Imagine a unique set of scales that measures the value of life. If a single human were on one side, how many chimpanzees (our closest genetic relatives) would need to be on the other side before the scales tipped in their direction?

This may seem like an abstract, irrelevant or even offensive question to some people. But it was made horrifically real by the death last week of Harambe, the Cincinnati Zoo gorilla who was shot after a young boy fell into his enclosure.

Zoo handlers were faced with the agonising decision to take Harambe’s life to ensure the young boy would not lose his. The response to this event online has varied from anger, to sadness, through to considerations of how much choice the zoo’s staff really had. How do we decide what our own lives are worth compared with other species?

Perhaps we can try to frame the comparison in relative terms. There are 7.4 billion human beings on the planet, whereas Western lowland gorillas are critically endangered. Does a human life hold more value than that of a member of a critically endangered animal species?

Harambe’s death suggests that the instinctive answer is yes, but is there a point at which some people’s moral scales might tip the other way? Our research suggests there might be.

The concept of ‘moral expansion’

No one expects an easy answer to this question. But the fact that we can even ask it shows that our moral sensibilities have expanded beyond the boundaries of our own species.

Many of us feel a deep moral responsibility not just to protect our fellow humans, but to guard the moral rights of entities the world over. This change, which has spanned the past few centuries, has resulted in some serious ethical challenges to the ways we interact with other species and the environment.

Recently, animal rights organisations in the United States have fought for the legal personhood status of chimpanzees like Tommy, while animal advocates have petitioned the United Nations for a Declaration of the Rights of Great Apes since 1993.

In the meantime, a river in New Zealand has been officially granted legal personhood status (similar to the status given to corporations), making the river a legal “person” with its own rights and interests under law.

In line with the concept of compassionate conservation, these examples highlight the narrowing of the gulf between the moral rights of humans, non-humans and the environment.

For supporters of these causes, human rights and corresponding moral standing should no longer be restricted to humans.

Are you willing to sacrifice?

The legal semantics are interesting, but what about when it really comes to the crunch? Our recent research has examined how widely people spread their moral concern to others. We found that this is a key predictor of the type of moral decision-making that compares the value of a human life to that of another animal.

We asked people the following question: how many other human beings would need to be in danger before you sacrificed your own life to ensure their survival? But our research didn’t stop at humans.

We also asked how many chimpanzees would need to be at risk. How many ants? How many redwood trees?

Responses to these questions were as varied as the responses to the shooting of Harambe.

Some people said they would sacrifice their life if it meant that just a few chimpanzees would keep theirs. Others said it wouldn’t matter how many animals or trees were in danger; a human life was simply worth more.

We found that we could predict people’s responses to specific questions based on their position on what we call the “moral expansiveness scale” (you can find out your own score here). Those whose moral outlook stretched further beyond humans were more likely to say they would sacrifice themselves to benefit other animals or nature.

A moral dilemma

Human beings are becoming increasingly morally expansive. As a species we are adopting a moral standard that represents ethical and altruistic responsibilities on a global scale. This is reflected in the extension of human rights to chimpanzees and the granting of legal rights to elements of our natural environment.

However, this trend is accompanied by an escalating moral conflict. The extension of our moral boundaries is happening just as the global human population is growing exponentially, leading to tension and competition over scarce resources.

As a consequence, we are increasingly likely to face ethical dilemmas over the value of human versus non-human life. It won’t be in the form of a quick decision to kill an animal to save the life of a child. These dilemmas will play out in courtrooms and parliaments, as human needs are pitted against environmental ones, and as the battle for natural resources brings threats of deforestation and species extinction.

As we edge ever closer to the brink of the Earth’s sixth mass extinction, perhaps we need to consider just exactly what a human life is worth.

Complete our survey to find out how morally expansive you are.