A few days ago prosecutors in the Michael Jackson case arguably hit their lowest point - by showing obvious embarrassment at a man touted as one of their star witnesses.

A few days ago prosecutors in the Michael Jackson case arguably hit their lowest point - by showing obvious embarrassment at a man touted as one of their star witnesses. On the stand was Stan Katz, the psychologist who first interviewed the teenage boy at the centre of the case and subsequently reported his allegations of sexual abuse to the police. The jury and press gallery were primed to hear a detailed account of what Gavin Arviso disclosed to Dr Katz about Jackson's alleged attempts to seduce him.

Instead, Tom Sneddon, the Santa Barbara County district attorney, did little more than establish the circumstances of Dr Katz's involvement and then, after barely half an hour, abruptly terminated his questioning. The prosecutor's reticence almost certainly had to do with Dr Katz's association with one of the most notorious child sex abuse investigations in American history - a catalogue of appalling professional errors and mass hysteria surrounding a Los Angeles area pre-school in the 1980s that scarred dozens of lives but failed to lead to a single criminal conviction.

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Dr Katz's organisation, Children's Institute International, believed at the time it had uncovered sex crimes and satanic rituals at the McMartin pre-school in Manhattan Beach. After interviewing 400 current and former students, it concluded that 369 of them had been sexually abused - lured into underground tunnels, forced to perform bizarre forms of devil-worship including the disinterment of coffins, raped at a car wash and filmed with their adult abusers for pornographic purposes.

The problem with CII's "discoveries" was not only that they failed to meet the basic test of plausibility. They were also based on highly coercive interviews, in which the children systematically denied anything was amiss until the interviewers started putting ideas into their heads. Over and over, the children were asked if they had participated in a certain "game" or if a teacher had touched them. If they said no, they were called "dumb". If they said yes, they were called "smart".

When the case reached trial, jurors were able to see the coercive techniques for themselves because the interviews had all been videotaped. Not a shred of corroborating evidence ever surfaced and the defendants, all members of the same family, were exonerated. Among family psychology professionals, the case is now a byword for how not to conduct a sex abuse investigation.

CII has never conducted a thorough review of its mistakes in the McMartin case, and indeed the woman who conducted the bulk of the sex abuse interviews - a social worker with no formal training in psychology or family therapy called Kee MacFarlane - was later promoted to become CII's director of education and training. She has since left the organisation, but continues to consult and lecture on child abuse.

All this, of course, plays very nicely into the hands of Mr Jackson's defence lawyers. Mr Sneddon's abbreviated questioning of Dr Katz denied them the opportunity to reveal the full horrors of the McMartin case to the jury last week, but they did get Dr Katz to acknowledge that he was personally involved.

The association with McMartin is not an automatic black mark on Dr Katz's reputation. The science of sex abuse evaluation was in its infancy in the 1980s, and he was not personally responsible for Ms MacFarlane's interview techniques.

Likewise, his involvement in the Jackson trial neither augments nor diminishes the credibility of Gavin Arviso and his brother Star, who allege that Jackson put his hand inside Gavin's underpants and masturbated him at least twice while the boys were guests at his Neverland ranch in central California.

But Dr Katz does present the latest of a series of headaches for the prosecutors, who badly need some authoritative figures with direct knowledge of the case to tell the court why the boys should be believed - despite the vagueness of some parts of their account and their acknowledged record of lies and rowdy behaviour.

For this purpose, Dr Katz is effectively useless, the risk being that he could only fuel the defence's contention that Mr Jackson's accusers are being egged on by a cabal of unscrupulous professionals out to fleece him for as much money as they can.

He and the private litigation lawyer Larry Feldman, who referred the Arvisos to him, were both involved in an earlier case against Mr Jackson, in which Jordy Chandler, then 13, accused the singer of molesting him. Chandler's family dropped their charges only after receiving a $20m (£10.6m) settlement.

It is far from proven that the allegations against Mr Jackson are part of a witch-hunt analogous to the McMartin case, but that does not mean that the defence won't try to argue it anyway.