[MarkPawelek (OP)]

http://www.telegraph.co.uk/science/science-news/10276672/Forensic-experts-biased-towards-side-which-pays-them.html

I'm not in the least bit surprised by this but how could we explain it using scientific reasoning?

[dlorde]

... how could we explain it using scientific reasoning?

It's said to be a form of the 'allegiance effect' (better known from psychotherapy), a cognitive bias in assessment that favours your own 'team' (or preferred methodology or treatment). Presumably, in the forensics case, there is a subconscious temporary affiliation with the people who pay, a sense of being recruited to their team or tribe. If you're working for them, you owe them some loyalty, or want to please them, reinforced by the implicit sense of authority a paymaster has...

You can see more explicit examples where individual loyalty is transferred when an employee moves from one company to a rival company, or when a team game player moves or is loaned to a new team.

Raises questions about a saying like "Loyalty can't be bought".

[evan_au]

There is another, conscious, effect at work here - a lawyer or promoter will contact experts who they think might be sympathetic with their position.
And if the expert's opinion does not agree with the story that is being sold, the expert is just never called up.

[alancalverd]

It is in the nature of the legal process that a court won't hear a case unless the prosecution has some evidence, so you always start with a bias, and the defence has to rebut the case, so they will call a scientist to challenge the prosecution's expert  or re-interpret the facts.

The facts themselves are rarely in dispute. You would have to be a very bold scientist to fabricate material evidence (though it happens) or to lie about a measurement that can be checked. But what the court needs is an opinion on the reliability and significance of the facts, or the minutiae of an obscure statutory regulation.

I always prefer batting for the defence. It's more satisfying than presenting a case that is brought out of malice or ignorance, but often more difficult to persuade the court that it has been misled by incompetent or irrelevant statistics - see http://www.bbc.co.uk/news/magazine-22310186 .

Opinions can be bought and presented or suppressed by the presenting barristers, so what happens in court may well be a lot more polarised than what happened in the laboratory. Sad, but inevitable in an adversarial system. The French procedure, using an investigating magistrate to ascertain the facts, may be less prone to bias as the experts are, I understand, hired by the court itself.   

[Lmnre]

The OP's study involved testimony/opinions from "experts" in the "soft sciences" of psychology and psychiatry.

I remember the day-care child-abuse hysteria that swept the US. Some experts were hired in multiple cases, and the children's testimony in those cases described the same abuse — clowns, "secret" rooms, morbid rituals, baby sacrifices, etc — allegations that may have only existed in the minds of the "experts" who apparently implanted false memories into the minds of little children.

However, we are now coming to realize that, in some ways, experts in physical evidence also can be far from impartial.

The case against chemist Annie Dookan alleges that she habitually ignored good science in order to find defendants more guilty than they actually were.

People are now beginning to question the reliability of the methodology of some kinds of forensic analysis, and they're finding that other physical "analysis" isn't so scientific. For example, the forensic disciplines of  hair-match analysis and bite mark analysis has come under serious scrutiny.

Analyses that result in opinions (versus analysis by instruments) are apparently more susceptible to being more subjective than objective.

[CliffordK]

It may also depend on how the study is phrased.

If the subjects are approached with a party (defense or prosecution) asking them to take a preliminary view of the data and provide an opinion, then it may be like a job interview with the expert witness trying to please the person hiring them.

It may or may not be different once they are on the witness stand.  But, there still may be some bias in favor of their employer.