guyde:
We authors use all the tricks of the trade to enhance the willing suspension of disbelief - by placing the tale in a time and place where the spanking we describe would be commonplace - or by giving our characters motivation not experienced everyday, and so on.
I think that I sometimes fail on my side of the bargain by making my characters act out of character, if that makes sense. What I mean is, having delineated the traits for a person, having he or she do something that that sort of person would never do: this alone can cause a rejection of the story as being "not believable".
I wonder if there are other areas in which we cause our readers to turn away, because no matter what twists and turns we may offer, the tale lacks sufficient credulity.
Well, if you set a story a century ago and then have an Irish mother administer severe corporal correction to her teenaged child for a major offense, and the boy cooperates without resistance or even protest, does that require a suspension of disbelief? IMHO not at all, that scenario is perfectly credible for its setting--in fact, if the woman simply sat him down and had a heart-to-heart discussion with him about his misconduct, it would strike me as much less believable since that kind of a parental approach would've been almost nonexistent in the established setting.
But suppose that it turned out that the child hadn't really misbehaved but simply was gratified by having his bare behind soundly whipped by his mother, and the woman understood that but pretended to believe his 'confession' of wrongdoing because she in turn enjoyed administering the chastisement? AFAIC that would be a relatively unlikely motivation a century ago, very few mother-son combinations would have had those feelings (or at least been able to identify them as such) back then. To make such a story reasonably believable, I'd venture that the reader would have to concede that the top-mother/bottom-son relationship described was highly atypical but did nonetheless occur in the particular situation being described.
However, establishing a character's personality and value system and then having him/her inexplicably behave contrary to them, that would be a serious credibility issue--I'd find it very difficult to suspend my disbelief unless a rational underlying reason for the unexpected, out-of-character behavior were eventually to be provided.
I can easily suspend my disbelief to embrace a clandestine world of magic, but if Harry Potter had suddenly switched to Lord Voldemort's side in the middle of THE DEATHLY HALLOWS, after opposing 'You-Know-Who' for six-and-a-half books, then I'd very likely have been incredulous--unless a solid reason (perhaps a faked 'defection' to learn the Death Eaters' plans) had surfaced later on. (No 'spoilers' here--for those who just watch the movies.)
Having a character behave in a manner that's statistically unlikely, such as all those children in Grace Brackenridge's stories who angle to repeatedly receive seriously stinging chastisements from adults and even other children, isn't necessarily a problem in terms of credulity to me--but if there's a sudden and unexplained change in attitude and/or actions, like a child who finds spankings to be only painfully punitive trying to get his/her naked buttcheeks blistered, there had better be a sensible reason provided for me to consider it a plausible circumstance.
Some spankophiles seem to believe that there's a scene in the novel GONE WITH THE WIND in which Rhett Butler spanks Scarlett O'Hara, but of course there isn't; IMHO it wouldn't be consistent with Scarlett's character for her to be 'tamed' via being corporally chastised by a 'dominant man'--which AFAIC is why, in spite of several verbal threats to that effect, Rhett never does lambaste her derriere. Based on Scarlett's character, being walloped by Rhett would make her angry and resentful but it would never 'tame' her--although that outcome does apparently occur fairly regularly in other historical romance novels.
Ahhhh well, that's my analysis anyway...

--C.K.