Moral circle expansion is an increase over time in the number and type of entities given moral consideration. The general idea of moral inclusion was discussed by ancient philosophers and since the 19th century has inspired social movements related to human rights and animal rights. Especially in relation to animal rights, the philosopher Peter Singer has written about the subject since the 1970s, and since 2017 so has the think tank Sentience Institute, part of the 21st-century effective altruism movement. There is significant debate on whether humanity actually has an expanding moral circle, considering topics such as the lack of a uniform border of growing moral consideration and the disconnect between people's moral attitudes and their behavior. Research into the phenomenon is ongoing.

The moral circle was discussed as early as the 2nd century by Stoic philosopher Hierocles, who described in On Appropriate Acts the concentric social circles of a human being, for whom duty to the innermost circle was strongest. The concept was developed more fully by William Lecky in his 1869 work History of European morals from Augustus to Charlemagne.

Edward Payson Evans, an early advocate for animal rights, published Evolutional Ethics and Animal Psychology in 1897. He argued that humans need to move past anthropocentric conceptions that view humans as fundamentally different and separate to all other beings and that, as a result, humans have no moral obligations toward them. The utilitarian philosopher and animal rights advocate J. Howard Moore argued for a sentientist philosophy in his 1906 book The Universal Kinship, asserting that humans should care about all sentient life based on shared evolutionary kinship:

The partially emancipated human being who extends his moral sentiments to all the members of his own species, but denies to all other species the justice and humanity he accords to his own, is making on a larger scale the same ethical mess of it as the savage. The only consistent attitude, since Darwin established the unity of life (and the attitude we shall assume, if we ever become really civilised), is the attitude of universal gentleness and humanity.

The concept was notably developed by the utilitarian philosopher Peter Singer in his 1981 book The Expanding Circle, which is titled after the concept of moral circle expansion. This book sets out a common theory of the expanding circle: humans started by only valuing those most similar to themselves, such as their family or social group, but then humans began to value other residents of their nation and finally humanity as a whole; the same process of expansion is now taking place with respect to animal rights. Singer wrote in the book that "The only justifiable stopping place for the expansion of altruism is the point at which all whose welfare can be affected by our actions are included within the circle of altruism." Singer also references the expanding circle in some of his other works.

Moral circle expansion has also been addressed by some later writers, whose definitions of it may not be exactly the same as Singer's. Robert Wright responds to Singer with a more critical conception in his 1994 book The Moral Animal:

The most cynical explanation of why so many sages have urged an expanded moral compass is the one set out near the beginning of this chapter: a large compass expands the power of the sages doing the urging.

T. J. Kasperbauer's 2018 book Subhuman defines the expansion of the moral circle in reference to an increase both in the number of things considered moral patients and how many kinds of things are considered moral patients. Kasperbauer also adds in that this degree of consideration for things newly in the moral circle must be large enough to be important.