Attorneys practicing tort law likely remember their first year of law school when they learned a plaintiff must establish all four elements of negligence by a preponderance of the evidence to prevail on a claim.  Those elements were, and still are: (1) duty, (2) breach, (3) causation, and (4) harm.  In determining causation, however, the standard that has to be met has not been clear in Massachusetts.  The recent Massachusetts Supreme Judicial Court decision of Doull et al. v. Foster et al., SJC-12921 (February 26, 2021), however, sets new precedent on causation.  In this landmark case, in which Morrison Mahoney lawyers Tory Weigand and Noel Dumas represented the appellees, the Court announced that when there are “multiple alleged causes of the harm” a traditional “but for” test is the appropriate causation analysis, as opposed to the alternative “substantial factor” test that is unnecessarily confusing and should no longer be used.

The “but for” test

It is a long-established Massachusetts principle of negligence law that a defendant cannot be held liable for a harm unless the defendant caused the harm.  To establish a defendant caused plaintiff’s harm or injury (also known as “causation”), plaintiff must prove the defendant’s actions were both the “cause in fact” and “proximate cause” of the plaintiff’s injury.  A “but for” test is employed to establish the “cause in fact” component of the analysis.  Simply put, if the plaintiff’s injury would not have occurred but for the defendant’s actions, then the defendant’s conduct is a “factual cause” of the injury or harm.  If the injury or harm would have occurred regardless of the defendant’s conduct, then there is no factual cause or cause in fact.

Multiple causes and the “substantial factor” test

The confusion and complexity of a causation analysis arises in situations where there are multiple causes of the injury to the plaintiff.  For example, at two camp sites 100 yards away from each other, the campers go to sleep for the night without properly extinguishing their fires.  During the night, each fire grows out of control, burning along two separate paths leading straight to a house.  Just before the house the fires merge with one another and the home is burnt to the ground.  If either fire could have independently destroyed the home neither fire could be a “but-for” cause of the harm because the home would have been destroyed by the other fire regardless. Therefore, the “but-for” standard would be ineffective as both sets of campers could be relieved of liability.  As a consequence, where there are multiple sufficient causes, Massachusetts courts have previously used an alternative test know as the “substantial factor” test to avoid such an unjust result.  Under this alternative causation standard, where several actions contributed to the plaintiff’s harm and each one alone would have been a factual cause of the plaintiff’s injury, a jury would have to make a determination of whether such was a “substantial factor” in bringing about the harm.

The Doull decision and the clarified analysis for causation

In 2015, Laura Doull died at the age of 43 from a rare condition known as chronic thromboembolic pulmonary hypertension (CTEPH), which resulted from a blood clot in her lungs.  Her medical history showed that in 2008, a nurse practitioner prescribed Doull a progesterone cream to treat perimenopause-related symptoms.  Through the spring of 2011, the nurse practitioner saw Doull on three separate occasions regarding complaints of shortness of breath.  The nurse practitioner attributed Doull’s shortness of breath to pre-existing asthma and allergies.  Despite the complaints, at no time did the supervising physician examine Doull.  In May 2011, after a seizure-like event, Doull was diagnosed with a pulmonary embolism, a condition where blood clots or other substances block portions of the pulmonary arties in the lung.  Evidence showed that a pulmonary embolism may cause shortness of breath, as well as CTEPH.  In 2011, Doull had surgery to remove the blockage in her lung, but it was unsuccessful.  She was then treated with various medications for the pulmonary hypertension.  The evidence showed she died from complications arising from CTEPH.  Prior to her death, Doull and her family members filed suit against both the nurse practitioner and the supervising physician alleging negligence, as well as other claims, with respect to her care.

At trial, the jury returned a verdict in favor of the defendants based largely on the fact that although the nurse practitioner negligently failed to diagnose Doull’s blood clots, and the supervising physician was negligent in his supervision, this negligence was not the cause of Doull’s death.  Rather, the jury accepted the testimony of defendants’ expert pulmonologist who testified that the chronic nature of Doull’s blood clots meant that her outcome would have remained the same had the nurse practitioner diagnosed her with the disease during the 3 visits in 2011.

In their appeal, plaintiff argued that the judge was required to instruct the jury on a substantial factor standard, instead of the but-for standard, because there were several possible causes of Doull’s death. Defendants disagreed, contending that the instruction given was consistent with both Massachusetts law and the approach taken by the Restatement (Third) of Torts.

In a lengthy opinion, the Massachusetts Supreme Judicial Court explained that there are two situations where the “substantial factor” test has traditionally been used.  The first, as illustrated in the above campfire example, is a situation involving multiple sufficient causes.  The second, is a situation involving toxic tort and asbestos cases.  The concern common to these two types of cases is the great difficulty, if not impossibility, of identifying “but-for” causes of the harm.  The Court, however, differentiated between these types of cases, and the Doull case which involved multiple potential causes of a harm.  In multiple potential harm cases, the Court opined the following:

There is nothing preventing a jury from assessing the evidence and determining which of the [potential] causes alleged by the plaintiff were actually necessary to bring about the harm, and which had nothing to do with the harm. . . If a substantial factor instruction is required whenever there is more than one potential cause [of harm], then the substantial factor standard could supplant the but-for standard as the primary standard for factual causation. What originated as an exception to but-for causation would swallow the rule.

The SJC further announced that even in cases involving multiple sufficient causes, the substantial factor test is no longer to be used.  Instead, a jury should be instructed when there are two or more competing causes, such as the camp fire example, each of which is sufficient without the other to cause the harm and each of which is in operation at the time the plaintiff’s harm occurs, the factual causation requirement is satisfied.  Thus, in cases where there are multiple, simultaneously operating, sufficient causes, the jury does not have to apply the “but-for test” in order to find causation.  The Court noted, however, that as the issue of causation in toxic tort and asbestos cases was not before it, the Court would not strike the use of the the substantial contributing factor instruction in those limited cases, but would be willing to consider whether to do so in the future.

The Doull decision’s impact

The Doull decision clarifies the instruction to be given in cases where there are multiple defendants, or multiple claims arising out of a tort.  In the past, plaintiffs would often argue that a “substantial factor” should be the standard, while defendants argued a “but for” standard.  The trial judge would then make a decision.  The Doull decision now provides clarification and certainty as to the correct instruction to be given.  No longer will juries have to figure out what a “substantial factor” means, but rather will simply use the straightforward but-for test. 


For over a year now, society has been dealing with the pandemic and everyone had to adjust to what is coined the “New Normal.”  Going to the grocery store or eating dinner out certainly looks different than before the pandemic.  With the decision in Doull, the SJC announced what the “New Normal” looks like when conducting a causation analysis.  So, as society adjusted their lifestyles to deal with the “New Normal,” lawyers and legal scholars alike will now have to adjust to the “New Normal” when analyzing a negligence claim.