The Expanding Circle: Ethics and Sociobiology

Singer argues that ethics has a biological origin but cannot be reduced to biology. Altruism began as a genetically based drive to protect kin and community members, but human beings possess a capacity for reason that allows them to generalize and universalize their altruistic tendencies beyond the groups to which they are biologically inclined. The book rejects both the sociobiological reduction that treats morality as nothing but genetic self-interest and the culturalist position that ethics belongs to a realm of pure thought unrelated to human nature. Singer's distinctive claim is that reason builds on evolved tendencies rather than opposing them, expanding the circle of beings whose interests we take seriously.

The 'expanding circle' itself became the book's enduring conceptual contribution. Singer describes the historical process by which the circle of beings whose interests people value has widened from self and family to tribe, nation, and race, and argues that this expansion should continue to include all sentient beings. The framework is used today in evolutionary ethics, animal rights discourse, and effective altruism. Steven Pinker has praised it as a concept that 'reconciles the existence of human nature with political and moral progress' [citation:1]. Robert Wright called it a brilliant book whose significance becomes clearer every year [citation:1]. The phrase has become shorthand for the idea that moral progress consists in widening the scope of equal consideration.

The Expanding Circle was one of the first sustained philosophical engagements with Wilson's Sociobiology: The New Synthesis (1975) by a moral philosopher . Wilson's work was initially ignored or dismissed by moral philosophers as reductionist and arrogant. Singer took a different approach: he restated and defended the sociobiological account of kin selection, reciprocal altruism, and group selection, while arguing that these biological insights do not exhaust ethics. His target was not sociobiology itself but the claim that ethics can be fully explained by it. Singer's book thus represents a constructive revision of Wilson's project, accepting its biological findings while insisting on the autonomy and importance of rational moral reasoning.

Kirkus Reviews offered a sharp critique: the book's defense of reason as the way out of biological determinism is 'unconvincing and similarly inadequate' . The reviewer argued that Singer nominates reason as the ultimate desideratum but never adequately defines it, asks whether there are conflicts of reason, or explains how reason is to be encouraged to prevail. The review also noted that Singer dismisses religion as irrelevant and biology as inadequate, leaving reason as a placeholder that does little explanatory work. Singer's defenders would respond that the book's argument is precisely that reason's capacity to universalize is a real feature of human cognition, not a metaphysical faculty, and that the burden of proof rests on those who deny its moral significance.

The 2011 Princeton University Press edition includes a new afterword in which Singer takes stock of his argument in light of three decades of research on the evolution of morality [citation:1]. He discusses how progress in biology and genetics has influenced his thinking and argues that recent findings reinforce his original theory. The afterword matters because it shows Singer treating the book not as a closed argument but as a hypothesis open to empirical revision. The 2011 edition also carries the subtitle 'Ethics, Evolution, and Moral Progress,' replacing the original 'Ethics and Sociobiology,' a change that reflects the shift in the book's reception from a intervention in the sociobiology wars to a foundational text in evolutionary ethics.

The Expanding Circle explicitly extends the argument of Animal Liberation (1975) into the domain of evolutionary ethics. Singer writes that it is as arbitrary to restrict equal consideration of interests to one's own species as to restrict it to one's own race, and that the only justifiable stopping place for the expansion of altruism is the point at which all beings capable of pleasure and pain are included . The book provides the theoretical and evolutionary grounding for the practical ethic of animal liberation. It also anticipates the effective altruism framework by linking moral progress to the rational expansion of concern beyond parochial boundaries, a theme Singer would develop in later works like The Life You Can Save.

Yes. The book was published in 1981, in the midst of the acrimonious sociobiology debates that had polarized intellectual life since Wilson's 1975 book. One reviewer described Singer as having 'broken out of this dilemma' between the 'absurd anti-culturalist reductionism' of writers like Wilson and Dawkins and the culturalist reaction that treated ethics as pure thought [citation:4]. The reviewer called it a 'lucid and sensible study' but also 'one to argue with, rather than from' [citation:4]. Singer's intervention was distinctive precisely because it refused the terms of the existing dispute, accepting the biological evidence while denying its sufficiency for ethics.