The doctrine of Utilitarianism was first promulgated in Bentham's Principles of Morals and Legislation where he advocated a consequentialist ethic useful for public morality based on the idea of collating and weighing objectively verifiable data.
Bentham proposed that the goal of ethics was to maximize utility for the greatest number of people possible, focusing on equating the like interests of involved parties. Utility, to Bentham, was happiness and its counterpart, the absence of suffering. Therefore, a moral decision is one that employs an ethical cost-benefit analysis, minimizing pains and expanding pleasures.
Utilitarianism has come far since its 18th Century iteration, and thus has many different forms.
- Act Utilitarianism is (the basic version) which notes that every single action that has moral relevance must be weighed regrading consequences for those involved.
- Rule Utilitarianism is another version, which stems from the 19th Century influence of John Stuart Mill, the son of yet another Utilitarian. John Stuart Mill is credited with a modification of basic Act Utilitarianism to create Rule Utilitarianism, which states that the morality of behaviour ought to be judged against rules which themselves are weighed against each other to determine long-term utility. In this sense, you ought not weigh individual situations, but situations relative to rules that would apply to maximize social utility.
- Act Utilitarianism and Rule Utilitarianism compared. There will always be cases when happiness overall is increased by breaking a rule, if the rules are well formulated such cases are rare. When a society follows reasonable rules people know where they stand and what they can expect from other people. Individuals also know how others expect them to act, this tends to increase happiness.
John Stuart Mill is also known for his application of Utilitarian reasoning to the defense first of a Liberal Democratic Republic operating on the principles of a relatively Free Market, which is evident from his writings On Liberty and eventually a more Socialistic Liberal Democracy as he became disillusioned with the Industrial Revolution's poverty conditions.
Alternative forms of Utilitarianism have evolved over the course of the late 19th century and early 20th Century, with notable philosophers such as Henry Sidgwick (b. 1838-1900), whose Methods of Ethics standardized and defended Utilitarian theory from its critics, laying the foundation for 20th century applications of the theory.
Modern Utilitarianism is a work in progress, but there is perhaps no better advocate and intellectual influential in the evolution and application of the doctrine than Preference Utilitarian Dr. Peter Singer, Professor of Bioethics at Princeton University, NJ. Preference Utilitarianism takes the basic premise of Utilitarianism and states that the goal is to maximize the greatest number of welfare preferences for the greatest number of people--slightly different, but importantly so, from classical "hedonistic" utilitarian calculus. Much of Singer's Philosophy of Utilitarianism can be seen in his famous college textbook: Practical Ethics.
Utilitarianism has been influential in public policy since its inception, but is largely in contrast to other Enlightenment theories, such as non-consequentialist Deontological Ethics (see Kantianism, Virtue Ethics, Rights-based Ethics, etc).