It is arguable […] that a further effect of our partiality for members of our own species is a tendency to decreased sensitivity to the lives and well-being of those sentient beings that are not members of our species.
One can discern an analogous phenomenon in the case of nationalism. It frequently happens that the sense of solidarity among the members of a nation motivates them to do for one another all that—and perhaps even more than—they are required to do by impartial considerations. But the powerful sense of collective identity within a nation is often achieved by contrasting an idealized conception of the national character with caricatures of other nations, whose members are regarded as less important or worthy or, in many cases, are dehumanized and despised as inferior or even odious. When nationalist solidarity is maintained. in this way—as it has been in recent years in such places as Yugoslavia and its former provinces—the result is often brutality and atrocity on an enormous scale. Thus, while nationalist sentiment may have beneficial effects within the nation, these are greatly outweighed from an impartial point of view by the dreadful effects that it has on relations between nations.
I believe that our treatment of the severely retarded and our treatment of animals follow a similar pattern. While our sense of kinship with the severely retarded moves us to treat them with great solicitude, our perception of animals as radically “other” numbs our sensitivity to them, allowing us to abuse them in various ways with an untroubled conscience. We are not, of course, aggressively hostile to them the way nationalists often are to the members of rival nations; we are simply indifferent. But indifference to their lives and well-being is sufficient, when conjoined with motives of self-interest, for the flourishing of various practices that involve both killing and the infliction of suffering on a truly massive scale and that go virtually unchallenged in all contemporary human societies: factory farming, slaughtering animals for food or to take their furs, using them for the testing of cosmetic products, killing them for sport, and so on. When one compares the relatively small number of severely retarded human beings who benefit from our solicitude with the vast number of animals who suffer at our hands, it is impossible to avoid the conclusion that the good effects of our species-based partiality are greatly outweighed by the bad.
Jeff McMahan, The Ethics of Killing: Problems at the Margins of Life, Oxford, 2002, p. 221