It's a little easier to prove retaliation claims because the sequence of events often tells the story. The plaintiff engages in protected activity (such as free speech, or a complaint about discrimination) and her termination takes place shortly thereafter, permitting an inference of retaliation. You know it when you see it, and the Courts deal with these cases one by one. Here's another one.
The case is Espinal v. Goord, decided on February 2. I wrote about Espinal in a different context (PLRA grievance exhaustion) here. But Espinal has another interesting holding: when to draw an inference that government officials have retaliated against someone in violation of the civil rights laws.
A state prison inmate, Espinal in June 1998 brought a civil rights lawsuit against corrections officers at Greenhaven Correctional Facility. That case was dismissed on June 14, 1999. On December 17, 1999, after Espinal had a non-violent dispute with another inmate, some corrections officers escorted him to the medical clinic for a "fight exam." Espinal says the officers beat the hell out of him for a half hour and warned him to keep quiet about the beating. They told him, "this is what happens to inmates when they submit law suits against us" and, according to the Court of Appeals, they threatened to kill Espinal.
So the question is, can Espinal sue the officers for retaliation under Section 1983 (the all-purpose civil rights statute)? The district court said no, but the Second Circuit says yes. The lawsuit that he previously filed is protected First Amendment activity (even though the Court of Appeals has also held that public employees who file certain lawsuits are not engaging in protected First Amendment activity because these cases are usually unique to the plaintiff and not matters of public concern). In the understatement of the year, referencing the relevant legal standard, Court of Appeals says that "we have no trouble finding on the record in this case that there is a triable issue of fact as to whether a severe beating by officers over the course of thirty minutes would deter a person of 'ordinary firmness' from exercising his rights." I would say so.
The harder question is whether the time lag between the first lawsuit and the beating permits the inference of retaliation. It could, the Second Circuit says. Borrowing from employment discrimination cases which stand for the same principle, the Second Circuit says that "a plaintiff can establish a causal connection that suggests retaliation by showing that protected activity was close in time to the adverse action." As some employment retaliation cases find a retaliatory link after eight months and others reject that link after only three months, there is no bright line rule on this; the courts know it when they see it. The Court of Appeals puts it this way: "This has allowed our Court to exercise its judgment about the permissible inferences that can be drawn from temporal proximity in the context of particular cases."
The Court says that although the retaliatory beating was 1.5 years after Espinal brought the first lawsuit, the clock can start ticking after it was dismissed, which was six months before the beating. That's close enough for retaliation because, the Court says, "[i]t is plausible that the officers waited to exact their retaliation at an opportune time -- as when Espinal was involved in a fight with another inmate -- in order to have a ready explanation for any injuries suffered by Espinal."
This is interesting reasoning. I have seen plaintiffs' lawyers attempt to link temporally separate events in order to pursue a retaliation claim on the theory that the bad actors used the first available opportunity to exact revenge. Some judges did not like this reasoning, accepting the argument instead that the two events were too far apart as a matter of law. In this pro se inmate retaliation case, the Court of Appeals has now accepted the contextual approach to retaliation claims.