In Blaire French’s 1998 novel The Ticking Tenure Clock, the central character, a junior political science professor, is struggling to finish a book on the “Orphans of the Left”: animal rights activists. According to French, animal rights activists in the 1990s saw themselves as part of a larger progressive family that sought to protect the vulnerable against exploitation by the powerful. Yet they also felt disowned by the other members of their family.

Twenty years later, the situation remains unchanged. Proponents of various social justice movements routinely express support for each other – feminist organisations often show support for Black Lives Matter, or for immigrant rights, or gay rights – but animal rights groups remain outside this circle of progressive solidarity. As Aimée Dowl notes, “None of the major feminist organisations in the United States devotes committee or internet space to or has polices dealing with animal rights issues”. Indeed, as John Sanbonmatsu indicates, “The left with few exceptions has historically viewed human violence towards other beings with indifference.”

This indifference has been a source of frustration and puzzlement to many animal rights activists. Part of the explanation is simple self-interest and inertia. The instrumentalisation of animals in our society makes possible a steady flow of pleasures – often very intimate pleasures relating to what we eat and wear – that are central to people’s identity. So even when people realise that the treatment of animals in our society is morally suspect, they employ various techniques of “moral disengagement” to look away from the issue.

There is, however, something deeper than self-interest and moral disengagement at work in the left’s resistance to animal rights. Embracing animal rights would challenge not only people’s identities and lifestyles, but also a central philosophical pillar of left politics: its “humanism”.

Humanism here is the idea that the value of humanity consists precisely in its difference from animality. It is because we are different from, and superior to, animals that we have inherent worth and are owed basic rights. What makes human lives valuable is not anything we share with other animals. Rather, it lies in our “distinctly human” qualities such as rationality, morality, or autonomy.

On this humanist account, respect for human dignity consists precisely in treating humans better than animals. Conversely, violating human dignity is defined as treating someone “like an animal”. In this sense, humanism is tied to species hierarchy: it is about elevating the human above the animal. A more accurate term therefore might be “human supremacism”.

Humanism supremacism is of course not unique to the left. It is found across the political spectrum in the West, with deep roots in the Judeo-Christian tradition. In many ways, it is simply a secular version of long-standing religious ideas that humans were uniquely created in God’s image. Yet the left’s commitment to humanism has been strengthened by the central role that dehumanisation played in the fight for social justice in the 20th century.

Indeed, many of the great injustices of the 20th century involved dehumanisation – treating Jews, blacks, immigrants, gay people, those with disabilities, and other outgroups as less than fully human. And a growing body of social science evidence confirms that many groups in society continue to be dehumanised, not in the literal sense that they are not seen as members of the species Homo sapiens, but rather in the sense that they are seen as less likely to possess the “distinctly human” qualities that allegedly distinguish us from animals.

Specifically, outgroups today are seen as driven by basic instincts we share with animals – say, pleasure or fear – but as less likely to display “distinctly human” feelings, such as remorse or gratitude. This leads to discrimination and prejudice. But it also renders outgroups vulnerable to violence, since it is assumed that, like animals, they need to be controlled through force.

Fighting dehumanisation is therefore a central challenge for the left. And it is widely assumed that human supremacism is an essential tool in that fight. The best way – and perhaps the only way – to fight the perception that some groups are not fully human is to emphasise the radical discontinuity between humans and animals: that is, to reassert the sanctity of the human.

This belief that human supremacism helps fight dehumanisation underpins many of the strategic choices of the left since World War II. As Claire Jean Kim has shown, the African-American civil rights movement made a deliberate decision in the 1950s to invest in sanctifying the species line as a resource to fight dehumanising racism. American society was accused of treating black people as if they were animals. And most social justice movements from the 1960s onwards followed the same humanist strategy, including those movements seeking to defend women’s rights, gay rights, and disability rights.

This is why animal rights activists have remained orphans of the left. The paradigmatic argument for animal rights depends on emphasising continuities between humans and animals; the paradigmatic argument to defend the rights of dehumanised groups depends on emphasising radical discontinuity between humans and animals.

It is not easy to see how this chasm can be overcome. But there is now growing interest in bridging the chasm.

One reason is that supremacism is difficult to defend in our post-Darwin world. Darwin’s theory of evolution showed that humans and other animals are continuous with respect to our interests and capacities. If humans share ancestry with other primates, and if we share 98.8 per cent of our DNA with chimpanzees, then whatever makes human lives valuable or worthy of respect almost certainly has analogues in the lives of other animals.

A second reason for overcoming the gap is the recognition on the left that animal agriculture is an environmental catastrophe, whose costs are borne disproportionately by the least well-off. So, the left needs to champion the transition from meat to plant-based diets: we need to wean ourselves off the flow of pleasures we derive from the animal-industrial complex.

But perhaps the most important reason for building bridges is that human supremacism may not be an effective strategy for fighting dehumanisation after all. There is growing evidence that shows that a belief in human supremacy and species hierarchy aggravates, rather than alleviates, the problem of dehumanisation. The more people believe that humans are superior to animals, the more likely they are to dehumanise immigrants, women, and racial minorities.

Interestingly, the link between species hierarchy and dehumanisation is causal, not just correlational. For instance, when participants in studies are given a newspaper story reporting on evidence for human superiority over animals, the outcome is the expression of greater prejudice against human outgroups. By contrast, those who are given a newspaper story reporting on evidence that animals are continuous with humans in the possession of valued traits and emotions become more likely to accord equality to human outgroups. In short, reducing the status divide between humans and animals helps to reduce prejudice and helps to strengthen belief in equality amongst human groups.

This suggests that human supremacism is not only unnecessary to counteract dehumanisation, but is in fact counterproductive. This, in turn, implies that the prospects for bringing animal rights back into the family of social justice movements are significantly improved. We do not need to throw animals under the bus in order to uphold the rights and dignity of women, racial minorities, or people with disabilities. In fact, the fight against species hierarchy and the fight against intra-human hierarchies are mutually supportive, not zero-sum.

Of course, as with any family reunification after a long period of estrangement, this will require a lot of good faith and hard work. If social justice movements have been too willing to throw animals under the bus, it is also true that the animal rights movement has sometimes adopted rhetoric and strategies that reproduce sexist, racist or ableist tropes.

Animal rights advocacy must acknowledge and be held accountable for its impact on gender and racial hierarchies, just as advocacy for gender and racial equality must acknowledge and be held accountable for its effects on animals. Establishing this “ethic of mutual avowal” may be difficult – but if animal rights remain the orphans of the left, the result will be disastrous, not just for animals, but for humans and for the planet.

Will Kymlicka is the Canada Research Chair in Political Philosophy. He is the author of Multicultural Citizenship and co-author with Sue Donaldson of Zoopolis: A Political Theory of Animal Rights.

This article is part of the Agora series, a collaboration between the New Statesman and Aaron James Wendland. Aaron is Assistant Professor of Philosophy at the Higher School of Economics and the co-editor of Wittgenstein and Heidegger and Heidegger on Technology. Follow him on Twitter: @ajwendland.