"Fool me once, shame on you; fool me twice, shame on me," is an old saw, but there's wisdom in it.

Unfortunately, that simple statement on trust and honesty often is overlooked by our courts, where junk science from expert witnesses-for-hire is all too common.

Many of the worst expert witnesses -- those whom trial lawyers can count on to say whatever is needed to win a case -- are repeat players. Some testify dozens of times a year, earning six-figure salaries for their efforts.

Take John Torkelson, a professional expert witness who recently pleaded guilty to perjury, admitting he lied in hundreds of cases against publicly traded companies. In most cases, his testimony concerned damages alleged by disgruntled shareholders.

Though claiming to be an independent expert, he was heavily invested in the cases and stood to lose millions if his side lost. In all, Torkelson's false testimony netted him tens of millions in fees.

Courts may have a tough time identifying crooked witnesses such as Torkelson, but when they do, they should act to prevent further injustice. A recent opinion by appeals court judge Richard Posner shows that courts have it within their power to keep these kinds of expert witnesses-for-hire out of any courtroom, for now and in the future.

Posner, a renowned economic thinker, was asked to examine two expert reports written by Stephen Buser, the chairman of a state university economics department, purportedly showing that the plaintiff in the case, Emerald Investments, lost millions when the defendant, Allmerica Insurance, violated a contract between the two companies.

Relying on the reports, Emerald demanded $150 million in damages. But the trial judge threw them out. So Emerald appealed that decision to Posner's court.

Posner's approach to handing this bogus expert evidence should be a lesson to all judges.

The first report he dismissed as "preposterous," with economic reasoning incapable of passing the laugh test. As Posner explained, the Emerald's business was arbitrage -- earning small gains by exploiting transient market anomalies -- so it made no sense for Buser to assume that these temporary opportunities would continue for 20 years. So far, so good -- there's nothing unusual about this kind of decision.

What's noteworthy is that Posner declined to even examine the second report. His reason is worth quoting in full:

"Buser's first report was so irresponsible as to justify the judge's decision to exclude the second report summarily. Buser had demonstrated a willingness to abandon the norms of his profession in the interest of his client. Such a person cannot be trusted to continue as an expert witness in the case in which he has demonstrated that willingness, and perhaps not in other cases either."

As one attorney commentating on the case noted, "With this throwaway line, Posner all but ensures that Buser will have to find a new way to supplement his teaching salary."

But that's not a bad thing, necessarily. Serving as an expert witness is lucrative, and some repeat-player witnesses try to exploit the legal system for personal profit.

It's admirable, then, that Posner went beyond just identifying the bad evidence to point the finger at the person who stood behind it. This puts other judges on notice and may even persuade some trial lawyers to drop the most pliable, and least reliable, expert witnesses from their rosters.

There's a role for state legislatures and Congress, too. Expert witnesses usually have to state their credentials and list the cases in which they've recently testified, but some proposals would go further to target professional witnesses. It makes sense, for example, that experts' primary employment should be actively practicing in the field in which they claim expertise, not working with trial lawyers.

One thing Congress should not do is take steps that make it harder for judges to identify bogus expert testimony. An amendment by Sen. Russ Feingold, D-Wisc., before Congress, for example, would make it difficult for federal judges to participate in educational programs run by universities and nonprofits, such as George Mason University's economics seminars.

Obviously, trial lawyers would prefer there be fewer economic experts such as Posner on the bench who can sniff out bad expert testimony. But helping trial lawyers win big judgments based on flimsy evidence is bad public policy and hinders justice.

Improving the quality of experts who testify increases the likelihood that courts and juries will reach decisions based on firm, reliable evidence, not bogus testimony by slick professional witnesses who sometimes have greater expertise at testifying in court than conducting any form of research.

Our courts exist to dispense justice. They can't do that with misleading evidence and slanted facts. Judges have a duty to make sure that they and their colleagues aren't fooled twice.

Andrew M. Grossman, is Senior Legal Policy Analyst in the Center for Legal and Judicial Studies at The Heritage Foundation