Almost half of men have no idea how often their wives achieve orgasm during sex.
A damning new survey of more than 3,200 newly-weds found a quarter of husbands over-estimate how often their wife reaches climax.
This presents a huge problem for couples, scientists said, as women whose husbands are clueless about their partner's orgasms tend to be unhappier in their sex lives and relationships.
The study, from researchers at Brigham Young University in Utah, used results from a survey of 1,683 heterosexual couples.
Respondents, all aged between 18 and 36 and within their first year of marriage, were asked how often they and their partner climaxed.
They answered on a scale of one to five, with a score of one signifying a range from never to 20 per cent of the time.
A top score of five meant women climaxed from 80 per cent to all of the time.
Men were asked to provide the same score for their wives, but got it wrong in 43 per cent of cases.
They were more likely to overestimate the answer, suggesting that almost three decades after Meg Ryan famously faked an orgasm at a diner table in When Harry Met Sally, women often still do this.
Commenting on the research, Ammanda Major, head of clinical practice at relationship counselling service Relate, said: 'When it comes to couples' love lives there can be a lot of misunderstandings.
"It is a problem if men are misperceiving how often women orgasm because this can lead to disappointment, over-expectation and frustration for both partners, and affect self-esteem.
"Not all women need to climax during sex to have had a good experience so understanding that and finding out what you each need from each other sexually is usually the best way of making sure everyone is happy."
More than one in five women achieve orgasm less than four in ten of their sexual encounters, the study results show.
Just three per cent of men have a 'happy ending' this infrequently.
While 25 per cent of men overestimated how often their wife climaxed, 17 per cent of those in the study underestimated the figure and had a better love life than they realised.
Unsurprisingly, women were far more aware of what was happening in the bedroom, with only one in seven misjudging how often their husband climaxed.
Couples also answered questions on how satisfied they were with the relationship and their sex lives, as well as how openly they talked to one another about sex.
Both sexes were happier in the bedroom and their relationship if they talked about sex.