Chapter 3

Causation Issues and Expert Testimony*

Randy C. JoshiCatherine F. MadridLee-Anne V. Mulholland

3.1 Introduction

Proof that a defendant's wrongful conduct caused a plaintiff to suffer damages is a key concept in the determination of culpability in civil litigation. Testimony of a qualified expert can help the trier of fact—that is, the jury, or the judge in a nonjury trial—to decide whether the plaintiff has proved this causal link.

Causation is a simple concept: Did the defendant's wrongful conduct produce the damage suffered by the plaintiff? Some situations have obvious causation, such as when a gunshot fired by the defendant hits the plaintiff. In others, however, such clarity does not exist. Does a public company's announcement that it will miss quarterly earnings projections cause its stock price to drop, as plaintiffs in a shareholder class action lawsuit might claim, or is some other factor at work, such as bad news about the economy that made the entire stock market plummet the same day?

The law follows logic. It requires that the plaintiff prove more than correlation to establish causation.1 Accordingly, courts reject expert testimony on causation that identifies merely a correlation between two variables.

Courts similarly refuse to find causation when an expert neglects to consider other factors that could have caused—or at least contributed to—the plaintiff's damages, even when the defendant has indisputably engaged in legally actionable misconduct. This refusal ...

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