We’re not great at thinking about the long-term consequences of catastrophes that threaten our existence

Imagine that tomorrow a catastrophe wipes out 99% of the world's population. That's clearly not a desirable scenario — we would all agree that a peaceful, continued existence is preferable. Now imagine that the disaster kills everyone, wiping out the human race. Most of us would rate that as an even worse occurrence.

But how do we see the relative severity of these different possibilities? Is there a bigger difference between nothing happening and 99% of people dying, or between 99% and 100% of people being wiped out?

This thought-experiment was first posed by the philosopher Derek Parfit, who thought most people would believe the first difference is greater — after all, going from business-as-usual to almost total annihilation is a big step. He, on the other hand, felt the second difference was greater by far: even if just a tiny fraction of humans survive, civilisation could continue for millions of years, but if humanity is wiped from the face of the Earth, then it's all over.

Now a new study in Scientific Reports has found that, like Parfit predicted, most people don't seem to share his view of human extinction as a "uniquely bad" catastrophe — until they are forced to go beyond their gut feeling and reflect on what extinction really means in the long term.

First, Stefan Schubert and colleagues at the University of Oxford surveyed 183 Americans on their opinions on human extinction. Almost 80% believed extinction would be bad, and participants also strongly agreed that human extinction should be prevented and that we have a moral obligation to do so.

The researchers then gave 1,251 members of the British public five variations on Parfit's thought experiment.  The first group were asked to rank the following three scenarios:

(A) There is no catastrophe.

(B) There is a catastrophe that immediately kills 80% of the world's population.

(C) There is a catastrophe that immediately kills 100% of the world's population.

Most participants ranked A as the best scenario and C as the worst.

The researchers then asked participants "In terms of badness, which difference is greater: the difference between A and B, or the difference between B and C?" Less than a quarter of people responded that that the difference between B and C was the greater of the two, suggesting that "most people did not find extinction uniquely bad".

The researchers suggest this may be because people instinctively focus on the immediate victims of a catastrophe, and the leap from no deaths to 80% seems greater than that from 80% to 100%. So for two further groups they tweaked the scenarios to encourage participants to focus less on the fact that billions of people would be dying. In one, participants read that the catastrophes would sterilise people rather than kill them, and in the other they read that the catastrophes would affect zebras rather than humans. In these cases, around 45% rated extinction as uniquely bad (that is, they rated the difference between B and C greater than the difference between A and B) — significantly more than in the first group.

Finally, encouraging participants to focus on the long-term consequences of the scenarios led to an even greater number of people seeing extinction as uniquely bad. One group was explicitly asked to consider the fact that only the extinction scenario would leave no future for humanity, while another were told that if 80% of people died, the remaining population would recover and go on to form a utopian society. In this condition more than three-quarters of participants rated the B vs C difference as the greatest.

Overall, the results suggest that although we believe human extinction is a bad thing, it's not until we're forced to think beyond the immediate, short-term consequences that we tend to agree that extinction is a far worse outcome than any other. This was supported by the additional finding that people were more likely to consider B vs C  as the greater difference if they scored higher on a test measuring "cognitive reflection" — the ability to override gut instinct and reflect on the answer to a question. "This could mean that deliberative thought-processes lead to finding extinction uniquely bad, whereas intuitive thought-processes lead to the opposite conclusion," the team writes.

Why does this real-world test of a philosophical thought experiment matter? We're living in a time where climate change and technological progress puts humans at growing risk of extinction, say the researchers, yet we're not doing enough to protect against these existential risks. The results suggest that there are cognitive biases in play that could partly explain why that's the case, and that encouraging people to reflect more carefully might help overcome these. After all, the team conclude, "if it is right that human extinction is uniquely bad, then we should arguably invest much more in making sure it does not happen."

Further reading

– The Psychology of Existential Risk: Moral Judgments about Human Extinction

About the author

Matthew Warren (@MattbWarren) is Editor of BPS Research Digest