There is no presumption of truthfulness. R. v. Bradey, 2015 ONCA 738 says:
 No presumption of honesty or truthfulness accompanies the witness to the witness box. It follows that an instruction that such a presumption applies is legally wrong: R. v. Luciano, 2011 ONCA 89, 267 C.C.C. (3d) 16, at para. 147; R. v. Thain, 2009 ONCA 223, 243 C.C.C. (3d) 230, at para. 32. Whether inclusion of such an instruction is fatal to the validity of a conviction will depend on the circumstances: Luciano, at para. 149.