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What is the principle of double effect?

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Double effect is an application of normative ethics used to determine the most ethical action when an act intended to help will also produce a morally harmful side effect. It is primarily used by Catholic theologians and bioethicists for medical treatments that may result in abortion or euthanasia and in discussions about just war strategies.

The principle of double effect originated in Thomas Aquinas’ Summa Theologiae. Under Question 64, Murder, Article 7, Aquinas discusses the morality of self-defense. He starts with Augustine’s conviction that killing in self-defense, though legal, is unethical. Augustine believed that it is wrong to kill merely to live without fear of unwillingly losing something of value, including one’s life; that one should not value possessions more than moral integrity. Aquinas then lists other arguments against killing in self-defense. One is that, if a person should not commit adultery in order to save his life, he certainly shouldn’t kill, since murder is worse than adultery. Another is that Romans 12:19 tells us not to seek revenge. The argument is that self-defense is worse than revenge because it acts not in response to an evil but in anticipation of it.

Aquinas then resolves that self-defense is supported by natural law, but only if conditionally appropriate violence is used. If the intended victim’s response is disproportionate to the threat, and death ensues, then the act of self-defense is immoral. It is unethical to take another’s life, but it is also unethical to so fear taking another’s life that you do not defend your own. Killing with intent is only moral when one is commissioned by an authority, as in the case of a soldier or an officer of the court.

Later ethicists adapted Aquinas’ conviction about self-preservation to cover all situations in which the act of bringing about a good may have a morally harmful side effect, particularly death. They arrived at four principles that characterize double effect:
1. The act itself must be morally good or neutral;
2. Although the agent may foresee morally grave harm, he must not desire it;
3. The good result must arise because of the action, not the negative effect;
4. The value of the good result must outweigh the harm of the negative effect.

The principles of double effect encompass the main categories of normative ethics. An act may be identified as wrong or right using deontology and Christian ethics. The desire of the agent reflects his character, which is addressed in virtue ethics. The order and nature of the good and bad effects may be related to ethical relativism. And the conviction that the good must outweigh the bad is an application of consequentialism.

Double effect can be applied to many different scenarios, but it is primarily used by medical professionals to determine if a treatment is ethical. Especially in Catholic hospitals, the two most common situations scrutinized involve abortion and euthanasia.

As applied to euthanasia, the principle of double effect weighs the relieving of pain, which is good, against a possibly shortened lifespan, which is bad. Double effect determines that medication may be given only if the primary goal is to relieve pain and not to hasten death.

The double effect framework is sometimes used to determine the extent of medical treatment. For example, if a woman has an ectopic pregnancy, it’s understood the baby will not live and the mother may not. It’s a simple procedure to remove the fetus early on, but that is technically an abortion. So a doctor may choose to remove the entire fallopian tube. The removal of the fallopian tube is usually unnecessary, but it allows the perspective that the death of the baby is not the cause of the mother’s survival but only a consequence.

Double effect is also used in discussions of matters of war. It is deemed ethical to bomb a military installation or a strategic target, such as a bridge, even if civilians are present, because the death of civilians, although foreseen, is undesired. The bombing of civilians to demoralize the government into surrender is deemed unethical, however, because the good (demoralization) is a direct consequence of the bad (civilian casualties). In cases such as these, steps are often added to mitigate the harm, like notifying the civilians of the impending bombing before destroying the infrastructure.

The trolley track switch, a traditional ethical dilemma, can also be analyzed using double effect. A trolley is barreling down a track, approaching a Y. Ahead stand five people who cannot move. Along an adjacent track stands one person. According to double effect, a bystander may throw the switch and divert the trolley so the one person dies rather than the five. The desire is to save the five people; the act is to switch the track. The death of the one person does not directly lead to the five being saved; it is a foreseen but undesired side effect, and one of proportionately less weight than the deaths of five people. The bystander may not, however, throw another person in front of the trolley to stop it. The crushed person would stop the trolley, but the saving of the five lives would be a direct result of the one person’s death and would therefore be unethical.

Double effect can be used on a number of other issues. Is it ethical to use vaccinations to prevent life-threatening disease if a handful of people will fall victim to conditions such as Guillain-Barre syndrome? Is it ethical for a soldier to dive onto a live grenade if his death will directly save those around him? Is eminent domain ethical if a new dam will control flooding and provide electricity, even if several houses are destroyed? Is it right to cut off a hand trapped by a boulder if the alternative is to die of exposure?

The principle of double effect is not an absolute tool. Acts, themselves, are not always right or wrong. And questions remain: can applying double effect contradict a living will? How much should the acting agent work to minimize the chance and extent of the harm?

Double effect is useful in that it gives agents the opportunity to slow down and think about the consequences of their actions. It also gives peace to medical professionals faced with difficult decisions that do not have an obvious biblical answer. Of course, difficult decisions should always involve prayer and submission to the leading of the Holy Spirit. God’s good is infinitely greater than man’s consideration. As James 1:5 says, “If any of you lacks wisdom, you should ask God, who gives generously to all without finding fault, and it will be given to you.”

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This page last updated: November 21, 2023