Are Witnesses who have something to gain from providing Testimony often Lying?

A witness who has something to gain from providing testimony is often lying.
Describe a specific situation in which a witness who has something to gain from providing testimony might not be lying. Discuss what you think determines whether or not a witness who stands to gain from providing testimony will lie.
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The American court system is adversarial in nature, wherein both plaintiff and defendant have the opportunity to present their sides before an impartial person or group of people. Each side can gather evidence, which may come in the form of photographs, videos, eye-witness and expert testimony, etc. Indeed, witnesses who have something to gain from providing testimony have the incentive to lie. However, the opposing lawyer has the right to cross-examine witnesses and use his power of persuasion and analysis to debunk the credibility of lying witnesses and sway the judge and/or jury to his side.

Witnesses may have something to gain when providing testimony. For instance, they may be trying to protect themselves and family members from physical, economic or reputational harm. Perhaps they may be attempting to escape their own complicity in the matter. They may also happen to be hired by the plaintiff or defendant to provide expert testimony. Just because witnesses have something to gain from providing testimony does not necessarily mean they are lying. Indeed, it is the cross-examining lawyer's duty, in front of the judge or jury, to expose any motives for stretching the truth. However, it may be improper ( or against the rules of evidence) to emphasize in court that expert witnesses are being paid. The fact that our judicial system is an adversarial process allows witnesses with "mixed motives" to appear before the court. They are prohibited by law from being untruthful but the opposing side, of course, never assumes the sincerity of witnesses and looks for probable ways to discredit his testimony.

While expert witnesses gain monetarily from providing testimony, they do not always "lie" per se. These witnesses are usually well-accomplished in their fields and have performed a good amount of academic and field research to be considered "experts." They are hired by lawyers to testify in areas from surgical practices to forensic crimes. These witnesses can present evidence in favor of their clients with the hopes of adding expert credibility to the evidence. While the witnesses will often not downright lie, they have the ability to sometimes stretch the truth and perhaps view the evidence from a narrow point of view which favors clients. As in the case of eye-witness testimony, it is the cross-examining lawyer's job to expose the evidence as unreliable and also provide counter-evidence in the form of opposing expert witnesses.

Witnesses may have good incentives to lie before the court. However, this fact in itself does not exclude them from providing testimony. Indeed, it is the cross-examining lawyer's responsibility to analyze where exactly this witness is coming from and expose any biases and holes that may be present in the testimony. Judges and juries must also remember that just because witnesses have something to gain from providing testimony does not necessarily mean that they are not telling the truth.