Causation is a term used to refer to the relationship between a person’s actions and the result of those actions. In a legal sense, causation is used to connect the dots between a person’s actions, such as driving under the influence, and the result, such as an accident causing serious injuries. Establishing causation is not, in itself, enough to determine legal liability, however. To explore this concept, consider the following causation definition.

Definition of Causation


  1. The relation of cause to effect
  2. The act of causing or producing something


1640-1650       Medieval Latin    causātiōn

What is Causation in the Law

The purpose of the legal system is to ensure fairness and justice in both civil disputes and criminal acts. While in a criminal matter, proving an accused person actually committed the crime for which he is charged is sometimes sufficient in itself, this is not the case in civil lawsuits. The point here is to hold the individual who committed a wrongful act responsible, forcing him to pay for the damages or harm his actions caused. By the same token, simply proving that the accuser, or “plaintiff,” was harmed is insufficient, as he must show the court that a wrongful act committed by the defendant actually caused that harm.

A plaintiff must prove that he suffered some type of harm or damages, that the defendant committed a wrongful act, and then make the connection between the defendant’s act and the plaintiff’s damages. This is causation, a necessary element in both criminal and civil matters. Once all of the evidence, including causation, have been shown at trial, the judge or jury must then make a determination about guilt (criminal), or liability (civil). In doing so, three questions must be answered:

1. Did the accused have a legal duty to act in a particular way?

This first element deals with whether the accused person was required to act in a particular manner. For example, Layla is disabled, and barely made it outside when her home caught fire. Layla’s neighbor Nate called the fire department, then stood with her outside until they arrived. Layla’s beloved cat did not make it out of the home, and she is heartbroken. Layla files a civil lawsuit against her neighbor for not running into her home to save her cat. Layla’s lawsuit fails at this first test, as Nate had no legal responsibility, or “duty to act,” in going into a burning home to save anyone, let alone a cat.

2.  Did the harm or damages result from the acts of the accused?

This element deals with whether the specific damages claimed by the plaintiff were caused by the defendant’s action. For example, Mary fails to look behind her, and backs into the rear bumper of Ronald’s truck in a parking lot. There is no question Mary should have been more careful, and that she caused the accident, but she couldn’t see any real damage to the bumper when they exchanged information. Ronald, however, is claiming she damaged the passenger side door. In this example of causation, the question for a judge to answer is whether Mary’s act caused the damages to Ronald’s door.

3. Were the acts of the accused unlawful, unreasonable, or otherwise against public policy?

Accidents and unexpected things happen in life. This element deals with whether the accused party actually did something wrong, or wrong enough to be held liable for some type of damages. For example, Ariel and her friends are at the public pool for the afternoon. When Mel cannonballs into the water near Ariel, drenching her and her phone, which was sitting on the lounge chair next to her, she becomes angry, and demands that Mel pay for a replacement phone.

In this example of causation, Mel’s act did result in the water damage to Ariel’s phone. However, was there anything wrong with Mel’s actions? Certainly he might have been more considerate. But, they were all at the public pool, where there is water, splashing, and other activities that could reasonably be expected. By this measurement, Mel did nothing wrong, and it is Ariel who should have been more careful.

Steps to Establishing Causation

While the question as to whether a defendant, either in a criminal case, or in a civil lawsuit, had a duty to act is often pretty straight-forward, proving factual and legal causation often takes a bit more effort.

Factual Causation

Factual causation is the second element of causation discussed above. It has to do with whether the defendant’s actions were the cause of the plaintiff’s injuries or damages. The courts use a “but-for” test to determine the answer to this question. Often, there are many factors at work in any given situation, any one of which may have been the cause of the damages, so the court must ask, “But for the defendant’s actions, would the harm have occurred?”

An example of factual causation occurs when Betty decides she has had enough of her husband’s abuse, and she plans to poison him by putting a poisonous substance in his dessert. Betty’s husband, Oscar, eats the poison-containing dessert, then begins another screaming argument with her. During the fight, Betty is surprised when Oscar suddenly falls to the ground, and he is pronounced dead when the Paramedics arrive. Investigators find traces of the poison, both in the dessert and in her husband’s body, so they arrest Betty and charge her with first degree murder.

During the autopsy, however, the medical examiner determines that Oscar died from a heart attack that stemmed from long-term heart disease, not from poisoning. In order to successfully prosecute Betty for killing her husband, the prosecution must answer the question, “But for Betty’s actions, would Nate have died?” In this he is considering whether Betty’s act was necessary for the harm to have occurred. In this case, the answer is “No.”

Although Betty has committed a crime in attempting to kill her husband, she did not actually cause his death. Oscar died when he himself became angry and had a heart attack. In this example of causation, the prosecutor would not be able to prove factual causation between the poison and the heart attack. They could, however, charge Betty with attempted murder, or some other crime.

Legal Causation

Even when people do things that might cause harm to someone, there has to be a limit as to how far that goes, or how long it remains a factor. In this, the law intervenes where the effects of a defendant’s action come to rest in a safe position, or in such a manner that it appears there is no longer a danger to others. Should some harm occur later from that action, the defendant may not be held liable for damages. This is known as “proximate cause.”

For example, 14-year old twins, Tom and Hank get their Frisbee stuck in a tree in the front yard one day. They try throwing a variety of objects at the Frisbee in an attempt to dislodge it. Finally, they throw a baseball bat at it, which nudges the Frisbee out of the tree. The bat, however, falls down one branch and gets stuck itself. Having had enough, the pair leave it in the tree. A few days later, the bat falls out of the tree in a gust of wind. Unfortunately, someone was walking down the sidewalk, under the tree, when it happened. The bat landed on the woman’s head, knocking her unconscious and giving her a concussion.

The woman files a lawsuit against the boys’ parents, claiming that, had they not given the bat to the boys to begin with, she would not have been injured. This is the problem with proximate cause, as it can be taken too far. In every situation, every injury, accident, or other cause of damage, there is a “cause,” but not all of them mean someone is liable for the damages.

For instance, Nora sails through a stop sign, and is broadsided by a car coming the other way. She learns that the other driver, Lisa, has no valid driver’s license, and so she shouldn’t have been driving at all. Nora attempts to sue Lisa for damages to her car, arguing that, but for Lisa’s driving illegally, there would have been no accident, and so no damage to her car.

Causation Example in Toxicity Lawsuit

In 2006, tenants of an apartment building in New York filed a lawsuit against the building’s owner, claiming they had suffered illnesses caused by toxic mold in the building. The causation requirement of the personal injury lawsuit made it necessary for the Plaintiffs to prove, not only that there was a certain level of toxic mold detected in the building, but that there was a definite link between that level of mold and the illnesses experienced by the tenants.

The Plaintiffs lined up an expert witness to offer an opinion as to the specific symptoms and illnesses caused by toxic mold. The court refused to allow that witness to testify, stating that, at the time, there was no comprehensive medical literature or scientific data firmly linking any specific level of mold to adverse health effects. The court acknowledged that it is common for courts to admit expert opinion that has its basis in well-established principle. However, that facts from which the expert makes his deduction or opinion must have been sufficiently established in the field to have become generally accepted as fact. This was not the case with the testimony and evidence presented in this case, and so the Plaintiffs were unable to show causation between the mold and the plaintiffs’ illnesses.

Related Legal Terms and Issues

  • Civil Lawsuit – A lawsuit brought about in court when one person claims to have suffered a loss due to the actions of another person.
  • Defendant – A party against whom a lawsuit has been filed in civil court, or who has been accused of, or charged with, a crime or offense.
  • Civil Liability – Responsibility for payment of damages, or for other court-imposed penalties in a civil lawsuit.
  • Plaintiff – A person who brings a legal action against another person or entity, such as in a civil lawsuit, or criminal proceedings.