Adekoya v. State  7 NWLR (Pt. 1565) 343 at 359, paras. A-B, per Peter-Odili, JSC:
“The evidence of PW4 was not contradicted during cross-examination. Learned counsel for the appellant had sought to discredit the testimony on the ground that PW4 was a tainted witness. In that regard, I would have to say that the mere fact that a witness is a blood relation of the victim does not translate without more to being a tainted witness. See Musa v. State (2012) 3 NWLR (Pt. 1286) 59; Ben v. State (2006) 12 SCM (Pt. 2) 71 at 88, (2006) 16 NWLR (Pt. 1006) 582.”
There is no statutory definition for “tainted witness” and none is contained in the Evidence Act 2011. In Ojo v. Gharoro (2006) 2 – 3 SC 105 at 124, Tobi, JSC (of blessed memory) described a “tainted witness” as “…a biased witness, that is to say a witness who, because of his prejudices and sentiments will invariably give evidence in favour of the party calling him, with little or no regard for the truth. A tainted witness could be an interested witness. And because of his interest, the witness develops a one sided inclination and it is the inclination towards the party who calls him to give evidence; no matter the obvious lies he tells in court. In determining whether a witness is an interested witness or a tainted witness, the court must examine the relationship of the witness to the party calling him.”
In Ezeugo v. The State (2013) LPELR-19984(CA), it was observed by the Court of Appeal that “a tainted witness may be defined as a witness who may not in the strict sense be an accomplice but who in giving his evidence is established to have some purpose of his own to serve and in respect of whom it is desirable that the warning as to corroboration be given.”
The above description obtains generally both in civil and criminal matters. See Ojo v. Gharoro (supra).
The use of the expression “tainted witness” was said to have been introduced by lawyers who intend to discredit the testimony of a particular witness during trial. The Supreme Court has since warned that the use of the words is capable of causing confusion in the area of the Law of Evidence and should therefore be kept within proper bounds. See Idigbe, J.S.C. in Garuba Mailayi & Anor v. The State (1968)1 All NLR 116 at 123; Ogunlana v. The State  5 NWLR (Pt. 395) 266.
The important point restated by the Supreme Court in the recent case of Adekoya v. State (supra) is that the evidence of a blood relative of a victim of crime should not be treated like the evidence of a tainted witness simply because the witness is a blood relative. Indeed such evidence can be relied upon where found to be cogent and credible and the court can act on same without the need for corroboration. In fact, in Adekoya, the Appellant contended that one of the witnesses, a 13 year old girl, was a “tainted witness”. The Appellant was charged for armed robbery attack on a woman and her two daughters (one of whom was the 13 year old girl) at there residence. The Court found the evidence of the little girl as credible and cogent and therefore refused the invitation to label her a “tainted witness”.
Interestingly, this principle has been applied even in murder cases. The Supreme Court has held that there is no law which precludes a blood relation of a deceased person from acting as a witness for the prosecution. It is only left for the trial court to examine the totality of the evidence to determine whether the relative gave a biased evidence in a bid to see that the accused person is convicted at all cost. See Nkebisi v. State  5 NWLR (Pt. 1188) 471 relied upon by the Court of Appeal in Uluebeka v. State  4 NWLR (Pt. 1237) 358 at 376.