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The substantial factor test can help address causation when multiple acts, conditions, exposures, or events may have contributed to the same harm, but its meaning and relationship to but-for causation vary significantly by jurisdiction. This article explains how states approach substantial factor and multiple-cause analysis, including concurrent and independently sufficient causes, and how engineering, medical, biomechanical, toxicology, and exposure experts may evaluate competing causal explanations using reliable, case-specific methodologies.
Why does multiple causes complicate causation analysis?
Causation becomes more complicated when an injury, disease, accident, or other loss may have resulted from several contributing factors. A defendant may point to another event, condition, exposure, or actor as the cause, while a plaintiff may contend that several causes operated together.
The substantial factor test is one causation framework used in some jurisdictions and types of claims to evaluate whether particular conduct contributed sufficiently to the harm. It is not a single nationwide rule. States differ in how they define substantial factor causation, how it relates to traditional but-for causation, and when the test should be used.
This guide explains the substantial factor test, how different jurisdictions approach it, how multiple causes may affect the analysis, and how qualified experts may evaluate competing causal explanations.
The substantial factor test generally asks whether particular conduct, exposure, or another alleged cause played a sufficiently significant role in producing the claimed harm.
The term can be misleading when used without reference to the governing law. A factor does not necessarily satisfy legal causation simply because it contributed in some way to an event. Depending on the jurisdiction and claim, courts may apply traditional but-for causation, a substantial-factor formulation, a rule for multiple sufficient causes, proximate-cause principles, or some combination of these concepts.
For that reason, attorneys and experts should identify the controlling causation standard before applying the phrase "substantial factor" to the facts of a case.
The traditional but-for test asks whether the harm would have occurred without the disputed conduct. In many negligence cases, it remains the starting point for factual causation.
Multiple causes do not necessarily defeat but-for analysis. Several acts may each be necessary to produce the same injury, meaning that removing any one of them would have prevented the harm.
A harder problem arises when two or more forces are each independently sufficient to cause the same harm. In that situation, ordinary but-for analysis may produce an artificial result because the remaining force would still have caused the injury.
Jurisdictions differ in how they address this problem, including whether substantial-factor terminology or another multiple-sufficient-cause rule applies.
Why does multiple causes complicate causation analysis?
Causation becomes more complicated when an injury, disease, accident, or other loss may have resulted from several contributing factors. A defendant may point to another event, condition, exposure, or actor as the cause, while a plaintiff may contend that several causes operated together.
The substantial factor test is one causation framework used in some jurisdictions and types of claims to evaluate whether particular conduct contributed sufficiently to the harm. It is not a single nationwide rule. States differ in how they define substantial factor causation, how it relates to traditional but-for causation, and when the test should be used.
This guide explains the substantial factor test, how different jurisdictions approach it, how multiple causes may affect the analysis, and how qualified experts may evaluate competing causal explanations.
The substantial factor test generally asks whether particular conduct, exposure, or another alleged cause played a sufficiently significant role in producing the claimed harm.
The term can be misleading when used without reference to the governing law. A factor does not necessarily satisfy legal causation simply because it contributed in some way to an event. Depending on the jurisdiction and claim, courts may apply traditional but-for causation, a substantial-factor formulation, a rule for multiple sufficient causes, proximate-cause principles, or some combination of these concepts.
For that reason, attorneys and experts should identify the controlling causation standard before applying the phrase "substantial factor" to the facts of a case.
The traditional but-for test asks whether the harm would have occurred without the disputed conduct. In many negligence cases, it remains the starting point for factual causation.
Multiple causes do not necessarily defeat but-for analysis. Several acts may each be necessary to produce the same injury, meaning that removing any one of them would have prevented the harm.
A harder problem arises when two or more forces are each independently sufficient to cause the same harm. In that situation, ordinary but-for analysis may produce an artificial result because the remaining force would still have caused the injury.
Jurisdictions differ in how they address this problem, including whether substantial-factor terminology or another multiple-sufficient-cause rule applies.
A single injury may involve several acts, conditions, exposures, or events. Their legal significance depends on how they interacted and on the causation standard governing the claim.
Multiple-cause disputes commonly arise in several forms.
Some harms occur only because several conditions act together.
For example, a vehicle collision may involve speed, braking, visibility, roadway conditions, and driver response. An accident reconstruction specialist may examine event data recorder information, impact locations, braking distances, photographs, roadway evidence, sight distance, and vehicle damage.
The analysis may help determine how those conditions interacted and whether the proposed accident sequence is physically consistent with the evidence. Whether any factor satisfies the governing legal causation standard remains a legal and case-specific question.
A different problem arises when two or more forces are each independently sufficient to produce the same harm. In that situation, the harm would have occurred even if one of the sufficient forces had been absent.
Depending on the jurisdiction, this situation may justify a substantial factor instruction, a multiple-sufficient-cause rule, or another modification of ordinary but-for analysis.
An expert may evaluate timing, mechanical forces, disease mechanisms, exposure levels, or other relevant evidence to assess whether the evidence supports finding that a particular force, acting independently, would have been sufficient to produce the outcome. The expert's technical analysis should remain distinct from the court's determination of the governing legal test.
Exposure cases may involve multiple products, workplaces, sources, or time periods, and states apply different causation rules and evidentiary requirements.
California's Rutherford v. Owens-Illinois, Inc., 16 Cal.4th 953, 67 Cal.Rptr.2d 16, 941 P.2d 1203 (1997), developed an asbestos-specific framework requiring threshold exposure to a defendant's asbestos-containing product and expert testimony showing, in reasonable medical probability, that the exposure was a substantial factor contributing to the risk of developing cancer.
Washington and other jurisdictions use different approaches, so counsel should confirm the controlling law before applying an exposure-causation rule from another state.
There is no uniform substantial factor rule across the United States. Some jurisdictions incorporate substantial factor terminology into ordinary causation instructions, while others use it more selectively in situations such as concurrent causes, toxic exposures, or cases in which ordinary but-for analysis is inadequate. Still other states have moved away from substantial factor terminology as a general factual-causation test.
California Civil Jury Instruction CACI No. 430 defines a substantial factor as a factor that a reasonable person would consider to have contributed to the harm. The contribution must be more than remote or trivial, but it does not have to be the only cause.
California also generally treats substantial factor causation as incorporating traditional but-for causation. In Viner v. Sweet, 30 Cal.4th 1232, 135 Cal.Rptr.2d 629, 70 P.3d 1046 (2003), the Supreme Court of California explained that the substantial factor test generally subsumes the but-for test while recognizing limited circumstances involving independently sufficient causes.
California also uses specialized causation instructions for particular claims. For example, CACI No. 435 addresses causation in asbestos-related cancer cases and should not be treated as a general exposure-causation instruction.
Texas uses substantial factor terminology as part of its cause-in-fact analysis, but its relationship to but-for causation depends on the circumstances.
Texas courts ordinarily require conduct to be a substantial factor in bringing about the injury and a cause without which the harm would not have occurred. In Bustamante v. Ponte, 529 S.W.3d 447 (Tex. 2017), however, the Supreme Court of Texas rejected an improperly stringent analysis that required each provider's negligence to independently satisfy but-for causation where the evidence supported combined negligence.
The court later clarified in Pediatrics Cool Care v. Thompson, 649 S.W.3d 152 (Tex. 2022), that Bustamante did not eliminate but-for causation. When multiple providers' negligent acts are so concurrent that they cannot be examined in isolation, the analysis considers whether each was a substantial factor and whether their combined negligence was a but-for cause of the injury.
New York commonly uses substantial factor terminology when discussing proximate causation.
New York courts have instructed juries that an act or omission may be considered a cause of an injury when it was a substantial factor in bringing about the injury. The state's causation framework also recognizes that more than one person's conduct may contribute to an injury.
The appropriate instruction still depends on the claim. Medical malpractice, comparative fault, premises liability, and other causes of action may involve additional causation requirements.
Washington generally begins with traditional but-for causation but uses substantial factor analysis in more limited circumstances.
Courts have applied the substantial factor standard in certain cases where the exact cause cannot be identified, including some disputes involving multiple sources of toxic materials. Its use is not automatic merely because several possible causes exist.
New Jersey expressly uses substantial factor analysis in certain concurrent-cause cases.
New Jersey courts have applied substantial factor analysis when concurrent causes are potentially capable of producing the harm. Under this approach, the defendant's conduct must have been a substantial factor in bringing about the injury, even when other causes also contributed.
New Jersey also uses specialized causation formulations in particular contexts, including certain exposure cases. The applicable causation standard depends on the nature of the claim and the issue presented.
Maryland distinguishes between ordinary but-for situations and cases involving multiple independent negligent acts.
Maryland appellate authority explains that the but-for test ordinarily applies when one negligent act is at issue. When two or more independent negligent acts combine to produce an injury, Maryland has applied a substantial factor analysis to factual causation.
The inquiry does not end with factual cause. Maryland also separately considers whether the defendant's conduct is a legally cognizable cause of the harm.
Massachusetts takes a different approach. In Doull v. Foster, 487 Mass. 1, 163 N.E.3d 976 (2021), the Supreme Judicial Court held that traditional but-for causation is the appropriate factual-causation standard in most cases, including cases involving multiple alleged causes, and discontinued use of the substantial factor test even in multiple-sufficient-cause cases.
Current Massachusetts model civil jury instructions therefore focus on whether the injury would have occurred without the defendant's negligence while recognizing that an injury may have more than one factual cause. This illustrates why substantial factor should not be treated as a nationwide default.
Experts may work with similar technical evidence across jurisdictions even when the governing legal question differs. An engineer's speed calculations, a physician's medical-causation opinion, or an exposure scientist's dose estimate may remain the same, while the legal significance of those findings may change.
Counsel should identify the applicable causation standard early so the expert addresses the correct issue. A report framed around a broad "substantial factor" concept may not fit a jurisdiction that primarily requires but-for causation or limits substantial-factor analysis.
The legal standard should guide the questions posed to the expert without turning the expert into the decision-maker on the legal issue.
Regardless of the terminology used by the jurisdiction, a causation opinion should explain why the evidence supports the proposed causal relationship.
Calling something a "substantial factor" does not substitute for analysis. Depending on the field and the issue, useful questions may include:
Did the alleged factor actually operate in producing the harm?
Is the proposed mechanism scientifically, medically, or physically supported?
Would the harm have occurred without the disputed factor when but-for causation applies?
Are several causes operating together, or could one or more causes independently produce the outcome?
Do alternative explanations better fit the available evidence?
Are the assumptions underlying the opinion supported by the record?
Can the methodology and reasoning be explained and evaluated?
The significance of each question depends on the expert's discipline and the causation standard applicable to the claim.
Expert testimony may be important when a causation issue depends on scientific, medical, engineering, technical, or other specialized knowledge. Whether expert testimony is legally required depends on the jurisdiction, claim, and complexity of the causal issue.
The appropriate expert discipline depends on the causal question being disputed.
Engineers and accident reconstruction specialists may evaluate:
Scene measurements, photographs, and video
Vehicle or equipment damage
Event data recorder information
Mechanical or structural failures
Roadway and environmental conditions
Speed, distance, force, and timing calculations
Applicable engineering principles
Testing, modeling, or simulations when appropriate
The analysis may help determine whether competing explanations are physically plausible and how particular conditions or actions contributed to an event.
When an injury has several possible causes, physicians and biomechanical experts may address different parts of the analysis.
Relevant information may include:
Preexisting medical conditions
Prior injuries
Imaging and diagnostic studies
Incident mechanics
Timing and progression of symptoms
Treatment records
Alternative medical explanations
Relevant medical and scientific literature
A biomechanical expert may assess whether forces and motions are consistent with a proposed injury mechanism, while a physician may address diagnosis and medical causation within the physician's qualifications. The scope of each opinion depends on the expert's qualifications, methodology, governing evidence law, and the issue before the court.
Chemical and environmental cases often involve several expert disciplines because establishing exposure and determining whether it caused disease are separate questions.
Depending on the case, industrial hygienists, exposure scientists, toxicologists, epidemiologists, physicians, and other specialists may evaluate:
Substance identity and concentration
Route, frequency, and duration of exposure
Workplace or environmental measurements
Product composition
Exposure pathways
Dose estimates
Background exposures
Toxicological and epidemiological research
Plausible alternative causes
The presence of a substance does not, by itself, establish that it caused a particular medical condition. Expert analysis should connect the exposure evidence to an appropriate scientific or medical methodology and to the causal question actually presented.
When offered as expert testimony, causation opinions must satisfy the forum's applicable expert-evidence requirements.
In federal court, Federal Rule of Evidence 702 provides that a qualified expert may testify if the proponent demonstrates to the court, more likely than not, that the expert's specialized knowledge will help the trier of fact, the testimony is based on sufficient facts or data, the testimony is the product of reliable principles and methods, and the opinion reflects a reliable application of those principles and methods to the facts.
In state court, counsel should apply the forum's governing evidence rules and expert-admissibility decisions.
For an opinion involving several possible causes, relevant reliability considerations may include:
Clearly identifying the causal question
Using a methodology appropriate to the discipline
Explaining how the evidence supports each analytical step
Addressing meaningful competing explanations when the methodology calls for it
Identifying important assumptions and limitations
Keeping conclusions within what the underlying data and methodology can support
The Reference Manual on Scientific Evidence also explains that an observed association, by itself, does not establish causation.
Experts may address technical or scientific causal relationships, while the court resolves legal and preliminary admissibility issues. When factual questions are submitted for decision, the factfinder resolves them under the applicable legal framework.
The substantial factor test can be important when several causes may have contributed to the same harm, but it is not a uniform nationwide causation rule. States differ in how they define the concept, how it relates to but-for causation, and when it may be used.
For litigation involving complex medical, scientific, engineering, or exposure issues, qualified experts can help evaluate competing causal explanations and connect the evidence to a defensible methodology. Attorneys should ensure that the expert analysis is framed around the causation law that actually governs the claim.
Can more than one cause contribute legally to the same harm?
Yes. Many jurisdictions recognize that an injury can have more than one factual or proximate cause. The test used to determine the legal significance of each cause varies by jurisdiction and claim.
Does every state use the substantial factor test?
No. States differ substantially. Some use substantial factor language in ordinary or concurrent-cause cases, some reserve the concept for limited situations, and others rely primarily on but-for causation.
Does a substantial factor have to be the main cause of the harm?
Not necessarily. In jurisdictions that use substantial factor terminology, the factor generally does not have to be the sole cause. The exact threshold and wording depend on controlling state law and the type of claim.
Is substantial factor causation the same as but-for causation?
Not always. Some jurisdictions treat substantial factor analysis as incorporating traditional but-for causation. Others use substantial factor reasoning only when ordinary but-for analysis is inadequate, such as some cases involving multiple independently sufficient causes.
When is expert testimony needed in a multiple-cause dispute?
Expert testimony may be needed when the causal issue involves medical, scientific, engineering, technical, or other matters beyond ordinary knowledge. Whether expert testimony is legally required depends on the jurisdiction and the issue presented.
What should an expert do when several alternative causes are possible?
A single injury may involve several acts, conditions, exposures, or events. Their legal significance depends on how they interacted and on the causation standard governing the claim.
Multiple-cause disputes commonly arise in several forms.
Some harms occur only because several conditions act together.
For example, a vehicle collision may involve speed, braking, visibility, roadway conditions, and driver response. An accident reconstruction specialist may examine event data recorder information, impact locations, braking distances, photographs, roadway evidence, sight distance, and vehicle damage.
The analysis may help determine how those conditions interacted and whether the proposed accident sequence is physically consistent with the evidence. Whether any factor satisfies the governing legal causation standard remains a legal and case-specific question.
A different problem arises when two or more forces are each independently sufficient to produce the same harm. In that situation, the harm would have occurred even if one of the sufficient forces had been absent.
Depending on the jurisdiction, this situation may justify a substantial factor instruction, a multiple-sufficient-cause rule, or another modification of ordinary but-for analysis.
An expert may evaluate timing, mechanical forces, disease mechanisms, exposure levels, or other relevant evidence to assess whether the evidence supports finding that a particular force, acting independently, would have been sufficient to produce the outcome. The expert's technical analysis should remain distinct from the court's determination of the governing legal test.
Exposure cases may involve multiple products, workplaces, sources, or time periods, and states apply different causation rules and evidentiary requirements.
California's Rutherford v. Owens-Illinois, Inc., 16 Cal.4th 953, 67 Cal.Rptr.2d 16, 941 P.2d 1203 (1997), developed an asbestos-specific framework requiring threshold exposure to a defendant's asbestos-containing product and expert testimony showing, in reasonable medical probability, that the exposure was a substantial factor contributing to the risk of developing cancer.
Washington and other jurisdictions use different approaches, so counsel should confirm the controlling law before applying an exposure-causation rule from another state.
There is no uniform substantial factor rule across the United States. Some jurisdictions incorporate substantial factor terminology into ordinary causation instructions, while others use it more selectively in situations such as concurrent causes, toxic exposures, or cases in which ordinary but-for analysis is inadequate. Still other states have moved away from substantial factor terminology as a general factual-causation test.
California Civil Jury Instruction CACI No. 430 defines a substantial factor as a factor that a reasonable person would consider to have contributed to the harm. The contribution must be more than remote or trivial, but it does not have to be the only cause.
California also generally treats substantial factor causation as incorporating traditional but-for causation. In Viner v. Sweet, 30 Cal.4th 1232, 135 Cal.Rptr.2d 629, 70 P.3d 1046 (2003), the Supreme Court of California explained that the substantial factor test generally subsumes the but-for test while recognizing limited circumstances involving independently sufficient causes.
California also uses specialized causation instructions for particular claims. For example, CACI No. 435 addresses causation in asbestos-related cancer cases and should not be treated as a general exposure-causation instruction.
Texas uses substantial factor terminology as part of its cause-in-fact analysis, but its relationship to but-for causation depends on the circumstances.
Texas courts ordinarily require conduct to be a substantial factor in bringing about the injury and a cause without which the harm would not have occurred. In Bustamante v. Ponte, 529 S.W.3d 447 (Tex. 2017), however, the Supreme Court of Texas rejected an improperly stringent analysis that required each provider's negligence to independently satisfy but-for causation where the evidence supported combined negligence.
The court later clarified in Pediatrics Cool Care v. Thompson, 649 S.W.3d 152 (Tex. 2022), that Bustamante did not eliminate but-for causation. When multiple providers' negligent acts are so concurrent that they cannot be examined in isolation, the analysis considers whether each was a substantial factor and whether their combined negligence was a but-for cause of the injury.
New York commonly uses substantial factor terminology when discussing proximate causation.
New York courts have instructed juries that an act or omission may be considered a cause of an injury when it was a substantial factor in bringing about the injury. The state's causation framework also recognizes that more than one person's conduct may contribute to an injury.
The appropriate instruction still depends on the claim. Medical malpractice, comparative fault, premises liability, and other causes of action may involve additional causation requirements.
Washington generally begins with traditional but-for causation but uses substantial factor analysis in more limited circumstances.
Courts have applied the substantial factor standard in certain cases where the exact cause cannot be identified, including some disputes involving multiple sources of toxic materials. Its use is not automatic merely because several possible causes exist.
New Jersey expressly uses substantial factor analysis in certain concurrent-cause cases.
New Jersey courts have applied substantial factor analysis when concurrent causes are potentially capable of producing the harm. Under this approach, the defendant's conduct must have been a substantial factor in bringing about the injury, even when other causes also contributed.
New Jersey also uses specialized causation formulations in particular contexts, including certain exposure cases. The applicable causation standard depends on the nature of the claim and the issue presented.
Maryland distinguishes between ordinary but-for situations and cases involving multiple independent negligent acts.
Maryland appellate authority explains that the but-for test ordinarily applies when one negligent act is at issue. When two or more independent negligent acts combine to produce an injury, Maryland has applied a substantial factor analysis to factual causation.
The inquiry does not end with factual cause. Maryland also separately considers whether the defendant's conduct is a legally cognizable cause of the harm.
Massachusetts takes a different approach. In Doull v. Foster, 487 Mass. 1, 163 N.E.3d 976 (2021), the Supreme Judicial Court held that traditional but-for causation is the appropriate factual-causation standard in most cases, including cases involving multiple alleged causes, and discontinued use of the substantial factor test even in multiple-sufficient-cause cases.
Current Massachusetts model civil jury instructions therefore focus on whether the injury would have occurred without the defendant's negligence while recognizing that an injury may have more than one factual cause. This illustrates why substantial factor should not be treated as a nationwide default.
Experts may work with similar technical evidence across jurisdictions even when the governing legal question differs. An engineer's speed calculations, a physician's medical-causation opinion, or an exposure scientist's dose estimate may remain the same, while the legal significance of those findings may change.
Counsel should identify the applicable causation standard early so the expert addresses the correct issue. A report framed around a broad "substantial factor" concept may not fit a jurisdiction that primarily requires but-for causation or limits substantial-factor analysis.
The legal standard should guide the questions posed to the expert without turning the expert into the decision-maker on the legal issue.
Regardless of the terminology used by the jurisdiction, a causation opinion should explain why the evidence supports the proposed causal relationship.
Calling something a "substantial factor" does not substitute for analysis. Depending on the field and the issue, useful questions may include:
Did the alleged factor actually operate in producing the harm?
Is the proposed mechanism scientifically, medically, or physically supported?
Would the harm have occurred without the disputed factor when but-for causation applies?
Are several causes operating together, or could one or more causes independently produce the outcome?
Do alternative explanations better fit the available evidence?
Are the assumptions underlying the opinion supported by the record?
Can the methodology and reasoning be explained and evaluated?
The significance of each question depends on the expert's discipline and the causation standard applicable to the claim.
Expert testimony may be important when a causation issue depends on scientific, medical, engineering, technical, or other specialized knowledge. Whether expert testimony is legally required depends on the jurisdiction, claim, and complexity of the causal issue.
The appropriate expert discipline depends on the causal question being disputed.
Engineers and accident reconstruction specialists may evaluate:
Scene measurements, photographs, and video
Vehicle or equipment damage
Event data recorder information
Mechanical or structural failures
Roadway and environmental conditions
Speed, distance, force, and timing calculations
Applicable engineering principles
Testing, modeling, or simulations when appropriate
The analysis may help determine whether competing explanations are physically plausible and how particular conditions or actions contributed to an event.
When an injury has several possible causes, physicians and biomechanical experts may address different parts of the analysis.
Relevant information may include:
Preexisting medical conditions
Prior injuries
Imaging and diagnostic studies
Incident mechanics
Timing and progression of symptoms
Treatment records
Alternative medical explanations
Relevant medical and scientific literature
A biomechanical expert may assess whether forces and motions are consistent with a proposed injury mechanism, while a physician may address diagnosis and medical causation within the physician's qualifications. The scope of each opinion depends on the expert's qualifications, methodology, governing evidence law, and the issue before the court.
Chemical and environmental cases often involve several expert disciplines because establishing exposure and determining whether it caused disease are separate questions.
Depending on the case, industrial hygienists, exposure scientists, toxicologists, epidemiologists, physicians, and other specialists may evaluate:
Substance identity and concentration
Route, frequency, and duration of exposure
Workplace or environmental measurements
Product composition
Exposure pathways
Dose estimates
Background exposures
Toxicological and epidemiological research
Plausible alternative causes
The presence of a substance does not, by itself, establish that it caused a particular medical condition. Expert analysis should connect the exposure evidence to an appropriate scientific or medical methodology and to the causal question actually presented.
When offered as expert testimony, causation opinions must satisfy the forum's applicable expert-evidence requirements.
In federal court, Federal Rule of Evidence 702 provides that a qualified expert may testify if the proponent demonstrates to the court, more likely than not, that the expert's specialized knowledge will help the trier of fact, the testimony is based on sufficient facts or data, the testimony is the product of reliable principles and methods, and the opinion reflects a reliable application of those principles and methods to the facts.
In state court, counsel should apply the forum's governing evidence rules and expert-admissibility decisions.
For an opinion involving several possible causes, relevant reliability considerations may include:
Clearly identifying the causal question
Using a methodology appropriate to the discipline
Explaining how the evidence supports each analytical step
Addressing meaningful competing explanations when the methodology calls for it
Identifying important assumptions and limitations
Keeping conclusions within what the underlying data and methodology can support
The Reference Manual on Scientific Evidence also explains that an observed association, by itself, does not establish causation.
Experts may address technical or scientific causal relationships, while the court resolves legal and preliminary admissibility issues. When factual questions are submitted for decision, the factfinder resolves them under the applicable legal framework.
The substantial factor test can be important when several causes may have contributed to the same harm, but it is not a uniform nationwide causation rule. States differ in how they define the concept, how it relates to but-for causation, and when it may be used.
For litigation involving complex medical, scientific, engineering, or exposure issues, qualified experts can help evaluate competing causal explanations and connect the evidence to a defensible methodology. Attorneys should ensure that the expert analysis is framed around the causation law that actually governs the claim.
Can more than one cause contribute legally to the same harm?
Yes. Many jurisdictions recognize that an injury can have more than one factual or proximate cause. The test used to determine the legal significance of each cause varies by jurisdiction and claim.
Does every state use the substantial factor test?
No. States differ substantially. Some use substantial factor language in ordinary or concurrent-cause cases, some reserve the concept for limited situations, and others rely primarily on but-for causation.
Does a substantial factor have to be the main cause of the harm?
Not necessarily. In jurisdictions that use substantial factor terminology, the factor generally does not have to be the sole cause. The exact threshold and wording depend on controlling state law and the type of claim.
Is substantial factor causation the same as but-for causation?
Not always. Some jurisdictions treat substantial factor analysis as incorporating traditional but-for causation. Others use substantial factor reasoning only when ordinary but-for analysis is inadequate, such as some cases involving multiple independently sufficient causes.
When is expert testimony needed in a multiple-cause dispute?
Expert testimony may be needed when the causal issue involves medical, scientific, engineering, technical, or other matters beyond ordinary knowledge. Whether expert testimony is legally required depends on the jurisdiction and the issue presented.
What should an expert do when several alternative causes are possible?
A qualified expert should use the methodology appropriate to the field, evaluate meaningful alternatives when relevant, and explain how the evidence supports or undermines the proposed causal explanation. Identifying a possible contributing factor does not automatically satisfy the governing legal causation standard.
Can an expert decide whether something was a substantial factor under the law?
An expert may address the scientific, medical, engineering, or technical relationship between a factor and the claimed harm. The governing law determines what legal causation standard applies, and the court or factfinder applies that standard to the evidence.
Legal Standards
Legal Standards
A qualified expert should use the methodology appropriate to the field, evaluate meaningful alternatives when relevant, and explain how the evidence supports or undermines the proposed causal explanation. Identifying a possible contributing factor does not automatically satisfy the governing legal causation standard.
Can an expert decide whether something was a substantial factor under the law?
An expert may address the scientific, medical, engineering, or technical relationship between a factor and the claimed harm. The governing law determines what legal causation standard applies, and the court or factfinder applies that standard to the evidence.