Intelligent design (ID), argument intended to demonstrate that living organisms were created in more or less their present forms by an “intelligent designer.”

Intelligent design was formulated in the 1990s, primarily in the United States, as an explicit refutation of the theory of biological evolution advanced by Charles Darwin (1809–82). Building on a version of the argument from design for the existence of God advanced by the Anglican clergyman William Paley (1743–1805), supporters of intelligent design observed that the functional parts and systems of living organisms are “irreducibly complex,” in the sense that none of their component parts can be removed without causing the whole system to cease functioning. From this premise, they inferred that no such system could have come about through the gradual alteration of functioning precursor systems by means of random mutation and natural selection, as the standard evolutionary account maintains; instead, living organisms must have been created all at once by an intelligent designer. In Darwin’s Black Box: The Biochemical Challenge to Evolution (1996), the American molecular biologist Michael Behe, the leading scientific spokesperson for intelligent design, offered three major examples of irreducibly complex systems that allegedly cannot be explained by natural means: (1) the bacterial flagellum, used for locomotion, (2) the cascade of molecular reactions that occur in blood clotting, or coagulation, and (3) the immune system.

Intelligent design was widely perceived as being allied with scientific creationism, the notion that scientific facts can be adduced in support of the divine creation of the various forms of life. Supporters of intelligent design maintained, however, that they took no position on creation and were unconcerned with biblical literalism. Consequently, they did not contest the prevailing scientific view on the age of Earth, nor did they dispute the occurrence of small evolutionary changes, which are amply observed and seemingly work by natural selection. Like earlier proponents of creationism, they wrote statutes or initiated lawsuits designed to permit the teaching of their view as an alternative to evolution in American public schools, where instruction in any form of religion is constitutionally forbidden. In the major case on the issue, Kitzmiller v. Dover Area School District (2005), concerning a school district in Dover, Pennsylvania, a federal court ruled that intelligent design was not clearly distinct from creationism and therefore should be excluded from the curriculum on the basis of earlier decisions, notably McLean v. Arkansas (1982).

Opponents of intelligent design argued that it rests on a fundamental misunderstanding of natural selection and that it ignores the existence of precursor systems in the evolutionary history of numerous organisms. Some noted that the argument had been refuted by Darwin himself in direct response to Paley. Beginning in the 1990s, conceptual advances in molecular biology shed additional light on how irreducible complexity can be achieved by natural means. Evolutionary biologists proposed various approaches to explain Behe’s three examples of complexity, including: (1) the self-organizing nature of biochemical systems, (2) the built-in redundancy of complex organic structures (if one crucial step is absent, other processes can achieve the same result), and (3) the role of versatile exploratory processes that, in the course of their normal physiological functioning, can help give rise to useful new structures of the body. Meanwhile, intelligent design appeared incapable of generating a scientific research program, which inevitably broadened the gap between it and the established norms of science.