Haas v. Estate of Mark Steven Carter, 525 P.3d 451

In 2014 Mark Carter struck a stopped car occupied by Roberta and Kevin Haas; both plaintiffs later underwent spine surgeries and had significant preexisting spinal conditions.
At trial plaintiffs sought two causation instructions: a but‑for instruction (given) and a substantial‑factor instruction (refused). The court also instructed on damages for a previous infirm condition.
Plaintiffs argued the jury faced multiple possible causes (preexisting degeneration, age, sneezing, etc.) and therefore needed a substantial‑factor instruction; defendants argued but‑for was adequate and preexisting conditions are non‑tortious susceptibilities or aggravating factors.
The jury returned a defense verdict; the Court of Appeals affirmed, and the Oregon Supreme Court granted review.
The Supreme Court affirmed: it held but‑for instructions correctly state cause‑in‑fact in most multiple‑cause cases; the substantial‑factor test is required only in limited scenarios (e.g., concurrent independent causes each sufficient to produce the harm).

Issue	Plaintiff's Argument	Defendant's Argument	Held
Whether the trial court erred by refusing a substantial‑factor causation instruction and giving only a but‑for instruction	Haas: when multiple factors may have contributed (including preexisting conditions), a substantial‑factor instruction is required so jurors can find liability even if defendant’s conduct was not the sole cause	Carter/estate: but‑for correctly states causation in most cases; substantial‑factor is only required in narrow scenarios (e.g., two concurrent sufficient causes); preexisting conditions are non‑tortious susceptibilities	Court: Affirmed. But‑for is ordinarily correct; substantial‑factor needed only where but‑for fails (e.g., concurrent sufficient causes). Preexisting conditions do not automatically require substantial‑factor instruction; damages instructions can address aggravation

Joshi v. Providence Health Sys., 342 Or. 152 (2006) (explains but‑for and substantial‑factor standards and that they apply to different types of cases)
McEwen v. Ortho Pharmaceutical, 270 Or. 375 (1974) (multiple defendants may each be a cause; substantial contribution theory)
Lasley v. Combined Transport, Inc., 351 Or. 1 (2011) (distinguishes factual causation from foreseeability/legal cause)
Simpson v. Sisters of Charity of Providence, 284 Or. 547 (1978) (upheld substantial‑factor instruction where instructions as a whole properly presented actual cause)
Sandford v. Chevrolet Div., Gen. Motors, 292 Or. 590 (1982) (definition of causation in fact and use of but‑for test)
Fazzolari v. Portland Sch. Dist. No. 1J, 303 Or. 1 (1987) (foreseeability as element of liability separate from factual causation)
State v. Gerhardt, 360 Or. 629 (2016) (clarifies that ‘‘substantial factor’’ for causation is distinct from comparing relative roles — foreseeability/foreseeable harm addresses policy)
Wright v. Turner, 368 Or. 207 (2021) (explains comparative fault/statutory apportionment vs. comparative causation)