After this spate of interest the study again fell into abeyance. The modern study of hypnotism is usually considered to have begun in the 1930s with Clark Leonard Hull at Yale. An experimental psychologist, his work Hypnosis and Suggestibility (1933) was a rigorous study of the phenomenon, using statistical and experimental analysis. Hull's studies emphatically demonstrated once and for all that hypnosis had no connection with sleep ("hypnosis is not sleep, … it has no special relationship to sleep, and the whole concept of sleep when applied to hypnosis obscures the situation"). The main result of Hull's study was to rein in the extravagant claims of hypnotists, especially regarding extraordinary improvements in cognition or the senses under hypnosis. Hull's experiments did show the reality of some classical phenomena such as hypnotic anaesthesia and post-hypnotic amnesia. Hypnosis could also induce moderate increases in certain physical capacities and change the threshold of sensory stimulation; attenuation effects could be especially dramatic.

In 1940s Andrew Salter(1914-1996) introduced to American therapy the Pavlovian method of contradicting, opposing, and attacking beliefs. In the conditioned reflex, he has seen the essence of hypnosis.He gave a rebirth to hypnotism by combining it with classical conditioning.

Studies continued after the Second World War. Barber, Hilgard, Orne and Sarbin also produced substantial studies. Ernest Hilgard and André Weitzenhoffer created the Stanford scales in 1961, a standardized scale for susceptibility to hypnosis, and properly examined susceptibility across age-groups and sex. Hilgard went on to study sensory deception (1965) and induced anesthesia and analgesia (1975).

Definitions:

A precise scientific definition of hypnosis is difficult to produce at this point, largely because there is still much controversy and skepticism on the subject among psychologists.

While many of the attributes of hypnosis may be well grounded in science, many other claims are commonly thought to be pseudoscience. Furthermore, as with most forms of therapy, there is always the risk of the Hawthorne Effect: the desire for a client to please the therapist or justify the expense of therapy, or hopeful thinking.

Among psychologists that accept hypnosis, some view hypnosis as an altered state of consciousness, others as a type of focused attention. The states invoked by clinical practitioners of these methods are very similar, implying that they are, in fact, describing the same phenomenon. The methods employed and the underlying methodologies have still not converged to the point where there is consensus on a single definition of hypnosis.

Altered State of Consciousness

Hypnosis is commonly viewed as a natural, altered state of consciousness, where the conscious (analytical) mind is bypassed, and the subconscious (creative) mind is accessed. This allows the client to use the power of visualization and suggestion, given by oneself or another, to change and improve behavior patterns.

Hypnosis, in itself, is just a state of mental and physical relaxation, along with a more focused sense of concentration. Hypnosis is not sleep (as is often popularly assumed), and most people find that they are more aware of smells, sounds, and feelings than usual. This concentrated awareness is what allows the hypnotherapist to plant positive suggestions and images in the mind of the client to bring about lasting changes.

Focused Attention

This school of thought holds that hypnosis as a state is very similar to other states of extreme concentration, where a person becomes oblivious to his or her surroundings while lost in thought. Often suggested as an example is when a driver suddenly finds oneself much further down the road without any memory of driving the intervening distance, or when a person is watching television and focuses so intently on the program that he or she ceases to be aware of the sides of the screen.

The act of hypnotizing, is, in effect, the act of manually inducing a similar state.

Suggestibility:

Psychologists have produced controversial studies that seem to show a strong correlation between the ease of putting someone in a state of hypnosis and their level of suggestibility (see Stanford scales).

Hypnosis has further been described as "The suspension of the critical factor" which expands on the idea of "increased suggestibility". A person who claims to be hypnotized may accept statements as true that he or she would normally reject.

For example, when told "you have forgotten your name," the subject in a normal state would react with disbelief, but under hypnosis people have claimed that they have, indeed, forgotten their own names.

It often appears as if the hypnotized participant accepts the authority of the hypnotist over his or her own experience. When asked after the conclusion of such a session, some participants appear to be genuinely unable to recall the incident, while others say that they had known the hypnotist was wrong but at the time it had seemed easier just to go along with his instructions. Some hypnotists would claim that this showed the difference between a deep and a shallow hypnotic trance while skeptics would question the validity of the demonstration.