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Would you kill one person to save five? It depends what age you are

Unthinkable: Changing morals may explain differences between age groups on the Israel-Palestine conflict

A famous thought experiment in philosophy is “the trolley problem”. Imagine you saw a tram about to crash into five people and you could save their lives by pushing a man off a footbridge into its path to halt its progress. Would that be morally justified?

Now imagine you could save the five people by instead pulling a switch to direct the tram on to another rail track where the man is standing. In both scenarios the choice is whether to kill one person to save five. But crucially, in the first, you intentionally kill the man (his death is not a side effect but the effect you want).

The problem is used to highlight a moral intuition. People see intentional harm as very different from “unintended but foreseen” harm. The intuition underpins what is called the “doctrine of double effect” (DDE). This says it is sometimes permissible to cause an unintended but foreseen harm if it brings about a good result. Under the DDE, the “switch” option could be seen as morally justifiable but not the “footbridge” option.

The difference between the two types of harm may seem abstract. But the DDE is highly relevant to political debates, especially the Israel-Hamas war. The slaughter committed by Hamas on October 7th was unambiguously intentional. The subsequent killing of a far greater number of Gazan civilians is portrayed by Israel as an unintentional but foreseen consequence of its attempt to free hostages and to destroy Hamas.


The issue is key to the genocide case before the International Court of Justice. To gain a prosecution there must be “proven intent” on the part of Israel “to physically destroy a national, ethnical, racial or religious group”. Israel told the court there was “undeniable intent” to combat Hamas “but that is not the intent to destroy all or part of a people as such”.

However, does the doctrine of double effect really stand up to scrutiny? In real-world scenarios, it can be difficult to determine intentions but there are several reasons to consider DDE a “helpful” principle, says Chris Finlay, an expert in “just war” theory.

“First, there is a pragmatic argument for doing so. Elizabeth Anscombe [a philosopher who explored the trolley problem after the second World War] warned that if we don’t offer ethical guidance on war that permits a sufficiently wide range of actions, then we run the risk that wars will go ahead anyway and soldiers will fight with no moral constraint at all.”

Secondly, “the DDE might sound quite permissive but it is actually a highly restrictive principle”. Finlay says “you’re only non-culpable under the doctrine provided the casualties you cause aren’t excessive.” Other criteria include proportionality, necessity and discriminating between combatants and non-combatants.

“Proportionality isn’t about retribution but about protecting living people and providing them with security,” says Finlay, a professor in political theory at Durham University who is originally from Omagh, Co Tyrone. “How many future attacks can you legitimately claim to be protecting people from? It can’t be an infinite number.”

In addition, he says, “the DDE absolutely prohibits using another innocent person’s body as an instrument to save other people’s lives. This is analogous to the terror bomber who kills civilians to coerce the government.”

A third reason why we might accept the DDE “is its psychological plausibility”. This goes back to the trolley problem, and the intuition that there is a category difference between the “footbridge” and the “switch” options.

Intriguingly, however, there is evidence to suggest that younger generations are less willing to accept this distinction.

A 2017 study found the endorsement rate for the “footbridge” option (intentionally killing the man to save five people) is much higher in millennials than in older generations. The trend is linked to a decline in religiosity as churchgoers are more likely to stick to the line that every life is sacred.

The change in attitude “begins with individuals born approximately in the 1960s, and accelerates among birth cohorts after 1990″, the US and Brazilian research team found. A study in 2020 found the endorsement rate in Ireland for the footbridge option had hit 56 per cent.

This generational shift may help to explain some of the attitudinal differences between age groups on the Israel-Palestine conflict.

In a poll conducted by Harvard University political scientists two months after the October 7th attack, 60 per cent of 18-24 year olds believed the atrocity was justified by the grievance of Palestinians, whereas 9 per cent of over 65s held this view. Some 27 per cent of 18-24 year olds said the Hamas attack was not terrorism, compared with 4 per cent of over 65s. There are many possible reasons for variations in opinion across generations but shifting moral intuitions may be part of it.

If this reflects a wider change in society, there are profound implications for moral debate. Of special relevance to Ireland is how to deal with our legacy of political violence, and whether it is okay to retrospectively defend terrorism – the kind of intentional killing that the DDE expressly forbids.

The lesson for Israel is somewhat different. While it may avoid prosecution for certain war crimes on legal principle, its leaders must be aware that the more the death toll rises in Gaza the more it turns future generations against the Israeli cause.