The Connecticut Appellate Court has held that expert testimony on the general unreliability of jailhouse informants is admissible, and that a trial court abused its discretion when it excluded a defense expert (me) in a trial in which the conviction depended on several jailhouse informants. The opinion is here: State v. Leniart. The discussion of the expert issue is in Part IV, beginning on page 42.
The opinion contains several key findings:
1. The Court “acknowledged the growing recognition by the legal community that jailhouse informant testimony is inherently unreliable and is a major contributor to wrongful convictions throughout this country.” (p. 43, quoting State v. Arroyo)
2. “Although credibility determinations ultimately must be left to the jury, expert testimony nevertheless is admissible if it can provide a jury with generalized information or behavioral observations that are outside the knowledge of an average juror and that would assist it in assessing a particular witness’ credibility. As long as the expert does not directly opine about a particular witness’ credibility or  testify in such a way as to vouch indirectly for or bolster the credibility of a witness, the expert’s testimony would not invade the province of the jury to decide credibility and may be admitted.” (p.49)
3. An understanding of jailhouse informant culture, including the expectation of benefits and the lengths to which informants may go to procure and fabricate evidence, is not within the ken and understanding of the average juror (p. 50).
4. Expert informant testimony is similar to expert testimony regarding the unreliability of eyewitness testimony which is now widely viewed as admissible (p.51-52).
5. Generalized jury instructions may be insufficient to educate jurors regarding the dangers of informant unreliability, since in eyewitness cases “generalized jury instructions were not an adequate substitute for expert testimony” (p. 52).