There are some things which are so stupid that only the well-educated and intelligent can believe them.

Behaviorism:

Methodological behaviorism... claims that psychology should concern itself with the behavior of organisms (human and nonhuman animals). Psychology should not concern itself with mental states or events or with constructing internal information processing accounts of behavior. According to methodological behaviorism, reference to mental states, such as an animal's beliefs or desires, adds nothing to what psychology can and should understand about the sources of behavior. Mental states are private entities which, given the necessary publicity of science, do not form proper objects of empirical study.

In other words, psychology (the study of the psyche!) should pay no attention at all to the idea of mental states.

Unsurprisingly:

Another stumbling block, in the case of analytical behaviorism, is the fact that the behavioral sentences that are intended to offer the behavioral paraphrases of mental terms almost always use mental terms themselves... In the example of my belief that I have a 2pm dental appointment, one must also speak of my desire to arrive at 2pm, otherwise the behavior of arriving at 2pm could not count as believing that I have a 2pm appointment. The term “desire” is a mental term.

Well duh.

Some really intelligent people believed this for a really long time: a large chunk of 20th century psychological research was poured down this sinkhole.

The accuracy of your mental model relates to the likelihood of getting correct results from your predictions. Leave out a key component, and you get bad results, or have trouble mapping your model onto reality. Ignore the fact that cars have drivers, and you can't explain many of the things vehicles do.

Clearly, we all experience internal states, and they can make a complete difference between choices we make. (If we're sad, we may eat more; if we're happy, we may call friends.) Surely even behaviorist researchers experienced these states -- in the very act of contradicting their importance!

By ignoring this, behaviorism wasted the efforts of many smart minds, and thus undoubtedly kept scientific psychology back decades, if not more.

(Clearly, I'm being unfair. I haven't presented you with all the surrounding mumbo-jumbo and intellectual doubletalk which is meant to dress up error as logical truth. Guilty as charged. Read Skinner's critics to see how his assumptions are unpacked as self-contradictory and in conflict with observable data.)

Why?

It's instructive to see what "mental states" led one to come up with or embrace such a seriously-wrong model of human activity. The Stanford article I'm quoting lists several reasons, most of which have everything to do with the religion (strongly-held metaphysics) of behaviorists, and nothing to do with science.

1. The tabula rasa ("blank slate"). Many leftists share an aversion to the idea that human being have tendencies or pre-programmed constraints. This tends to get in the way of their dream of re-engineering society from the ground up. So we pretend the science must fit our dreams:

Behaviorism, therefore, appeals to theorists who deny that there are innate rules by which organisms learn. To Skinner and Watson organisms learn without being innately or pre-experientially provided with implicit procedures by which to learn. Learning does not consist, at least initially, in rule-governed behavior. Learning is what organisms do in response to stimuli.

The authors note that computer science has, unfortunately, adopted this same dead-end approach to learning -- implying that AI research is currently being retarded by the same mistakes and biases (motivated similarly, no doubt).

2. Aversion to free will which raises all kinds of problematic tangential issues, again, especially if you plan on re-making society:

Skinner is the only major figure in the history of behaviorism to offer a socio-political world view based on his commitment to behaviorism. Skinner constructed a theory as well as narrative picture in Walden Two (1948) of what an ideal human society would be like if designed according to behaviorist principles (see also Skinner 1971). Skinner's social worldview illustrates both his aversion to free will, to homunculi [a "little man" in our heads], to dualism as well as his reasons for claiming that a person's history of environmental interactions controls his or her behavior.

One remarkable feature of human behavior which Skinner deliberately rejects is that people creatively make their own environments...

3. Utopianism, justified by eschatological fears.

Skinner wrote: “It is now widely recognized that great changes must be made in the American way of life... The choice is clear: either we do nothing and allow a miserable and probably catastrophic future to overtake us, or we use our knowledge about human behavior to create a social environment in which we shall live productive and creative lives..."

So we have bad science rooted in the usual utopian drek -- and that, rooted (as usual) in Skinner's own atheism and the influence of people like Bertrand Russell. Religion can indeed retard science -- but it's usually the religion of people who are convinced they "have no religion", and are capable only of seeing it in others.

After commenting on B.F. Skinner's influence on his own atheism, Professor Mike Adams notes:

The notion that psychology might provide an explanation for atheism – rather than theism - never really occurred to me during my years as a psychology student (from 1983 until 1989 when I received my M.S. in psychology). But, in March of 1989, a woman named Martha Hamilton – the mother of my “second mother” Lisa Chambers – responded to my praise of B.F. Skinner and the behaviorists with the following comment: “It just sounds like a bunch of people trying to get out of serving God.”

That sounds simplistic, but it's also a model which explains a number of irrational beliefs which people like Skinner pretended to hold. (I say "pretended to" because its clear his own life contradicted his alleged beliefs.) Adams expands on this, citing more rigorous psychological evidence.

Psychologist, heal thyself.