The material below is taken from my lecture notes for my undergraduate Introduction to Linguistics course at Penn:

In figuring out how the brain works, one standard line of inquiry is to look at how it fails. This approach was first taken to the problem of speech generation by Sigmund Freud, in his 1901 work The Psychopathology of Everyday Life.

Freud focused on the substitution of words either in speech (lapsus linguae, slips of the tongue) or in writing (lapsus calami, slips of the pen). The substitution is contrary to the conscious wishes of the person speaking or writing, and in fact sometimes is subversive of these wishes. The speaker or writer may be unaware of the error, and may be embarrassed when the error is pointed out. Freud believed that such "slips" come from repressed, unconscious desires.

Freud's general term for such errors was "faulty action (Fehleistung)," which has been translated as the pseudo-Greek scientism parapraxis. The colloquial label is "Freudian slip."

In Freud's analysis, a slip of the tongue is a form of self-betrayal. Here are a few of the examples he cited.

The President of the Austrian Parliament said " I take notice that a full quorum of members is present and herewith declare the sitting closed!"

The hotel boy who, knocking at the bishop's door, nervously replied to the question "Who is it?" "The Lord, my boy!"

A member of the House of Commons referred to another as the honourable member for "Central Hell," instead of "Hull."

Another professor says, "In the case of the female genital, in spite of the tempting ... I mean, the attempted ..... "

When a lady, appearing to compliment another, says "I am sure you must have thrown this delightful hat together" instead of "sewn it together", no scientific theories in the world can prevent us from seeing in her slip the thought that the hat is an amateur production. Or when a lady who is well known for her determined character says: "My husband asked his doctor what sort of diet ought to be provided for him. But the doctor said he needed no special diet, he could eat and drink whatever I choose," the slip appears clearly as the unmistakable expression of a consistent scheme."

Slips of the tongue often give this impression of abbreviation; for instance, when a professor of anatomy at the end of his lecture on the nasal cavities asks whether his class has thoroughly understood it and, after a general reply in the affirmative, goes on to say: "I can hardly believe that this is so, since persons who can thoroughly understand the nasal cavities can be counted, even in a city of millions, on one finger ... I mean, on the fingers of one hand." The abbreviated sentence has its own meaning: it says that there is only one person who understands the subject.

There has been quite a bit of research on slips of the tongue since 1901, and (to the extent that Freud's theory is susceptible of empirical test) this research tends to undermine Freud's conception, and to substitute another one. The characteristics of slips are the result of the information-processing requirements of producing language. If this theory is correct, then slips tell us much less than Freud thought about unconscious intentions, and much more about language structure and use.

Linguistic theory tells us that there is a hierarchy of units below the level of the sentence: phrase, word, morpheme, syllable, syllable-part (such as onset or rhyme), phoneme, phonological feature. Slips can occur at each of these levels. In addition, slips can be of several types: substitution (of one element for another of the same type), exchange (of two elements of the same type within an utterance), shift (of an element from one place to another within the utterance), perseveration (re-use of an element a second time, after the 'correct' use), anticipation (re-use of an element, before the 'correct' use).

In a review article entitled "Speaking and Misspeaking" (published in v.1 of An Invitation to Cognitive Science), Gary Dell gives the following made-up examples, all related to the target utterance "I wanted to read the letter to my grandmother."

Why should mistakes of these kinds occur? The basic facts of the case suggest the reason: talking is a hard thing to do! In fact, fluent speech articulation has been called our most complex motor skill.

Language is a complex and hierarchical system. Language use is creative, so that new utterance is put together on the spot out of the piece-parts made available by the language being spoken. A speaker is under time pressure, typically choosing about three words per second out of a vocabulary of 40,000 or more, while at the same time producing perhaps five syllables and a dozen phonemes per second, using more than 100 finely-coordinated muscles, each of which has a maximum gestural repetition rate of about three cycles per second or less. Word choices are being made, and sentences constructed, at the same time that earlier parts of the same phrase are being spoken.

Given the complexities of speaking, it's not surprising that about one slip of the tongue on average occurs per thousand words said.

A more parsimonious theory would be that speech errors occur simply because talking is a cognitively complex and difficult task, with many opportunities for mixups in memory and execution. This does not exclude examples of the kind that interested Freud -- a substituted word might be "primed" by association with unexpressed wishes or fears, or exempted from normal "editing" processes by the same associations.

In fact, M.T. Motley was able to create "Freudian slip" effects of this kind in the experimental induction of speech errors. He used one of the standard techniques for inducing phonemic exchange errors, which works as follows. The subject is asked to read a list of word pairs such as "dart board." Some of these are target pairs, in which the experimental hopes to induce an error, and some are bias pairs. A "bias pair" has something in common (say initial phonemes) with the desired error. Three bias pairs precede every target pair. A sample of sequence of this kind is

Under these conditions, subjects produce about 10-15% spoonerisms on the target items. The experimenter can then systematically examine the factors that make errors more or less likely. For instance, errors are generally more likely when the results are real words () than when the results are not (), or when the rest of the target words are phonologically similar (e.g. , where the same vowel follows, vs. , where different vowels follow).

In Motley's 1980 experiment, he used manipulation of the experimental context as the independent variable. The subjects were male undergraduates, and the context was either electrical or sexual. In the "electrical" context, the subjects were attached to (fake) electrodes and told that mild shocks would be administered if they performed badly. In the "sexual" context, the test was administered by a provocatively-dressed and conventionally attractive female experimenter (it's not clear if subjects' sexual preference was controlled).

Motley then looked at the likelihood of errors whose output has electrical associations (as in the case of the word pair shad bock), as opposed to sexual ones (as in the word pair tool kits). He found that errors tended to correspond to the contextual conditions: in the electrical context, electrical errors were more common, while in the sexual condition, sexual errors were more common.

Motley's results show that genuinely Freudian slips -- errors that reveal unexpressed thoughts -- do happen. At least, slips of the tongue can be primed or biased in the direction of topics or concepts that are on the speaker's mind. However, the same experiment shows that it is easy to cause slips of the tongue for purely phonological reasons, without any semantic or even lexical priming. We can conclude that many speech errors -- perhaps most speech errors -- do not real the speaker's secret fears and desires, but rather the innocent (if still hidden) properties of his or her language production system.

The material above is taken from my lecture notes for my undergraduate Introduction to Linguistics course at Penn.