Substantial Contributing Cause in California Workers’ Comp

In California workers’ comp, your job doesn’t have to be the only cause, or even the main cause, of a physical injury for it to be covered. The California Supreme Court has held that employment need only be a contributing cause of the injury, shown to a reasonable medical probability. Stricter percentage tests apply only in specific situations, such as psychiatric injuries, and a separate causation question decides how much permanent disability the employer pays for.

What “substantial contributing cause” means

You will often hear lawyers and adjusters talk about whether work was a “substantial contributing cause.” The phrase is a shorthand, not a single statutory test. In practice, three different causation questions come up in a California claim, and each has its own standard.

The contributing-cause test for physical injuries

Labor Code §3600 makes employers liable for injuries arising out of and in the course of employment, with or without negligence. In a 2015 decision, the California Supreme Court explained that employment need only be one of the contributing causes without which the injury would not have occurred, and that the worker must show industrial causation is reasonably probable, not certain. The court called this standard less restrictive than the one used in personal injury lawsuits. Our guide to AOE/COE explains the work-connection rule in more detail.

You still have to prove causation by a preponderance of the evidence (Labor Code §3202.5), and in most disputed cases that proof comes from a medical report.

Where causation disputes come up most

• Cumulative trauma. Repetitive lifting, typing, kneeling or vibration over months or years. Labor Code §3208.1 recognizes these as cumulative injuries. See cumulative trauma claims.
• Pre-existing conditions. If work aggravated or accelerated a condition you already had, such as arthritis or a prior back problem, the aggravation can be compensable. See pre-existing conditions and workers’ comp.
• Occupational disease. Hearing loss, lung disease and skin conditions from workplace exposures.
• Heart attacks and strokes. These can be compensable when work exertion or conditions contributed. Certain public safety workers, such as police officers and firefighters, get a statutory presumption for heart trouble (Labor Code §3212 and related sections). Most private-sector workers do not and must prove causation with medical evidence. See heart attacks and workers’ comp.

Psychiatric injuries: a higher bar

Labor Code §3208.3 sets stricter rules for psychiatric claims. Actual events of employment generally must be “predominant as to all causes combined,” which is commonly understood to mean more than half. If the injury resulted from being a victim of a violent act or direct exposure to a significant violent act, work events need only be a “substantial cause,” which the statute defines as at least 35 to 40 percent of the causation from all sources combined.

Other limits apply, including a general six-month employment requirement (with an exception for sudden and extraordinary events) and a bar on claims substantially caused by lawful, nondiscriminatory, good-faith personnel actions. Read more in our explainer on Labor Code §3208.3.

Apportionment: the second causation fight

Once your injury is accepted, causation comes up again when permanent disability is rated. Under Labor Code §4663, apportionment is based on causation, and a doctor’s report on permanent disability must estimate what percentage of the disability the work injury caused and what percentage other factors caused. If the doctor can’t do that, the report must explain why. You must disclose prior permanent disabilities or impairments if asked.

A prior permanent disability award can also be conclusively presumed to still exist under Labor Code §4664. Apportionment must rest on substantial medical evidence that explains how and why the other factors caused part of the disability. See our explainer on apportionment under §4663.

Hypothetical examples

• Warehouse back injury. Ray, 58, has lifted boxes for 15 years. An MRI shows degenerative disc disease, and the insurer says it’s just age. If a doctor explains that the repetitive lifting contributed to his back condition, the injury can still be compensable. Apportionment may later reduce his permanent disability if the doctor attributes part of it to natural degeneration.
• Data entry and carpal tunnel. Kim had mild wrist symptoms before starting a high-volume data entry job. Two years later she needs surgery. If her job substantially worsened the condition, the aggravation can be a work injury.
• Delivery driver heart attack. Sam, a private-sector delivery driver, has a heart attack while carrying heavy loads on a hot day. He gets no presumption, but if a cardiologist concludes the exertion contributed to the heart attack, it can be compensable.

Common insurer defenses and how workers respond

• “It’s degenerative” or “it’s age.” A degenerative condition doesn’t defeat a claim if work contributed to or aggravated it. The real fight is often over apportionment.
• “It’s idiopathic.” Insurers sometimes argue an injury came from a purely personal cause. The facts of what you were doing at work, and where, matter.
• “It happened off the job.” Consistent reporting dates, coworker witnesses and medical records describing the work activity help counter this.
• Weight, smoking or genetics. These may be relevant to apportionment, but a doctor must explain how they caused part of the disability. A bare conclusion isn’t enough.

Building the medical evidence

1. Give every doctor a complete job history. Describe your duties, hours, weights lifted and how long you’ve done the work.
2. Report symptoms early and consistently. Gaps and inconsistent histories give insurers room to argue another cause.
3. Disclose prior injuries honestly. Hiding them can undermine your credibility, and you may be required to disclose them anyway.
4. Make sure the report explains its reasoning. A medical opinion generally must be stated within reasonable medical probability and explain how and why the doctor reached it.
5. Use the QME process if you disagree. The DWC’s QME guide explains how to request an evaluator, and the Information and Assistance Unit offers free help.

Frequently asked questions

Does work have to be more than 50% of the cause of my injury?

Not for a physical injury. Work generally needs to be a contributing cause. The more-than-half “predominant cause” standard applies to most psychiatric injury claims.

Can I get workers’ comp for a condition I already had?

Yes, if work aggravated or accelerated it. Your permanent disability may be apportioned between the work injury and the pre-existing condition.

What is the difference between causation of injury and apportionment?

Causation of injury decides whether the claim is covered at all. Apportionment decides how much of your permanent disability the employer pays for once the injury is accepted.

Who decides causation?

A workers’ compensation judge decides, based mainly on medical reports from your treating doctor, a QME or an AME, plus your testimony and other evidence.

Talk to a California workers’ comp lawyer

Employees First Labor Law represents workers exclusively, including injured workers whose claims were denied as degenerative, age-related or not work-related. We offer free virtual consultations to review your medical reports and options. Contact us online or call (310) 853-3461.

This article is general information about California law as of 2026, not legal advice for your situation. Reading it does not create an attorney-client relationship.