Introduction
Causation is one of the basic
conceptual tools of legal analysis. And for most purposes, we can get
along with a notion of causation that is both vague and ambiguous.
In the world of medium sized physical objects (automobiles,
pedestrians, etc.), our judgments about causation rarely depend on
conceptual niceties. The driver’s negligence caused the death of the
pedestrian but did not
cause Barak Obama to win the Iowa caucuses in 2008. In these cases,
various notions of causality converge. The person on the street, the
scientist, and lawyer can all agree in such cases that for all
practical purposes X caused Y but not Z. But sometimes the various
notions of cause come apart exposing ambiguities and vagueness in both
ordinary and legal talk about causes and effects. This post
provides a very basic introduction to causation for law students
(especially first-year law students) with an interest in legal theory.
Cause-in-Fact & Legal Cause
Let’s put the most
important distinction on the table right away. Contemporary legal
theory and judicial practice assume that there is a distinction between
legal cause on the one hand and cause-in-fact on the other. What does that mean? That’s a huge question, of course, but we can state one conclusion straight away: that X is a cause-in-fact of Y does not
entail that X is a legal cause of Y. Less obviously, that X is a legal
cause of Y does not entail that X is a cause-in-fact of Y. The various
ways that cause-in-fact and legal cause can come apart leads many to
the conclusion that legal cause simply has nothing to do with
causation, but this turns out to be an exaggeration. I know this all
sounds very airy. So let’s get down to brass tacks!
Cause-in-Fact
What do we mean when we say that X is a cause-in-fact of Y? Many law students learn that the answer to this question is but-for causation. If it is the case that but for X, Y would not have occurred,
then X is a but-for cause of Y and hence X is a cause-in-fact of Y.
This simple story works most of the time, and as a rough and ready rule
of thumb, it isn’t half bad. But it turns out that if you try to use but-for
causation as a hard and fast rule for determining whether X is the
cause of Y, you will run into trouble, sooner or later. In torts and
criminal law, but-for causation runs into trouble somewhere in the
midst of the first-year course. In a sense, the point of this Lexicon post is to provide a set of tools that for understanding the troubles that overreliance on but-for causation can cause.
Necessary and Sufficient Causes
The first item in the causation toolkit is the distinction between necessary and sufficient cause. The basic ideas are simple and familiar. X is a necessary
cause of Y, if Y would not have occurred without X. Ben’s running the
red light is a necessary cause of the damage to Alice’s car, just in
case the damage would not have occurred without Ben’s having run the
light. The idea of "necessary cause" is the same idea expressed by the
phrase "but-for cause."
X is a sufficient
cause of Y, if Y would have occurred so long as X occurred. Alice’s
shooting Ben through the heart is a sufficient cause of Ben’s death,
just in case the shot thru the head by itself would have caused Ben’s
death. This is true, even though Ben would have died anyway, because
Cynthia shot him through the head at the same time Alice shot him
through the heart.
This rough ready distinction between necessary and sufficient causes
really won't do all the necessary work. For example, Alice's shooting
Ben through the heart is not truly sufficient, by itself, no matter
what other conditions obtain, to cause Ben's death. If Ben had been on
an operating table awaiting a heart transplant, then he might have
lived despite the slot. One suggestion for dealing with cases like
this is to identify "sufficient causes" as a "necessary element of a
sufficient set." Things get even more complex from for the purposes of
this introduction the point is simply to see the indadequacy of
"necessary and sufficient cause" as a tool for dealing with complex
cases in a precise way.
The Role of Counterfactuals
The notions of necessary and sufficient
causation are familiar to almost everyone. We use these ideas all the
time in everyday life. But the very familiarity of these concepts
creates a temptation to take them for granted. There is an important
feature of these ideas that our day-to-day use of them does not make
explicit. Both necessary and sufficient causation are counterfactual concepts. What does that mean?
“Counterfactual” is simply the fancy name for “what if” thinking. What
if Ben had stopped at the red light? Would the damage to Alice’s car
still have occurred? What if the Ben had gotten immediate medical
attention? Would the shot through the head still have killed him? Every
statement regarding a necessary or sufficient cause can be interpreted
as making a counterfactual (“what if”) claim.
What-if reasoning is itself familiar and ordinary. When we say,
Ben’s running the red light was a necessary cause of the damage to
Alice’s car, we are claiming that if the world had been different and
Ben had not run the red light, then Alice’s car would not have been
damaged. We imagine what the world would have been like if Ben had
stopped at the red light, and Alice had proceeded through the
intersection without being struck by Ben’s car. Counterfactual
reasoning can get more complicated that this, but for our purposes we
can use everyday what-if reasoning as our model of role of
counterfactuals in necessary and sufficient causation.
Overdetermination
Once we’ve gotten the notions of necessary and sufficient causes, we can move on to the idea of overdetermination.
An effect is overdetermined if it has more than one sufficient cause.
Take the case of Alice shooting Ben through the heart. We have
postulated that the bullet passing through the heart was a sufficient
cause of Ben’s death, but it may not have been a necessary cause.
Suppose that Alice was a member of a firing squad, and that at the
exact same moment that Alice’s bullet passed through Ben’s heart,
another Bullet, fired by Cynthia, passed through Ben’s cerebral cortex
and that this would have resulted in Ben’s death, even if Alice’s had
not fired or her bullet had missed. Ben’s death now results from two
sufficient causes, but neither Alice’s shot nor Cynthia’s shot was
necessary. If Alice had not fired, Cynthia’s shot would have killed
Ben. If Cynthia had not fired, Alice’s shot would have killed Ben.
Overdetermination is important, because it undermines the
idea that but-for causation tells us everything we need to know about
cause-in-fact. We might say that both Alice and Cynthia’s shooting
caused Ben’s death or we might say they were both partial causes of Ben’s death, but we would not be likely to say that neither Alice nor Cynthia’s shot was the cause.
The firing squad example was described as a case of simultaneous
overdetermination—both sufficient causes occurred at the same time.
What if Cynthia shot a few seconds before Alice and Ben died before
Alice’s shot pierced his heart? In that case, Cynthia’s shot would have
preempted the causal role of Alice’s shot. If Cynthia had missed, then
Alice’s shot would have killed Ben. This kind of case is sometimes
called preemptive causation.
Coincidence
Overdetermination poses one kind of problem for but-for causation, coincidence
poses another a different sort of difficulty. Suppose the driver of a
trolley is speeding. As a result the trolley is in just wrong place and
time and a tree falls, injuring a passenger. If trolley had gone just a
little faster or just a little slower, the tree would have missed the
trolley and the injury would not have occurred. Given these
circumstances, speeding was a but-for cause (a necessary cause) of the
tree injuring the passenger. So what? Coincidence is no problem
for cause-in-fact, but it does pose a problem for the legal system.
Intuitions vary, but lots of folks are inclined to believe that one
should not be legally responsible for harms that one causes as a result
of coincidences.
Coincidence is related to a variety of other problems with but-for
causation. Take our example of Ben running the stoplight and hitting
Alice’s car. Running the stoplight was one but-for cause of this
accident, but there are many others. For example, Alice’s being in the
intersection was also a but-for cause. And how did Alice come to be in
the intersection at just the time when Ben was running the red light?
If her alarm clock hadn’t gone off, she would have slept in and arrived
in the intersection long after Ben, so her alarm clock’s ringing was
another but-for cause. And you know how the story goes from here.
As we trace the chain of but-for causes back and out, we discover that
thousands and millions and billions of actions and events are but-for
causes of the accident.
Alternatives to But-For Cause-in-Fact
Although "but-for causation" is frequently presented as the only
account of cause in law school classes, it is far from the only view.
The alternatives to but-for causation frequently view causal
relationships as involving more than but-for cause. The best way to
learn about these alternatives to read the references cited below: the
Stanford Encyclopedia of Philosophy articles provide a good starting
point.
Legal Cause
What do we about the problems with problems created by but-for cause? One way that the law responds is with the idea of legal cause or proximate cause.
In this post, we cannot hope to achieve a deep understanding of legal
cause, but we can get a start. Here are some of the ideas that help me
to understand legal cause.
First, there is a terminological issue: causation may be confused
with responsibility. “Legal cause” is only partially about cause. We
start with the idea of cause-in-fact (understood in light of the
distinction between necessary sufficient cause). This idea of cause
seems, on the surface, to fit into the structure of various legal
doctrines. So we imagine that if a defendant breaches a duty of care
and causes a harm, then defendant is legally responsible for the harm.
This works for lots of cases, but then we start thinking about other
cases like overdetermination and coincidence. “Legal cause” is the way that we adjust our ideas about legal responsibility
to overcome the counterintuitive results that would follow from a
simple reliance on but-for causation. In other words, “legal cause” may
be a misnomer. It might be clearer if we used the phrase “legal
responsibility” (or some other phrase) to describe the ways in which we
adjust the law.
Second, legal cause is frequently associated with
the idea of foreseeability. For example, in coincidence cases, the harm
(the tree injuring the passenger) is not a foreseeable consequence of
the wrongful act (driving the trolley at an excessive speed). If the
purpose of the law is deterrence, then no good purpose may be served by
assigning legal responsibility in cases where the effect is
unforeseeable.
Third, legal cause is sometimes associated with
the idea of proximity in time and space. Of course, the phrase
“proximate cause” emphasizes this connection. We usually don’t want to
hold defendants responsible for the remote and attenuated effects of
their actions. We frequently do want to hold defendants responsible for
the immediate and direct effects of their actions. “Proximity” seems to
capture this point, but an overemphasis on proximity in time and space
leads to other problems. Some immediate consequences do not give rise
to legal responsibility: the trolley driver may have started speeding
just seconds before the tree fell. Some causal chains that extend for
long distances over great durations do
give rise to legal responsibility: Osama bin Laden’s responsibility for
9/11 would not be vitiated by the fact that he set events in motions
years in advance and thousands of miles away.
Probability
Our
investigation of causality so far has elided an important set of
issues—the connections between causation and probability. These
connections are far too large a topic for this post, but even a
superficial analysis requires that we consider two perspectives–ex ante and ex post.
Ex post
questions about causation arise in a variety of contexts, but for the
legal system, a crucial context is provided by litigation and
especially trial. In many cases, there is no doubt about causation.
When Ben’s car speeds through the red light and hits Alice’s car, we
don’t have much doubt about what caused the damage. But in many types
of cases, causation will be in doubt. Did the chemical cause cancer?
Was the desk job the cause of the back injury? Sometimes the evidence
will answer these questions with certainty (or perhaps, with something
that is so close to certainty that we treat it as certainty for legal
and practical purposes). But in other cases, the evidence will leave us
with a sense that that the defendant’s action is more or less likely to
have caused the harm to the defendant. Such probabilities may be
expressed either qualitatively or quantitatively. That is, we might say
that it is “highly likely” that X caused Y or we might say that there is a 50% chance (p = .5) that X caused Y.
Ex ante
issues of causation also arise for the law. For example, the legal
system may be required to assign a value to a risk of harm that has not
yet been realized. David has been exposed to asbestos, but may or may
not develop cancer. In this case, probabilities refer to the likelihood
of future events.
Decision theory and mathematics have elaborate
formal machinery for representing and calculating probabilities. In
this short post, we cannot even scratch this surface, but there are two
or three bits of notation that every legal theorist should know:
–The
letter “p” is frequently used to represent probability. Most law
students encounter this notation in Justice Hand’s famous opinion in
the Carroll Towing case (B < PL or “burden less than loss
discounted by probability). The notation p(x) = 0.1 can be read “the
probability of x equals 1/10.” And the notation, p=0.5 can be read
“probability equals one in two.”
–The
symbol “|” is frequently used to represent conditional probabilities.
Suppose we want to represent the probability that X will occur given
that Y has occurred, we can use this notation: p(X|Y). So we could
represent the sentence, “The probability of Cancer given Exposure to
Asbestos is ten percent,” as p(C|EA)=0.1.
Types and Tokens
So
far, we have been focusing mostly on cases where an individual instance
of harm is caused by some particular wrongful action. But of course, we
frequently think about causation as a more general relationship. For
example, in science we might speak of “causal laws.” There is no
standard terminology for this distinction: we might use the phrase
“individual causation” and “systematic causation.” One helpful bit of
terminology for getting at this idea is to differentiate “types” and
“tokens.” Ben’s running the rend light at a particular time and
location is an event token and it is a token of a type of events, i.e. the type “running a red light.”
Once
we have the distinction between types and tokens in place, we can
define individual causation as a causal relationship between a token
(e.g. a token event) and another token (e.g. a token action). And we
can define systematic causation as a causal relationship between a type
(e.g. a type of event) and another type (e.g. a type of action).
Science studies causal relationships between types; trials frequently
involve questions about the causation of one token by another. This
leads to another important point: the question whether an individual
harm was caused by an individual action will sometimes depend on the
question whether a systematic causal relationship exists; for example,
the question whether this factory’s release of a chemical caused an
individual case of cancer may require a jury to resolve a “scientific”
question about systematic causation.
Conclusion
Even though this is a long entry by the standards of the Legal Theory Lexicon
it is a very compressed and incomplete treatment of the concept of
causation. Given the way legal education is organized (around doctrinal
fields like torts, criminal law, and evidence), most law students never
get a truly comprehensive introduction to causation. Torts may
introduce the distinction between cause-in-fact and legal cause;
criminal law, problems of overdetermination; and evidence, the
relationship between probability and causation. If this post
accomplishes anything of value, I hope that it serves as
warning—causation is a deep and broad topic about which there is much
to learn.
Bibliography
H.L.A. Hart & Tony Honore, Causation in the Law (2d ed. 1985). For many years, this was the book on causation and the law. Currently out of print, but used copies are available on Amazon.com.
Michael Moore, Causation and Responsibility (2009). This is the best contemporary treatment, and a must read for legal theorists who want to know about causation.
Causation (Oxford Readings in Philosophy) (Ernest Sosa & Michael Tooley eds. 1993). A
fine collection of essays, with contributions by J.L Mackie, Michael
Scriven, Jaegwon Kim, G.E.M. Anscombe, G.H. von Wright, C.J. Ducasse,
Wesley C. Salmon, David Lewis, Paul Horwich, Jonathan Bennett, Ernest
Sosa, and Michael Tooley.
Links
Antony Honore, Causation in the Law, Stanford Encyclopedia of Philosophy
Peter Menzies, Counterfactual Theories of Causation, Stanford Encyclopedia of Philosophy
Jonathan Schaffer, The Metaphyiscs of Causation, Stanford Encyclopedia of Philosophy
(This entry was last modified on July 26, 2009.)