See, e.g., Estlund, Wrongful Discharge, supra note 42, at 1670-71. The governing statutes also bar retaliation against a person who has "made a charge, testified, assisted, or participated... in an investigation, proceeding, or litigation" under these laws or has "opposed any practice made unlawful" by them. See, e.g., 29 U.S.C. § 623(d) (2000) (ADEA). To prove a prima facie case of retaliation, the plaintiff must demonstrate: (1) he engaged in activity protected by the statute; (2) he suffered an adverse employment action; and (3) a causal link exists between (1) and (2). See Modjeska, supra note 79, § 1.04, at 1-11 to 1-12 (3d ed. 2002); see, e.g., Slattery v. Swiss Reinsurance Am. Corp., 248 F.3d 87, 94 (2d Cir.) (finding that the claim filed was not the cause of the employee's dismissal), cert. denied, 534 U.S. 951 (2001). In my experience, plaintiffs often find it easier to prove a charge of retaliation than one of substantive discrimination. Significantly, to prevail on the former, a plaintiff need only show that he sincerely and reasonably believed that discrimination had occurred--he need not show that he was correct. See, e.g., Alexander v. Gerhardt Enters., Inc., 40 F.3d 187, 195- 96 (7th Cir. 1994).