The Expanding Child

The scenario

Imagine a growing child has become lodged inside you. It is not there by any wrongdoing of its own. But it is expanding, and unless you act, it will kill you. The only way to save yourself is to kill the child.

Most people say you are permitted to defend yourself. The child is a threat, even if an innocent one. Your life is yours to defend.

What it reveals about self-defense

Standard just war and self-defense doctrine holds that killing in self-defense is justified only against an unjust aggressor, someone who is culpably threatening you. The expanding child complicates this. The child has done nothing wrong. It poses no threat through any choice or fault of its own.

If we say you may still kill it, we are conceding that innocence alone does not make a threat immune from lethal defense. The right to self-preservation can, in some circumstances, override the innocent third party's right to life. Thomson's point: once you accept this, the moral logic of self-defense changes shape.

The abortion connection

Thomson used the expanding child alongside her famous violinist scenario to argue that even if a fetus has a right to life, that right may not automatically override the pregnant person's right to their own body. The analogy is deliberate: in both cases, the threat is innocent, the harm is severe, and the person threatened did not necessarily choose the situation.

The argument does not settle every question about abortion. But it cuts directly against the claim that fetal innocence is by itself sufficient to forbid termination. Rights, Thomson argues, can conflict, and even innocent parties can lose when another person's survival is at stake.