"Until recently, little research has explored female sexual function (Berman et al. 1999), and yet women commonly report sexual dysfunction. For example, over 50% of women in the UK report at least one sexual problem lasting a month or more during the previous year (Mercer et al. 2003), and around a quarter have never or rarely achieved orgasm during the previous three months (Dunn et al. 2002). There is controversy over whether the problem has a real medical basis, or whether it has partly been created by the media, pharmaceutical advertising and cultural expectations (Moynihan 2003). Whereas studying male sexual function is widely accepted (with a recent increase following the release of drugs like Viagra), the female orgasm is less well studied. This lack of scientific interest may be partly attributable to the previous consensus that the female orgasm has no clear role in reproduction. This view was challenged by research showing that the orgasm helped facilitate sperm retention (Morris 1967; Fox et al. 1970). Further evidence for the orgasm's reproductive role comes from studies linking it with the menstrual cycle (Matteo & Rissman 1984; Baker & Bellis 1995). More recently, this was corroborated in other studies showing that desire for pregnancy subconsciously predicted timing of the female orgasm to be just after that of the male (Singh et al. 1998). Studies such as these are obviously difficult to conduct, and there is still a lack of agreement among experts about the role of the female orgasm in humans. While differences in sexual function between women were known to exist, these have been largely attributed to cultural, religious and psychological factors. No study has explored family history or genetic factors—which might indicate or refute a biological or evolutionary basis for the variation. We therefore performed a classical twin study comparing the similarities in identical and non-identical twins to explore whether there were observable genetic influences on variation in female orgasmic function."