The principle of double effect—also known as the rule of double effect; the doctrine of double effect, often abbreviated as DDE or PDE, double-effect reasoning; or simply double effect—is a set of ethical criteria which Christian philosophers, and some others, have advocated for evaluating the permissibility of acting when one's otherwise legitimate act (for example, relieving a terminally ill patient's pain) may also cause an effect one would otherwise be obliged to avoid (sedation and a slightly shortened life). The first known example of double-effect reasoning is Thomas Aquinas' treatment of homicidal self-defense, in his work Summa Theologica.[1]

This set of criteria states that an action having foreseen harmful effects practically inseparable from the good effect is justifiable if the following are true:

Intentional harm versus side effects

The principle of double effect is based on the idea that there is a morally relevant difference between an "intended" consequence of an act and one that is foreseen by the actor but not calculated to achieve his motive. So, for example, the principle is invoked to consider the terror bombing of non-combatants having as its goal victory in a legitimate war morally out of bounds, while holding as ethically in bounds an act of strategic bombing that similarly harms non-combatants with foresight as a side effect of destroying a legitimate military target. Because advocates of double effect propose that consequentially similar acts can be morally different, double effect is most often criticized by consequentialists who consider the consequences of actions entirely determinative of the action's morality.

In their use of the distinction between intent and foresight without intent, advocates of double effect make three arguments. First, that intent differs from foresight, even in cases in which one foresees an effect as inevitable. Second, that one can apply the distinction to specific sets of cases found in military ethics (terror bombing/strategic bombing), medical ethics (craniotomy/hysterectomy), and social ethics (euthanasia). Third, that the distinction has moral relevance, importance, or significance.

The doctrine consists of four conditions that must be satisfied before an act is morally permissible:

  1. The nature-of-the-act condition. The action must be either morally good or indifferent.
  2. The means-end condition. The bad effect must not be the means by which one achieves the good effect.
  3. The right-intention condition. The intention must be the achieving of only the good effect, with the bad effect being only an unintended side effect.
  4. The proportionality condition. The bad effect must not be disproportionate to the good effect.

The second of these four conditions is an application of the more general principle that good ends do not justify evil means.[3]



The principle of double effect is frequently cited in cases of pregnancy and abortion. A doctor who believes abortion is always morally wrong may still remove the uterus or fallopian tubes of a pregnant woman, knowing the procedure will cause the death of the embryo or fetus, in cases in which the woman is certain to die without the procedure (examples cited include aggressive uterine cancer and ectopic pregnancy). In these cases, the intended effect is to save the woman's life, not to terminate the pregnancy, and the effect of not performing the procedure would result in the greater evil of the death of both the mother and the foetus.[4][5][6]

In cases of terminally ill patients who would hasten their deaths because of unbearable pain, or whose caregivers would do so for them (euthanasia, medical aid in dying, etc.), a principle of "double effect death" could be applied to justify the deliberate administration of a pain-killer in potentially unsafe doses—not in an attempt to end life but the pain suffered is considered harmful to the patient. The U.S. Supreme Court has voiced support for this principle in its deliberations over the constitutionality of medical aid in dying.[7]


Consequentialists, in particular, reject the notion that two acts can differ in their moral permissibility if both have exactly the same consequences, or expected consequences. John Stuart Mill, a nineteenth-century advocate of the utilitarian version of consequentialism, argues that it is a mistake to confuse the standards for right action with a consideration of our motives to perform a right action: "He who saves a fellow creature from drowning does what is morally right, whether his motive be duty, or the hope of being paid for his trouble; he who betrays the friend that trusts him, is guilty of a crime, even if his object be to serve another friend to whom he is under greater obligations." According to Mill, scrutiny of motives will show that almost all good behavior proceeds from questionable intentions. Therefore, Mill argues, our moral analysis should ignore matters of motivation, and so we should reject DDE, which appeals to a distinction between intended and unintended consequences.[8] Mill further claims that scrutiny of motives will reveal a man's character, but utilitarianism does not judge character, only the rightness or wrongness of actions.

See also


  1. ^ Summa Theologiae, IIa-IIae Q. 64, art. 7
  2. ^ T. A. Cavanaugh, Double-Effect Reasoning: Doing Good and Avoiding Evil, p.36, Oxford: Clarendon Press
  3. ^ Mark Timmons, Moral Theory: An Introduction (Rowman & Littlefield 2003 ISBN 978-0-8476-9768-7), p. 80
  4. ^ McIntyre, Alison. "Doctrine of Double Effect". In Edward N. Zalta (ed.). Stanford Encyclopedia of Philosophy (Summer 2006 ed.). Retrieved 2007-08-18.
  5. ^ "Principle of Double Effect". Catholics United for the Faith. 2003. Archived from the original on 2007-09-27. Retrieved 2007-08-18. ((cite web)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  6. ^ Delgado, George. "Pro-Life Open Forum, Apr 10, 2013 (49min40s)". Catholic answers. Archived from the original on 2 April 2015. Retrieved 2 September 2014. ((cite web)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  7. ^ Vacco v. Quill and Washington v. Glucksberg, both in 1997. See: Tucker, Kathryn E., “Legal Advocacy to Improved Care and Expand Options at the End of Life,” in Physician-Assisted Dying: The Case for Palliative Care & Patient Choice, edited by T.E. Quill and M.P. Battin (Johns Hopkins University Press, 2004). The Court made it clear that a “medical death” hastened by palliative measures was permissible.
  8. ^ John Stuart Mill, Utilitarianism (London: Parker, Son and Bourn, 1863), page 26.