The principle of least effort is a broad theory that covers diverse fields from evolutionary biology to webpage design. It postulates that animals, people, even well designed machines will naturally choose the path of least resistance or "effort". It is closely related to many other similar principles see Principle of least action or others listed below. This is perhaps best known or at least documented among researchers in the field of library and information science. Their principle states that an information seeking client will tend to use the most convenient search method, in the least exacting mode available. Information seeking behavior stops as soon as minimally acceptable results are found. This theory holds true regardless of the user's proficiency as a searcher, or their level of subject expertise. Also this theory takes into account the user’s previous information seeking experience. The user will use the tools that are most familiar and easy to use that find results. The principle of least effort is known as a “deterministic description of human behavior.” The principle of least effort applies not only in the library context, but also to any information seeking activity. For example, one might consult a generalist co-worker down the hall rather than a specialist in another building, so long as the generalist's answers were within the threshold of acceptability.
The principle of least effort is analogous to the path of least resistance.
However, the term "principle of least effort" did not originate with Mann. It can be traced at least as early as the 1949 book, Human Behaviour and the Principle of Least Effort: An Introduction to Human Ecology by Harvard linguist George Kingsley Zipf.
Likewise, one of the most common measures of information seeking behavior, library circulation statistics, also follows the 80-20 rule. This suggests that information seeking behavior is a manifestation not of a normal distribution curve, but a power law curve.
The principle of least effort is exceptionally important when considering design for libraries and research in the context of the modern library. Libraries must take into consideration the users desire to find information quickly and easily. As a result, the principle must be considered to design individual OPAC’s as well as other library tools.
The principle is a guiding force for the push to provide access to electronic media in libraries. The principle of least effort was further explored in a study of library behavior of graduate students by Zao Liu and Zheng Ye (Lan) Lang published in 2004. The study sampled Texas A&M distance learning graduate students to test what library resources they used, and why they used those particular resources. In this study the Internet was used the most, while libraries were the next most used resource for conducting class research. The study found that most students used these resources due to their quickness and ability to access from home. The study found that the principle of least effort was the primary behavior model of most distance learning students. This means that modern libraries, especially academic libraries need to analyze their electronic databases in order to successfully cater to the needs of the changing realities of information science.
Professional writers employ the principle of least effort during audience analysis. The writer analyzes the reader's environment, previous knowledge, and other similar information which the reader may already know. In technical writing, recursive organization, where parts resemble the organization of the whole, helps readers find their way. Consistency of navigational features is a common concern in software documentation. (cf. Hackos)