Double Effect

We may perform an act that has at least two effects, one good and one evil, if it meets four criteria:

1. The act itself, independent of its consequences, must be good or at least morally neutral.

2. The good effect must not result from the evil effect. The evil effect must be an incidental by-product of the good effect.

3. The evil effect must not be intended but only permitted.

4. There must be a proportionately grave reason to permit the evil effect.

All four criteria must be met. If even one is not met, the act is evil.

For example, we may kill in self defense. Jesus said, Jn 10:10 “I came so that they may have life, and have it abundantly.” Human life from conception to natural death is consistent with God’s will and therefore intrinsically good. Our heavenly Father commanded, Ex 20:13 “You shall not kill,” so killing is intrinsically evil. If our intention at the moment of firing a weapon at an armed robber is “This is the only way I can possibly save the lives of my wife and myself,” there is no sin. However, if our intention is, “Great, I always wanted to see a man die, and now I can do it legally,” the act is grave sin.

We would apply the criteria this way: [1] The act of firing a gun, independent of its consequences, is morally neutral. [2] The continuation of our life did not result from the intruder’s death because we would have continued living even if he had not entered our home. [3] Our intention was to protect life. [4] Saving a human life is certainly grave reason to permit the death of an attacker.

CCC 2263 “The act of self-defense can have a double effect: the preservation of one’s own life; and the killing of the aggressor.... The one is intended, the other is not.”

CCC 2264 “Someone who defends his life is not guilty of murder even if he is forced to deal his aggressor a lethal blow.”

CCC 2265 “Legitimate defense can be not only a right but a grave duty for one who is responsible for the lives of others.”

The ectopic pregnancy is another well-known example of the principle of double effect. The surgeon’s effort is to save both mother and child. If he is able to save only one of his two patients, he has done all he could to protect human life.