In social science and economics, public interest is "the welfare or well-being of the general public" and society.

While it has earlier philosophical roots and is considered to be at the core of democratic theories of government, often paired with two other concepts, convenience and necessity, it first became explicitly integrated into governance instruments in the early part of the 20th century.

The public interest was rapidly adopted and popularised by human rights lawyers in the 1960s and has since been incorporated into other fields such as journalism and technology.

Economist Lok Sang Ho, in his Public Policy and the Public Interest, argues that the public interest must be assessed impartially and, therefore, defines the public interest as the "ex ante welfare of the representative individual". Under a thought experiment, by assuming that there is an equal chance for one to be anyone in society and, thus, could benefit or suffer from a change, the public interest is by definition enhanced whenever that change is preferred to the status quo ex ante. This approach is "ex ante", in the sense that the change is not evaluated after the fact but assessed before the fact without knowing whether one would actually benefit or suffer from it.

This approach follows the "veil of ignorance" approach, which was first proposed by John Harsanyi but popularized by John Rawls in his 1971 Theory of Justice. Historically, however, the approach can be traced to John Stuart Mill, who, in his letter to George Grote, explained that "human happiness, even one's own, is in general more successfully pursued by acting on general rules, than by measuring the consequences of each act; and this is still more the case with the general happiness, since any other plan would not only leave everybody uncertain what to expect, but would involve perpetual quarrelling..."

This approach is attractive because only under impartiality can there be the possibility of consensus. This of course is still conditional: i.e., conditional on everybody doing the same thought experiment as Rawls suggested. Other than this, if everybody considers his own private interests from his point of view, then social welfare will have to be defined in an ex post fashion by summing up or otherwise defined over individually rated welfares or social states that would come about because of different social choices. This is the approach of social welfare functions. Whether these functions are based on individually ranked social states or individual cardinal uitilities, the absence of consensus over social welfare functions is unavoidable.

The Institute of Chartered Accountants in England and Wales argues that applying a detailed definition is likely to result in unintended consequences, in Acting in the Public Interest(2012). Instead, each circumstance needs to be assessed based on criteria such as the relevant public, wants, and constraints. The key to assessing any public interest decision is transparency of the decision-making process, including balancing competing interests.

The need to consider the circumstances carefully in all dimensions is well taken, as is the need for transparency of the standards and procedures for policy making. It needs to be noted that in practice adversarial politics means that "balancing competing interests" amounts to politicians navigating through the web of divergent interests to procure their best political interests. The outcome will be a contest of political clout among different competing interests. Whether this promotes the public interest remains controversial.