It is not a question of asking "why would the complainant lie?"

R. v. Horton 1999 BCCA 0150 is an older British Columbia decision just released electronically. It is a very helpful decision in considering "he said, she said" cases especially where this is a denial of wrongdoing:

[14] However, the more significant difficulty with the decision lies in the treatment of the evidence of the accused and his witnesses. The trial judge concluded his reasons thus:

In my opinion, [the complainant's] testimony had

the ring of truth to it and his manner of giving that

testimony had an air of authenticity about it as

well.

I turn, now, to the testimony of Mr. Horton. It

amounted to a bare denial. That is not to be critical

of him, because there is little else he could say

since there were only two persons present. It was his

word against that of young [K.]; but some parts of

his evidence caused me concern.

...

[16] The fundamental error of approach is disclosed by the first and fourth paragraphs quoted supra. Having found the complainant's evidence to have the ring of truth, the trial judge, as the first sentence of the last paragraph shows, placed the burden on the accused to prove that he was not guilty. The case is essentially indistinguishable from the decision of the Ontario Court of Appeal in R. v. S.(W.) (1994), 90 C.C.C. (3d) 242, in which the court, in allowing an appeal from a conviction under s.151, directed an acquittal. Finlayson J.A. for the court, said at pp.253-254:

The trial judge in the present case gave full

reasons and he recited the evidence accurately. He

also referred to the appropriate authorities. I

cannot say that he misdirected himself in a material

way. His overall approach to this particular case,

however, was wrong. Instead of questioning the

veracity and accuracy of the witnesses who, because

of the nature of the charge, were called to support a

negative, he should have been more critical of the

complainant who put forward the affirmative that the

offences took place: see R. v. Norman, supra, at pp.

172-3. This is another example of the way the trial

judge shifted the onus to the appellant in spite of

the Crown's burden to prove all elements of the crime

beyond a reasonable doubt.

...

[18] In R. v. S.(W.), Finlayson J.A. referred to and adopted relevant language in two decisions of this court. The first is R. v. K.(V.) (1991), 68 C.C.C. (3d) 18, 4 C.R. (4th) 338, (B.C.C.A.) in which case Wood J.A. said at p.35:

I have already alluded to the danger, in a case

where the evidence consists primarily of the

allegations of a complainant and the denial of the

accused, that the trier of fact will see the issue as

one of deciding whom to believe. Earlier in the

judgment, I noted the gender- related stereotypical

thinking that led to assumptions about the

credibility of complainants in sexual cases which we

have at long last discarded as totally inappropriate.

It is important to ensure that they are not replaced

by an equally pernicious set of assumptions about the

believability of complainants which would have the

effect of shifting the burden of proof to those

accused of such crimes.

...

[20] It is not a question of asking "why would the complainant lie?" or "which witness do I prefer?" In the circumstances here, where there is no finding against the truthfulness of the complainant, the ultimate question becomes "having regard to all of the evidence, or the lack of it, can the trier of fact be satisfied of guilt beyond a reasonable doubt."