computers more useful by making human computer interaction more similar to what we subconsciously expect.

References

The Stanford Encyclopedia of Philosophy (http://plato.stanford.edu/)
Churchland, Paul M. 1981. "Eliminative Materialism and the Propositional Attitudes". The Journal of Philosophy.