Introduction

But-For Causation

Introduction

How does the law decide that a defendant caused a death? The Model Penal Code answers in § 2.03, its section on the causal relationship between conduct and result.¹ Subsection (1) says that conduct is the cause of a result when two conditions are met, and the first of them, § 2.03(1)(a), requires that "it is an antecedent but for which the result in question would not have occurred."² An antecedent is simply something that happens earlier. The words "but for" tell you to remove that earlier event from the story, leave everything else as it was, and ask whether the result still happens. If it does, the conduct was not a but-for cause. The phrase "result in question" ties the inquiry to the particular harm charged, such as this death occurring in this way. The second condition, § 2.03(1)(b), adds whatever further causal requirements the Code or the definition of the offense imposes, so clause (a) is the starting condition and other requirements may be layered on top of it.³

Courts ordinarily link a defendant to a forbidden result in two steps. The first is actual cause, also called cause in fact or factual cause, which usually means the but-for relationship just described.⁴ The second is legal cause, often called proximate cause, which asks whether the result that occurred resembles the result the defendant intended or risked closely enough, and came about in a similar enough way, that holding the defendant responsible is fair.⁵ This reading teaches only the first step. Legal cause is a separate inquiry with its own tests, and it is taken up elsewhere. The first step comes first for a practical reason: if the defendant's conduct made no difference to the result, there is nothing left to say about foreseeability or fairness. The prosecution must prove causation beyond a reasonable doubt whenever a result is an element of the crime.⁶ In most cases clearing the but-for hurdle is easy, because the prosecutor need only show that without the defendant's act the result would not have occurred when and as it did.⁷ The word "usually" in the definition of cause in fact matters, however, because a minority of cases strain the formula and receive attention later in this chapter.⁸

The causation question arises only when the crime is defined to require a specified result in addition to conduct.⁹ Where an offense is complete once the prohibited act is done, there is no harm for the defendant's conduct to have produced, and the but-for test never comes into play. This chapter therefore begins by separating result crimes from conduct crimes, then states the but-for test and works through its mechanics, and then shows why satisfying the test is only a threshold and where the formula misfires when more than one force could have produced the same harm. The first task is to learn when the test applies at all.