> An expert witness, particularly in common law countries such as the United Kingdom, Australia, and the United States, is a person whose opinion by virtue of education, training, certification, skills or experience, is accepted by the judge as an expert. The judge may consider the witness's specialized (scientific, technical or other) opinion about evidence or about facts before the court within the expert's area of expertise, to be referred to as an "expert opinion".

Thanks for reinforcing my point.

Usually both sides have an expert witness, the implications of that are self-evident. Don't even need to factor in knowledge about human nature to understand what is happening.

To be fair, the deviation from the truth an expert witness may grant tends to be bounded. Whereas the deviation a lawyer may attempt is unbounded. In many cases the bounds don't matter much as the case contains sufficient uncertainly to be a contest of spin targeted at the psychology of the jury.

What you're describing is selection bias: Obviously I'm going to choose an engineer that agrees with me, and I'm not going to bring one that doesn't agree with me. But I'm not allowed to pay him to defend a side.

With lawyers, paying to defend a side is the whole point.