Experimental psychology refers to work done by those who apply experimental methods to the study of behavior and the processes that underlie it. Experimental psychologists employ human participants and animal subjects to study a great many topics, including, among others sensation & perception, memory, cognition, learning, motivation, emotion; developmental processes, social psychology, and the neural substrates of all of these.
Early experimental psychology 
Wilhelm Wundt 
Experimental psychology emerged as a modern academic discipline in the 19th century when Wilhelm Wundt introduced a mathematical and experimental approach to the field. Wundt founded the first psychology laboratory in Leipzig, Germany. Other early experimental psychologists, including Hermann Ebbinghaus and Edward Titchener, included introspection among their experimental methods.
Charles Bell 
Charles Bell was a British physiologist, whose main contribution was research involving nerves. He wrote a pamphlet summarizing his research on rabbits. His research concluded that sensory nerves enter at the posterior (dorsal) roots of the spinal cord and motor nerves emerge from the anterior (ventral) roots of the spinal cord. Eleven years later, a French physiologist Francois Magendie published the same findings without being aware of Bell’s research. Due to Bell not publishing his research, the discovery was called the Bell-Magendie law. Bell’s discovery disproved the belief that nerves transmitted either vibrations or spirits.
Ernst Heinrich Weber 
Weber was a German physician who is credited with being one of the founders of experimental psychology. His main interests were the sense of touch and kinesthesis. His most memorable contribution is the suggestion that judgments of sensory differences are relative and not absolute. This relativity is expressed in "Weber's Law," which suggests that the just-noticeable difference, or jnd is a constant proportion of the ongoing stimulus level. Weber's Law is stated as an equation:
where is the original intensity of stimulation, is the addition to it required for the difference to be perceived (the jnd), and k is a constant. Thus, for k to remain constant, must rise as I increases. Weber’s law is considered the first quantitative law in the history of psychology.
Gustav Fechner 
Fechner published in 1860 what is considered to be the first work of experimental psychology, "Elemente der Psychophysik." Some historians date the beginning of experimental psychology from the publication of "Elemente." Weber was not a psychologist, and it was Fechner who realized the importance of Weber’s research to psychology. Fechner was profoundly interested establishing a scientific study of the mind-body relationship, which became known as psychophysics. Much of Fechner's research focused on the measurement of psychophysical thresholds and just-noticeable differences, and he invented the psychophysical method of limits, the method of constant stimuli, and the method of adjustment, which are still in use.
Oswald Külpe 
Oswald Külpe is the main founder of the Würzburg School in Germany. He was a pupil of Wilhelm Wundt for about twelve years. Unlike Wundt, Külpe believed experiments were possible to test higher mental processes. In 1883 he wrote Grundriss der Psychologie, which had strictly scientific facts and no mention of thought. The lack of thought in his book is odd because the Würzburg School put a lot of emphasis on mental set and imageless thought.
Würzburg School 
The work of the Würzburg School was a milestone in the development of experimental psychology. The School was founded by a group of psychologists led by Oswald Külpe, and it provided an alternative to the structuralism of Edward Titchener and Wilhelm Wundt. Those in the School focussed mainly on mental operations such as mental set (Einstellung) and imageless thought. Mental set affects perception and problem solving without the awareness of the individual; it can be triggered by instructions or by experience. Similarly, according to Külpe, imageless thought consists of pure mental acts that do not involve mental images. An example of mental set was provided by William Bryan, an American student working in Külpe’s laboratory. Bryan presented subjects with cards that had nonsense syllables written on them in various colors. The subjects were told to attend to the syllables, and in consequence they did not remember the colors of the nonsense syllables. Such results made people question the validity of introspection as a research tool, and let to a decline of voluntarism and structuralism. The work of the Würzburg School later influenced many Gestalt psychologists, including Max Wertheimer.
George Trumbull Ladd 
Experimental psychology was introduced into the United States by George Trumbull Ladd, who founded Yale University's psychological laboratory in 1879. In 1887, Ladd published Elements of Physiological Psychology, the first American textbook that extensively discussed experimental psychology. Between Ladd's founding of the Yale Laboratory and his textbook, the center of experimental psychology in the US shifted to Johns Hopkins University, where George Hall and Charles Sanders Peirce were extending and qualifying Wundt's work.
Charles Sanders Peirce 
With his student Joseph Jastrow, Charles S. Peirce randomly assigned volunteers to a blinded, repeated-measures design to evaluate their ability to discriminate weights. Peirce's experiment inspired other researchers in psychology and education, which developed a research tradition of randomized experiments in laboratories and specialized textbooks in the 1800s. The Peirce–Jastrow experiments were conducted as part of Peirce's pragmatic program to understand human perception; other studies considered perception of light, etc. While Peirce was making advances in experimental psychology and psychophysics, he was also developing a theory of statistical inference, which was published in "Illustrations of the Logic of Science" (1877–78) and "A Theory of Probable Inference" (1883); both publications that emphasized the importance of randomization-based inference in statistics. To Peirce and to experimental psychology belongs the honor of having invented randomized experiments, decades before the innovations of Neyman and Fisher in agriculture.
Peirce's pragmaticist philosophy also included an extensive theory of mental representations and cognition, which he studied under the name of semiotics. Peirce's student Joseph Jastrow continued to conduct randomized experiments throughout his distinguished career in experimental psychology, much of which would later be recognized as cognitive psychology. There has been a resurgence of interest in Peirce's work in cognitive psychology. Another student of Peirce, John Dewey, conducted experiments on human cognition, particularly in schools, as part of his "experimental logic" and "public philosophy."
20th century 
In the middle of the 20th century, behaviorism became a dominant paradigm within psychology, especially in the United States. This led to some neglect of mental phenomena within experimental psychology. In Europe this was less the case, as European psychology was influenced by psychologists such as Sir Frederic Bartlett, Kenneth Craik, W.E. Hick and Donald Broadbent, who focused on topics such as thinking, memory and attention. This laid the foundations for the subsequent development of cognitive psychology.
In the latter half of the 20th century, the phrase "experimental psychology" had shifted in meaning due to the expansion of psychology as a discipline and the growth in the size and number of its sub-disciplines. Experimental psychologists use a range of methods and do not confine themselves to a strictly experimental approach, partly because developments in the philosophy of science have had an impact on the exclusive prestige of experimentation. In contrast, an experimental method is now widely used in fields such as developmental and social psychology, which were not previously part of experimental psychology. The phrase continues in use, however, in the titles of a number of well-established, high prestige learned societies and scientific journals, as well as some university courses of study in psychology.
The Four Canons of Science 
In order to understand the scientific approach to experimental psychology as well as other areas of scientific research, it is useful to know the four fundamental principles that appear to be accepted by almost all scientists.
One of the first cannons of science is the assumption of determinism. This canon assumes that all events have meaningful, systematic causes. The principle of determinism has a close corollary, that is, that the idea that science is about theories. Scientists accept this canon largely on faith and also to the fact that theories wouldn't be very useful in the absence of determinism, because in the absence of determinism, orderly, systematic causes wouldn't exist.
The canon of empiricism simply means to make observations. This is the best method of figuring out orderly principles. This is a favorite tool among scientist and psychologists because they assume that the best way to find out about the world is to make observations.
The third basic assumption of most scientific schools of thought is parsimony. The canon of parsimony says that we should be extremely frugal in in developing or choosing between theories by steering away from unnecessary concepts. Almost all scientist agree that if we are faced with two competing theories, that both do a great job at handling a set of empirical observations, we should prefer the simpler, or more parsimonious of the two. The central idea behind parsimony is that as long as we intend to keep simplifying and organizing, we should continue until we have made things as simple as possible. One of the strongest arguments made for parsimony was by the medieval English philosopher William of Occam. For this reason, the principle of parsimony if often referred to as Occam's razor.
The final and most important canon of science is the assumption that scientific theories should be testable using currently available research techniques. This canon is closely related to empiricism because the techniques that scientists typically use to test their theories are empirical techniques. In addition to being closely related to empiricism, the concept of testability is even more closely associated falsifiability. The idea of falsifiability is that scientists go an extra step by actively seeking out tests that could prove their theory wrong. Among psychologists, the concepts of testability and falsifiability are extremely important because many really theories like the work of Freud and other psychoanalysts were difficult to put to any kind of objective test.
Some well-known behaviorists such as Edward C. Tolman and Clark Hull popularized the idea of operationism, or operational definitions. Operational definitions are definitions of theoretical constructs that are stated in terms of concrete, observable procedures. Operational definitions solve the problem of what is not directly observable by connecting unobservable traits or experiences to things that can be observed. Operational definitions make the unobservable observable.
Validity and Reliability in Experimental Psychology 
Validity is the relative accuracy or correctness of a study. Like many other concepts that are often broad in nature, validity takes a variety of forms and ranges greatly in meaning including internal, external, conceptual, and construct validity.
Internal validity refers to the extent to which a set of research findings provides compelling information about causality When a study is high in internal validity, there can be a confident conclusion that variations in the independent variable caused any observed changes in the dependent variable. Internal validity is highly important to testing theories because theories are all about causality.
External Validity refers to the extent to which a set of research findings provides an accurate description of what typically happens in the real world. When a study is high in external validity, or generalizability, the conclusion can confidently be made that the findings of the study will apply to other people, other physical or social environments, or even other cultures. One concern of researchers is generalizability with respect to people. In this case, researchers want to know that the results that they may get in one sample will also occur in other samples or for other kinds of people. Another concern regarding generalizability is generalizability with respect to situations. This form of external validity has to do with the degree to which a set of research findings applies to real world settings or contexts. Passive observational studies that are conducted on diverse groups of people in real-world situations tend to be very high in external validity.
A third important form of validity is construct validity. Construct validity refers to the extent to which the independent and dependent variables in a study really represent the abstract hypothetical variables of interest. In simpler terms, it has to do with whether the manipulated and/or measured variables in a study accurately reflect the variables the researcher hoped to manipulate. Construct validity is also a direct reflection of the quality of one’s operational definitions. If a researcher has done a good job of converting the abstract to the observable, construct validity is high.
Another form of validity is called conceptual validity. Conceptual validity refers to how well a specific research hypothesis maps onto the broader theory that it was designed to test. Conceptual and construct validity have a lot in common with one another being that they both have to do with how well a specific manipulation a measure maps onto what the researcher should have done, but conceptual validity lies on a much broader scale. Construct validity has more to do with specific manipulations and measures in specific studies, and conceptual validity has more to do with research hypothesis and even research programs.
Another crucial aspect of almost all research is reliability. This refers to the consistency or repeatability of a measure or an observation. One of the most sensible ways to assess the reliability of a measure is to assess test-retest reliability by measuring a group of participants at one time and then having them having them tested a second time to see if the results are consistent. It is also important to note that by definition, a reliable measure need not be valid.
Experimental psychologists study human behavior and animal behavior in a number of different ways. Human participants often respond to visual, auditory or other stimuli, following instructions given by an experimenter; animals may be similarly "instructed" by rewarding appropriate responses. Since the 1990s, computers running various software packages have automated much of the stimulus presentation and behavioral measurement in the laboratory. Experiments with both humans and animals typically measure reaction time, choices among two or more alternatives, and/or response probability, rate, or strength. Experiments with humans may also obtain written responses before, during, and after experimental procedures; they may also record movements, facial expressions, or other behaviors of participants.
The complexity of human behavior and mental processes, the ambiguity with which they can be interpreted and the unconscious processes to which they are subject gives rise to an emphasis on sound methodology within experimental psychology.
Control of extraneous variables, minimizing the potential for experimenter bias, counterbalancing the order of experimental tasks, adequate sample size, the use of operational definitions, emphasis on both the reliability and valid of results, and proper statistical analysis are central to experimental methods in psychology. Because an understanding of these matters is important to the interpretation of data almost all fields of psychology, undergraduate programs in psychology usually include mandatory courses in Research Methods and Statistics.
Other methods 
A pilot study may be run before a major experiment, in order to test out different procedures or determine optimal values of the experimental variables before the researcher moves on to the main experiment. It can help the researcher find weaknesses in the experiment.
A crucial experiment is an experiment that is meant to test all possible hypotheses simultaneously. If one hypothesis is confirmed, then it will also reject another hypothesis. This type of experiment could confirm multiple hypotheses, which will then lead a researcher to do more experiments that will lead to one confirmed hypothesis.
In a field study, participants work in a naturalistic setting outside the laboratory. Field studies can vary from a description of behaviors in situations not under experimental control (for example, interactions of people at a party) to a true experiment with variables planned in advance (for example, use of different toys in a nursery school). In either case, control is typically more lax than it would be in a laboratory setting.
While other methods of research—case study, interview, and naturalistic observation—are used by psychologists, the use of well-defined, controlled experimental variables with appropriate randomization and isolation from unwanted variables remains the preferred method for testing hypotheses in scientific psychology.
Scales of measurement 
Measurement can be defined as "the assignment of numerals to objects or events according to rules."  Almost all psychological experiments involve some sort of measurement, if only to determine the reliability and validity of results, and of course measurement is essential if results are to be relevant to quantitative theories.
The rule for assigning numbers to a property of an object or event is called a "scale". Following are the basic scales used in psychological measurement.
Nominal measurement 
In a nominal scale, numbers are used simply as labels – a letter or name would do as well. Examples are the numbers on the shirts of football or baseball players. The labels are more useful if the same label can be given to more than one thing, meaning that the things are equal in some way, and can be classified together.
Ordinal measurement 
An ordinal scale arises from the ordering or ranking objects, so that A is greater than B, B is greater than C, and so on. Many psychological experiments yield numbers of this sort; for example, a participant might be able to rank odors such that A is more pleasant than B, and B is more pleasant than C, but these rankings ("1, 2, 3 ...") would not tell by how much each odor differed from another. Some statistics can be computed from ordinal measures - for example, median, percentile, and order correlation - but others, such as standard deviation, cannot properly be used.
Interval measurement 
An interval scale is constructed by determining the equality of differences between the things measured. That is, numbers form an interval scale when the differences between the numbers correspond to differences between the properties measured. For instance, one can say that the difference between 5 and 10 degrees on a Fahrenheit thermometer equals the difference between 25 and 30, but it is meaningless to say that something with a temperature of 20 degrees Fahrenheit is "twice as hot" as something with a temperature of 10 degrees. (Such ratios are meaningful on an absolute temperature scale such as the Kelvin scale. See next section.) "Standard scores" on an achievement test are said to be measurements on an interval scale, but this is difficult to prove.
Ratio measurement 
A ratio scale is constructed by determining the equality of ratios. For example, if, on a balance instrument, object A balances two identical objects B, then one can say that A is twice as heavy as B and can give them appropriate numbers, for example "A weighs 2 grams" and "B weighs 1 gram". A key idea is that such ratios remain the same regardless of the scale units used; for example, the ratio of A to B remains the same whether grams or ounces are used. Length, resistance, and Kelvin temperature are other things that can be measured on ratio scales. Some psychological properties such as the loudness of a sound can be measured on a ratio scale.
Research Design 
The simplest experimental design is a one-way design. In this type of design, there is one and only one independent variable. Furthermore, the simplest kind of one-way design is called two-group design. In a two-group design, there is only one independent variable and this variable has two levels. A two-group design mainly consists of an experimental group (a group that receives treatment) and a control group (a group that doesn’t receive treatment). In addition to two group designs, experimenters often make use of another kind of one-way design called the one-way, multiple groups design. This is another design in which there is only a single independent variable, but the independent variable takes on three or more levels. This type of design is useful in studies such as those that measure perception. Although these types of designs may be simple, they do have limitations.
One major limitation of one-way designs is the fact that they allow researchers to look at only one independent variable at a time. The problem is that a great deal of human behavior is a result of multiple variables acting together. Because of this, R.A Fisher popularized the use of factorial designs. Factorial designs are designs that contain two or more independent variables that are completely crossed. This means that every level of the independent variable appears in combination with every level of every other independent variable. There are a broad variety of factorial designs, so researchers have specific descriptions for the different designs. The label given to a factorial design specifies how many independent variables exist in the design and how many levels of each independent variable exist in the design. Therefore a 2x3 factorial design has two independent variables (because there are two numbers in the description), the first of which has two levels and the second having three levels.
Main Effects and Interactions
The simple straightforward effects of independent variables in factorial studies are referred to as main effects. Main effects are the factorial equivalent of the only kind of effect that you can detect in a one-way design. This refers to the overall effect of an independent variable, averaging across all levels of the other independent variables. Main effects are simple. They only have to do with one variable. In addition to providing information about main effects, studies can also produce a second, very important kind of information called interactions. Interactions exist when the effect of one independent variable on a dependent variable depends on the level of a second independent variable.
The two basic approaches to research design include between-subjects design and within-subjects design. Between-subjects designs are designs in which each participant serves in one and only one condition of an experiment. In contrast, within-subjects or repeated measures designs are those in which each participant serves in more than one or perhaps all of the conditions of a study. Within-subjects have some huge advantages over between-subjects designs especially when it comes to complex factorial designs that have many conditions. Within-subjects designs eliminate person confounds. When researchers use this type of design, they eliminate person confounds in a much more direct approach. They ask the same people to serve in the different experimental conditions in which they happen to be interested. In a sense, these designs take advantage of the only perfect form of matching and in doing so, they totally eliminate person confounds. While there are advantages to this type of design, there are disadvantages as well. There are three closely related biases that are applicable to within-subjects designs. The first bias has to do with the fact that people’s psychological states change as they spend time working on one or more tasks. More specifically, sequence effects can pose serious problems. Sequence effects occur when the simple passage of time begins to take its toll on people’s responses. A second closely related problem has to do with carry-over effects. Carry-over effects occur when people’s responses to one stimulus in a study directly influence their responses to a second stimulus. Another kind of carry-over effect can occur when participants knowingly or unknowingly learn something by performing an experimental task. When a participants’ experience with one task makes it easier for them to perform a different task that comes along later, they have benefited from practice effects. This is a problem because researchers cannot tell if people’s superior performance on the second task happened because of an experimental manipulation or because of simple practice.
Experimental Instruments 
Instruments used in experimental psychology evolved along with technical advances and with the shifting demands of experiments. The earliest instruments, such as the Hipp Chronoscope and the kymograph, were originally used for other purposes. The list below exemplifies some of the different instruments used over the years.
Hipp Chronoscope/Chronograph 
This instrument, dating from around 1850, uses a vibrating reed to tick off time in 1000ths of a second. Originally designed for experiments in physics,it was later adapted to study the speed of bullets. After then being introduced to physiology, it was finally used in psychology to measure reaction time and the duration of mental processes.
The first stereoscope was invented by Wheatstone in 1838. It presents two slightly different images, one to each eye, at the same time. Typically the images are photographs of the same object taken from camera positions that mimic the position and separation of the eyes in the head. When one looks through the steroscope the photos fuse into a single image that conveys a powerful sense of depth and solidity.
Developed by Carl Ludwig in the 19th century, the kymograph is a revolving drum on which a moving stylus tracks the size of some measurement as a function of time. The kymograph is similar to the polygraph, which has a strip of paper moving under one or more pens. The kymograph was originally used to measure blood pressure and it later was used to measure muscle contractions and speech sounds. In psychology, it was often used to record response times.
This device is a photographic recorder. It used mirrors and light to record the photos. Inside a small box with a slit for light there are two drive rollers with film connecting the two. The light enters through the slit to record on the film. Some photokymographs have a lens so an appropriate speed for the film can be reached.
The galvanometer is an early instrument used to measure the strength of an electic current. Hermann von Helmholtz used it to detect the electical signals generated by nerve impulses, and thus to measure the time taken by impulses to travel between two points on a nerve.
This apparatus was designed to produce several fixed frequencies at different levels of intensity. It could either deliver the tone to a subject’s ear or transmit sound oscillations to the skull. An experimenter would generally use an audiometer to find the auditory threshold of a subject. The data received from an audiometer is called an audiogram.
These determine the color composition by measuring its tricolor characteristics or matching of a color sample. This type of device would be used in visual experiments.
Algesiometers and Algometers 
Both of these are mechanical stimulation for pain. They have a sharp needle-like stimulus point so it does not give the sensation of pressure. Experimenters use these when doing an experiment on analgesia.
An olfactometer is any device that is used to measure the sense of smell. The most basic type in early studies was placing a subject in a room containing a specific measured amount of an odorous substance. More intricate devices involve some form of sniffing device, such as the neck of a bottle. The most common olfactometer found in psychology laboratories at one point was the Zwaardemker olfactometer. It had two glass nasal tubes projecting through a screen. One end would be inserted into a stimulus chamber, the other end is inserted directly into the nostrils.
Probably one of the oldest instruments for studying memory would be the maze. The common goal is to get from point A to point B, however the mazes can vary in size and complexity. Two types of mazes commonly used with rats are the radial arm maze and the Morris water maze. The radial arm maze consists of multiple arms radiating from a central point. Each arm has a small piece of food at the end. The Morris water maze is meant to test spatial learning. It uses a large round pool of water that is made opaque. The rat must swim around until it finds the escape platform that is hidden from view just below the surface of the water.
Electroencephalograph (EEG) 
The EEG is an instrument that can reflect the summed electrical activity of neural cell assemblies in the brain. It was originally used as an attempt to improve medical diagnoses. Later it became a key instrument to psychologists in examining brain activity and it remains a key instrument used in the field today.
Functional Magnetic Resonance Imaging (fMRI) 
The fMRI is an instrument that can detect changes in blood oxygen levels over time. The increase in blood oxygen levels shows where brain activity occurs. These are rather bulky and expensive instruments which are generally found in hospitals. They are most commonly used for cognitive experiments.
Positron Emission Tomography (PET) 
PET is also used to look at the brain. It can detect drugs binding neurotransmitter receptors in the brain. A down side to PET is that it requires radioisotopes to be injected into the body so the brain activity can be mapped out. The radioisotopes decay quickly so they do not accumulate in the body.
Institutional Review Board (IRB) 
In the United States, Institutional Review Boards (IRBs) play an important role in monitoring the conduct of psychological experiments. Their presence is reequired by law at institutions such a universities where psychological research occurs. Their purpose is to make sure that experiments do not violate ethical codes or legal requirements; thus they protect human subjects from physical or psychological harm and assure the humane treatment of animal subjects. An IRB must review the procedure to be used in each experiment before that experiment may begin. The IRB also assures that human participants give informaed consent in advance; that is, the participants are told the general nature of the experiment and what will be required of them. There are three types of institutional review boards, which can be seen on the main IRB page
Some research areas that employ experimental methods 
The use of experimental methods was perhaps the main characteristic by which psychology became distinguishable from philosophy in the late 19th century. Ever since then experiments have been an integral part of most psychological research. Following is a sample of some major areas that use experimental methods.
Cognitive psychology 
Some of the major topics studied by cognitive psychologists are memory, learning, problem solving, and attention. Most cognitive experiments are done in a lab instead of a social setting; this is done mainly to provide maximum control of experimental variables and minimal interference from irrelevant events and other aspects of the situation. A great many experimental methods are used; frequently used methods are described on the main pages of the topics just listed. In addition to studying behavior, experimenters may use fMRI or PET so they are able to see what areas of the brain are active during cognitive processing.
Sensation and Perception 
The main senses of the body (sight, touch, smell, auditory, and taste) are what generally get tested for sensation and perception. An experimenter may be interested in the effect color has on people, or what kind of sound is pleasing to a person. These answers require experimental methods to get an answer. Depending on what sense is being tested an experimenter has many experimental instruments to choose from to use in their experiment. These instruments include audio oscillator, attenuator, stroboscope, photometer, colorimeter, algesiometer, algometer, and olfactometer. Each instrument allows the experimenter to record data on what they are researching and helps expand the knowledge of sensation and perception.
Behavioral Psychology 
Behavioral psychology has had a vast array of experimentation completed and much more still going on today. A few notable founders of experiments in behavioral psychology include John B. Watson, B.F. Skinner, and Ivan Pavlov. Pavlov used experimental methods to study the digestion system in dogs, which led to his discovery of classical conditioning. Watson also used experimental methods in his famous experiments with Little Albert. Skinner invented the operant conditioning chamber at first to study rat behavior, and later pigeon behavior, under varying schedules of reinforcement. It was experiments like these that helped the science of behavior become what it is today.
Social Psychology 
Social psychology often employs the experimental method in an attempt to understand human social interaction. Social psychology conducts its experiments both inside and outside of the laboratory. Notable social psychology experiment is the Stanford prison experiment conducted by Philip Zimbardo in 1971, although the extremity of this field experiment is not prototypical of the field. Another notable study is the Stanley Milgram obedience experiment, often known as the Milgram experiment.
There have been several criticisms of experimental psychology.
Frankfurt School 
One school opposed to experimental psychology has been associated with the Frankfurt School, which calls its ideas "Critical Theory." Critical psychologists claim that experimental psychology approaches humans as entities independent of the cultural, economic, and historical context in which they exist. These contexts of human mental processes and behavior are neglected, according to critical psychologists, like Herbert Marcuse. In so doing, experimental psychologists paint an inaccurate portrait of human nature while lending tacit support to the prevailing social order, according to critical theorists like Theodor Adorno and Jürgen Habermas (in their essays in The Positivist Debate in German Sociology).
Critical theory has itself been criticized, however. While the philosopher Karl Popper "never took their methodology (whatever that may mean) seriously" (p. 289), Popper wrote counter-criticism to reduce the "'irrationalist' and 'intelligence-destroying'" "political influence" of critical theorists on students (Karl Popper pages 288–300 in [The Positivist Debate in German Sociology]). The critical theorists Adorno and Marcuse have been severely criticized by Alasdair MacIntyre in Herbert Marcuse: An Exposition and Polemic. Like Popper, MacIntyre attacked critical theorists like Adorno and especially Marcuse as obscurantists pontificating dogma in the authoritarian fashion of German professors of philosophy of their era—before World War II—(page 11); Popper made a similar criticism of critical theory's rhetoric, which reflected the culture of Hegelian social studies in German universities (pp. 293–94). Furthermore, MacIntyre ridiculed Marcuse as being a senile revival of the young Hegelian tradition criticized by Marx and Engels (pp. 18–19, 41, and 101); similarly, "critical theory"'s revival of young Hegelianism and its criticism by Karl Marx was noted by Popper (p. 293). Marcuse's support for the political re-education camps of Maoist China was also criticized as totalitarian by MacIntyre (pp. 101–05). More recently, the Critical Theory of Adorno and Marcuse has been criticized as being a degeneration of the original Frankfurt school, particularly the work of empirical psychologist Erich Fromm, who did surveys and experiments to study the development of personality in response to economic stress and social change (Michael Macoby's Preface to Fromm's Social Character in a Mexican Village).
See also 
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