Pseudoscience consists of statements, beliefs, or practices that are claimed to be both scientific and factual but are incompatible with the scientific method.^[12][1] Pseudoscience is often characterized by contradictory, exaggerated or unfalsifiable claims; reliance on confirmation bias rather than rigorous attempts at refutation; lack of openness to evaluation by other experts; absence of systematic practices when developing hypotheses; and continued adherence long after the pseudoscientific hypotheses have been experimentally discredited. The term pseudoscience is considered pejorative,^[15] because it suggests something is being presented as science inaccurately or even deceptively. Those described as practicing or advocating pseudoscience often dispute the characterization.^[13]

The demarcation between science and pseudoscience has philosophical and scientific implications.^[16] Differentiating science from pseudoscience has practical implications in the case of health care, expert testimony, environmental policies and science education.^[17] Distinguishing scientific facts and theories from pseudoscientific beliefs, such as those found in astrology, alchemy, alternative medicine, occult beliefs, religious beliefs and creation science, is part of science education and scientific literacy.^[17][18]

Pseudoscience can be harmful. Anti-vaccine activists present pseudoscientific studies that falsely call into question the safety of vaccines. Homeopathic remedies with no active ingredients have been promoted as treatment for deadly diseases.

The word pseudoscience is derived from the Greek root pseudo meaning false^[19][20] and the English word science, from the Latin word scientia, meaning "knowledge". Although the term has been in use since at least the late 18th century (e.g. in 1796 by James Pettit Andrews in reference to alchemy^[21][22]) the concept of pseudoscience as distinct from real or proper science seems to have become more widespread during the mid-19th century. Among the earliest uses of "pseudo-science" was in an 1844 article in the Northern Journal of Medicine, issue 387:

An earlier use of the term was in 1843 by the French physiologist François Magendie, that refers to phrenology as "a pseudo-science of the present day".^[23][24] During the 20th century, the word was used pejoratively to describe explanations of phenomena which were claimed to be scientific, but which were not in fact supported by reliable experimental evidence. From time-to-time, though, the usage of the word occurred in a more formal, technical manner in response to a perceived threat to individual and institutional security in a social and cultural setting.^[25]

Pseudoscience is differentiated from science because – although it claims to be science – pseudoscience does not adhere to accepted scientific standards, such as the scientific method, falsifiability of claims, and Mertonian norms.

A number of basic principles are accepted by scientists as standards for determining whether a body of knowledge, method, or practice is scientific. Experimental results should be reproducible and verified by other researchers.^[27] These principles are intended to ensure experiments can be reproduced measurably given the same conditions, allowing further investigation to determine whether a hypothesis or theory related to given phenomena is valid and reliable. Standards require the scientific method to be applied throughout, and bias to be controlled for or eliminated through randomization, fair sampling procedures, blinding of studies, and other methods. All gathered data, including the experimental or environmental conditions, are expected to be documented for scrutiny and made available for peer review, allowing further experiments or studies to be conducted to confirm or falsify results. Statistical quantification of significance, confidence, and error^[28] are also important tools for the scientific method.

During the mid-20th century, the philosopher Karl Popper emphasized the criterion of falsifiability to distinguish science from nonscience.^[29] Statements, hypotheses, or theories have falsifiability or refutability if there is the inherent possibility that they can be proven false. That is, if it is possible to conceive of an observation or an argument which negates them. Popper used astrology and psychoanalysis as examples of pseudoscience and Einstein's theory of relativity as an example of science. He subdivided nonscience into philosophical, mathematical, mythological, religious and metaphysical formulations on one hand, and pseudoscientific formulations on the other, though he did not provide clear criteria for the differences.^[30]

Another example which shows the distinct need for a claim to be falsifiable was stated in Carl Sagan's publication The Demon-Haunted World when he discusses an invisible dragon that he has in his garage. The point is made that there is no physical test to refute the claim of the presence of this dragon. Whatever test one thinks can be devised, there is a reason why it does not apply to the invisible dragon, so one can never prove that the initial claim is wrong. Sagan concludes; "Now, what's the difference between an invisible, incorporeal, floating dragon who spits heatless fire and no dragon at all?". He states that "your inability to invalidate my hypothesis is not at all the same thing as proving it true",^[31] once again explaining that even if such a claim were true, it would be outside the realm of scientific inquiry.

During 1942, Robert K. Merton identified a set of five "norms" which he characterized as what makes a real science. If any of the norms were violated, Merton considered the enterprise to be nonscience. These are not broadly accepted by the scientific community. His norms were:

During 1978, Paul Thagard proposed that pseudoscience is primarily distinguishable from science when it is less progressive than alternative theories over a long period of time, and its proponents fail to acknowledge or address problems with the theory.^[33] During 1983, Mario Bunge has suggested the categories of "belief fields" and "research fields" to help distinguish between pseudoscience and science, where the former is primarily personal and subjective and the latter involves a certain systematic method.^[34] The 2018 book The Skeptics' Guide to the Universe lists hostility to criticism as one of the major features of pseudoscience.^[35]

Philosophers of science such as Paul Feyerabend argued that a distinction between science and nonscience is neither possible nor desirable.^[36][2] Among the issues which can make the distinction difficult is variable rates of evolution among the theories and methods of science in response to new data.^[3]

Larry Laudan has suggested pseudoscience has no scientific meaning and is mostly used to describe our emotions: "If we would stand up and be counted on the side of reason, we ought to drop terms like 'pseudo-science' and 'unscientific' from our vocabulary; they are just hollow phrases which do only emotive work for us".^[39] Likewise, Richard McNally states, "The term 'pseudoscience' has become little more than an inflammatory buzzword for quickly dismissing one's opponents in media sound-bites" and "When therapeutic entrepreneurs make claims on behalf of their interventions, we should not waste our time trying to determine whether their interventions qualify as pseudoscientific. Rather, we should ask them: How do you know that your intervention works? What is your evidence?"^[40]

For philosophers Silvio Funtowicz and Jerome R. Ravetz "pseudo-science may be defined as one where the uncertainty of its inputs must be suppressed, lest they render its outputs totally indeterminate". The definition, in the book Uncertainty and quality in science for policy (p. 54),^[41] alludes to the loss of craft skills in handling quantitative information, and to the bad practice of achieving precision in prediction (inference) only at the expenses of ignoring uncertainty in the input which was used to formulate the prediction. This use of the term is common among practitioners of post-normal science. Understood in this way, pseudoscience can be fought using good practices to assesses uncertainty in quantitative information, such as NUSAP and – in the case of mathematical modelling – sensitivity auditing.

The history of pseudoscience is the study of pseudoscientific theories over time. A pseudoscience is a set of ideas that presents itself as science, while it does not meet the criteria to be properly called such.^[42][43]

Distinguishing between proper science and pseudoscience is sometimes difficult. One proposal for demarcation between the two is the falsification criterion, attributed most notably to the philosopher Karl Popper.^[44] In the history of science and the history of pseudoscience it can be especially difficult to separate the two, because some sciences developed from pseudosciences. An example of this transformation is the science chemistry, which traces its origins to pseudoscientific or pre-scientific study of alchemy.

The vast diversity in pseudosciences further complicates the history of science. Some modern pseudosciences, such as astrology and acupuncture, originated before the scientific era. Others developed as part of an ideology, such as Lysenkoism, or as a response to perceived threats to an ideology. Examples of this ideological process are creation science and intelligent design, which were developed in response to the scientific theory of evolution.^[45]

A topic, practice, or body of knowledge might reasonably be termed pseudoscientific when it is presented as consistent with the norms of scientific research, but it demonstrably fails to meet these norms.^[12]

A large percentage of the United States population lacks scientific literacy, not adequately understanding scientific principles and method.^{[8][9][68][10]} In the Journal of College Science Teaching, Art Hobson writes, "Pseudoscientific beliefs are surprisingly widespread in our culture even among public school science teachers and newspaper editors, and are closely related to scientific illiteracy."^[70] However, a 10,000-student study in the same journal concluded there was no strong correlation between science knowledge and belief in pseudoscience.^[71]

In his book The Demon-Haunted World Carl Sagan discusses the government of China and the Chinese Communist Party's concern about Western pseudoscience developments and certain ancient Chinese practices in China. He sees pseudoscience occurring in the United States as part of a worldwide trend and suggests its causes, dangers, diagnosis and treatment may be universal.^[72]

During 2006, the U.S. National Science Foundation (NSF) issued an executive summary of a paper on science and engineering which briefly discussed the prevalence of pseudoscience in modern times. It said, "belief in pseudoscience is widespread" and, referencing a Gallup Poll,^[73] stated that belief in the 10 commonly believed examples of paranormal phenomena listed in the poll were "pseudoscientific beliefs".^[74] The items were "extrasensory perception (ESP), that houses can be haunted, ghosts, telepathy, clairvoyance, astrology, that people can communicate mentally with someone who has died, witches, reincarnation, and channelling".^[74] Such beliefs in pseudoscience represent a lack of knowledge of how science works. The scientific community may attempt to communicate information about science out of concern for the public's susceptibility to unproven claims.^[74] The National Science Foundation stated that pseudoscientific beliefs in the U.S. became more widespread during the 1990s, peaked about 2001, and then decreased slightly since with pseudoscientific beliefs remaining common. According to the NSF report, there is a lack of knowledge of pseudoscientific issues in society and pseudoscientific practices are commonly followed.^[75] Surveys indicate about a third of all adult Americans consider astrology to be scientific.^[76][77][78]

In a report Singer and Benassi (1981) wrote that pseudoscientific beliefs have their origin from at least four sources.^[79]

Another American study (Eve and Dunn, 1990) supported the findings of Singer and Benassi and found pseudoscientific belief being promoted by high school life science and biology teachers.^[80]

The psychology of pseudoscience attempts to explore and analyze pseudoscientific thinking by means of thorough clarification on making the distinction of what is considered scientific vs. pseudoscientific. The human proclivity for seeking confirmation rather than refutation (confirmation bias),^[81] the tendency to hold comforting beliefs, and the tendency to overgeneralize have been proposed as reasons for pseudoscientific thinking. According to Beyerstein (1991), humans are prone to associations based on resemblances only, and often prone to misattribution in cause-effect thinking.^[82]

Michael Shermer's theory of belief-dependent realism is driven by the belief that the brain is essentially a "belief engine" which scans data perceived by the senses and looks for patterns and meaning. There is also the tendency for the brain to create cognitive biases, as a result of inferences and assumptions made without logic and based on instinct — usually resulting in patterns in cognition. These tendencies of patternicity and agenticity are also driven "by a meta-bias called the bias blind spot, or the tendency to recognize the power of cognitive biases in other people but to be blind to their influence on our own beliefs".^[83] Lindeman states that social motives (i.e., "to comprehend self and the world, to have a sense of control over outcomes, to belong, to find the world benevolent and to maintain one's self-esteem") are often "more easily" fulfilled by pseudoscience than by scientific information. Furthermore, pseudoscientific explanations are generally not analyzed rationally, but instead experientially. Operating within a different set of rules compared to rational thinking, experiential thinking regards an explanation as valid if the explanation is "personally functional, satisfying and sufficient", offering a description of the world that may be more personal than can be provided by science and reducing the amount of potential work involved in understanding complex events and outcomes.^[84]

There is a trend to believe in pseudoscience more than scientific evidence.^[85] Some people believe the prevalence of pseudoscientific beliefs is due to widespread "scientific illiteracy".^[86] Individuals lacking scientific literacy are more susceptible to wishful thinking, since they are likely to turn to immediate gratification powered by System 1, our default operating system which requires little to no effort. This system encourages one to accept the conclusions they believe, and reject the ones they do not. Further analysis of complex pseudoscientific phenomena require System 2, which follows rules, compares objects along multiple dimensions and weighs options. These two systems have several other differences which are further discussed in the dual-process theory. The scientific and secular systems of morality and meaning are generally unsatisfying to most people. Humans are, by nature, a forward-minded species pursuing greater avenues of happiness and satisfaction, but we are all too frequently willing to grasp at unrealistic promises of a better life.^[87]

Psychology has much to discuss about pseudoscience thinking, as it is the illusory perceptions of causality and effectiveness of numerous individuals that needs to be illuminated. Research suggests that illusionary thinking happens in most people when exposed to certain circumstances such as reading a book, an advertisement or the testimony of others are the basis of pseudoscience beliefs. It is assumed that illusions are not unusual, and given the right conditions, illusions are able to occur systematically even in normal emotional situations. One of the things pseudoscience believers quibble most about is that academic science usually treats them as fools. Minimizing these illusions in the real world is not simple.^[88] To this aim, designing evidence-based educational programs can be effective to help people identify and reduce their own illusions.^[88]

Philosophers classify types of knowledge. In English, the word science is used to indicate specifically the natural sciences and related fields, which are called the social sciences.^[89] Different philosophers of science may disagree on the exact limits – for example, is mathematics a formal science that is closer to the empirical ones, or is pure mathematics closer to the philosophical study of logic and therefore not a science?^[90] – but all agree that all of the ideas that are not scientific are non-scientific. The large category of non-science includes all matters outside the natural and social sciences, such as the study of history, metaphysics, religion, art, and the humanities.^[89] Dividing the category again, unscientific claims are a subset of the large category of non-scientific claims. This category specifically includes all matters that are directly opposed to good science.^[89] Un-science includes both bad science (such as an error made in a good-faith attempt at learning something about the natural world) and pseudoscience.^[89] Thus pseudoscience is a subset of un-science, and un-science, in turn, is subset of non-science.

Science is also distinguishable from revelation, theology, or spirituality in that it offers insight into the physical world obtained by empirical research and testing.^[91][92] The most notable disputes concern the evolution of living organisms, the idea of common descent, the geologic history of the Earth, the formation of the solar system, and the origin of the universe.^[93] Systems of belief that derive from divine or inspired knowledge are not considered pseudoscience if they do not claim either to be scientific or to overturn well-established science. Moreover, some specific religious claims, such as the power of intercessory prayer to heal the sick, although they may be based on untestable beliefs, can be tested by the scientific method.

Some statements and common beliefs of popular science may not meet the criteria of science. "Pop" science may blur the divide between science and pseudoscience among the general public, and may also involve science fiction.^[94] Indeed, pop science is disseminated to, and can also easily emanate from, persons not accountable to scientific methodology and expert peer review.

If the claims of a given field can be tested experimentally and standards are upheld, it is not pseudoscience, however odd, astonishing, or counterintuitive the claims are. If claims made are inconsistent with existing experimental results or established theory, but the method is sound, caution should be used, since science consists of testing hypotheses which may turn out to be false. In such a case, the work may be better described as ideas that are "not yet generally accepted". Protoscience is a term sometimes used to describe a hypothesis that has not yet been tested adequately by the scientific method, but which is otherwise consistent with existing science or which, where inconsistent, offers reasonable account of the inconsistency. It may also describe the transition from a body of practical knowledge into a scientific field.^[29]

Karl Popper stated it is insufficient to distinguish science from pseudoscience, or from metaphysics (such as the philosophical question of what existence means), by the criterion of rigorous adherence to the empirical method, which is essentially inductive, based on observation or experimentation.^[47] He proposed a method to distinguish between genuine empirical, nonempirical or even pseudoempirical methods. The latter case was exemplified by astrology, which appeals to observation and experimentation. While it had astonishing empirical evidence based on observation, on horoscopes and biographies, it crucially failed to use acceptable scientific standards.^[47] Popper proposed falsifiability as an important criterion in distinguishing science from pseudoscience.

To demonstrate this point, Popper^[47] gave two cases of human behavior and typical explanations from Sigmund Freud and Alfred Adler's theories: "that of a man who pushes a child into the water with the intention of drowning it; and that of a man who sacrifices his life in an attempt to save the child."^[47] From Freud's perspective, the first man would have suffered from psychological repression, probably originating from an Oedipus complex, whereas the second man had attained sublimation. From Adler's perspective, the first and second man suffered from feelings of inferiority and had to prove himself, which drove him to commit the crime or, in the second case, drove him to rescue the child. Popper was not able to find any counterexamples of human behavior in which the behavior could not be explained in the terms of Adler's or Freud's theory. Popper argued^[47] it was that the observation always fitted or confirmed the theory which, rather than being its strength, was actually its weakness. In contrast, Popper^[47] gave the example of Einstein's gravitational theory, which predicted "light must be attracted by heavy bodies (such as the Sun), precisely as material bodies were attracted."^[47] Following from this, stars closer to the Sun would appear to have moved a small distance away from the Sun, and away from each other. This prediction was particularly striking to Popper because it involved considerable risk. The brightness of the Sun prevented this effect from being observed under normal circumstances, so photographs had to be taken during an eclipse and compared to photographs taken at night. Popper states, "If observation shows that the predicted effect is definitely absent, then the theory is simply refuted."^[47] Popper summed up his criterion for the scientific status of a theory as depending on its falsifiability, refutability, or testability.

Paul R. Thagard used astrology as a case study to distinguish science from pseudoscience and proposed principles and criteria to delineate them.^[95] First, astrology has not progressed in that it has not been updated nor added any explanatory power since Ptolemy. Second, it has ignored outstanding problems such as the precession of equinoxes in astronomy. Third, alternative theories of personality and behavior have grown progressively to encompass explanations of phenomena which astrology statically attributes to heavenly forces. Fourth, astrologers have remained uninterested in furthering the theory to deal with outstanding problems or in critically evaluating the theory in relation to other theories. Thagard intended this criterion to be extended to areas other than astrology. He believed it would delineate as pseudoscientific such practices as witchcraft and pyramidology, while leaving physics, chemistry and biology in the realm of science. Biorhythms, which like astrology relied uncritically on birth dates, did not meet the criterion of pseudoscience at the time because there were no alternative explanations for the same observations. The use of this criterion has the consequence that a theory can be scientific at one time and pseudoscientific at a later time.^[95]

In the philosophy and history of science, Imre Lakatos stresses the social and political importance of the demarcation problem, the normative methodological problem of distinguishing between science and pseudoscience. His distinctive historical analysis of scientific methodology based on research programmes suggests: "scientists regard the successful theoretical prediction of stunning novel facts – such as the return of Halley's comet or the gravitational bending of light rays – as what demarcates good scientific theories from pseudo-scientific and degenerate theories, and in spite of all scientific theories being forever confronted by 'an ocean of counterexamples'".^[16] Lakatos offers a "novel fallibilist analysis of the development of Newton's celestial dynamics, [his] favourite historical example of his methodology" and argues in light of this historical turn, that his account answers for certain inadequacies in those of Karl Popper and Thomas Kuhn.^[16] "Nonetheless, Lakatos did recognize the force of Kuhn's historical criticism of Popper – all important theories have been surrounded by an 'ocean of anomalies', which on a falsificationist view would require the rejection of the theory outright... Lakatos sought to reconcile the rationalism of Popperian falsificationism with what seemed to be its own refutation by history".^[96]

The boundary between science and pseudoscience is disputed and difficult to determine analytically, even after more than a century of study by philosophers of science and scientists, and despite some basic agreements on the fundamentals of the scientific method.^[12][97][98] The concept of pseudoscience rests on an understanding that the scientific method has been misrepresented or misapplied with respect to a given theory, but many philosophers of science maintain that different kinds of methods are held as appropriate across different fields and different eras of human history. According to Lakatos, the typical descriptive unit of great scientific achievements is not an isolated hypothesis but "a powerful problem-solving machinery, which, with the help of sophisticated mathematical techniques, digests anomalies and even turns them into positive evidence".^[16]

Laudan maintained that the demarcation between science and non-science was a pseudo-problem, preferring to focus on the more general distinction between reliable and unreliable knowledge.^[100]

The demarcation problem between science and pseudoscience brings up debate in the realms of science, philosophy and politics. Imre Lakatos, for instance, points out that the Communist Party of the Soviet Union at one point declared that Mendelian genetics was pseudoscientific and had its advocates, including well-established scientists such as Nikolai Vavilov, sent to a Gulag and that the "liberal Establishment of the West" denies freedom of speech to topics it regards as pseudoscience, particularly where they run up against social mores.^[16]

It becomes pseudoscientific when science cannot be separated from ideology, scientists misrepresent scientific findings to promote or draw attention for publicity, when politicians, journalists and a nation's intellectual elite distort the facts of science for short-term political gain, or when powerful individuals of the public conflate causation and cofactors by clever wordplay. These ideas reduce the authority, value, integrity and independence of science in society.^[101]

Distinguishing science from pseudoscience has practical implications in the case of health care, expert testimony, environmental policies, and science education. Treatments with a patina of scientific authority which have not actually been subjected to actual scientific testing may be ineffective, expensive and dangerous to patients and confuse health providers, insurers, government decision makers and the public as to what treatments are appropriate. Claims advanced by pseudoscience may result in government officials and educators making bad decisions in selecting curricula.^[11]

The extent to which students acquire a range of social and cognitive thinking skills related to the proper usage of science and technology determines whether they are scientifically literate. Education in the sciences encounters new dimensions with the changing landscape of science and technology, a fast-changing culture and a knowledge-driven era. A reinvention of the school science curriculum is one that shapes students to contend with its changing influence on human welfare. Scientific literacy, which allows a person to distinguish science from pseudosciences such as astrology, is among the attributes that enable students to adapt to the changing world. Its characteristics are embedded in a curriculum where students are engaged in resolving problems, conducting investigations, or developing projects.^[18]

Friedman mentions why most scientists avoid educating about pseudoscience, including that paying undue attention to pseudoscience could dignify it.^[102] On the other hand, Park emphasizes how pseudoscience can be a threat to society and considers that scientists have a responsibility to teach how to distinguish science from pseudoscience.^[103]

Pseudosciences such as homeopathy, even if generally benign, are used by charlatans. This poses a serious issue because it enables incompetent practitioners to administer health care. True-believing zealots may pose a more serious threat than typical con men because of their affection to homeopathy's ideology. Irrational health care is not harmless and it is careless to create patient confidence in pseudomedicine.^[104]

On December 8, 2016, Michael V. LeVine, writing in Business Insider, pointed out the dangers posed by the Natural News website: "Snake-oil salesmen have pushed false cures since the dawn of medicine, and now websites like Natural News flood social media with dangerous anti-pharmaceutical, anti-vaccination and anti-GMO pseudoscience that puts millions at risk of contracting preventable illnesses."^[105]

The anti-vaccine movement has persuaded large number of parents not to vaccinate their children, citing pseudoscientific research that links childhood vaccines with the onset of autism.^[106] These include the study by Andrew Wakefield, which claimed that a combination of gastrointestinal disease and developmental regression, which are often seen in children with ASD, occurred within two weeks of receiving vaccines.^[107][108] The study was eventually retracted by its publisher while Wakefield was stripped of his license to practice medicine.^[106]