Doi:10.1098/rsbl.2005.0308

scientific interest may be partly attributable to the previous consensus that the female orgasm has no clear role in reproduction. This view was challengedby research showing that the orgasm helped facilitate sperm retention (Further evidence for the orgasm’s reproductive role comes from studies linking it with the menstrual cycle( More recently, this was corroborated in other studiesshowing that desire for pregnancy subconsciously predicted timing of the female orgasm to be just afterthat of the male Studies such as these are obviously difficult to conduct, and there is still a lack of agreement among experts about the role 1Twin Research and Genetic Epidemiology Unit, St Thomas’ Hospital, While differences in sexual function between 2Primary Care Sciences Research Centre, Keele University, women were known to exist, these have been largely attributed to cultural, religious and psychological *Author for correspondence (tim.spector@kcl.ac.uk) factors. No study has explored family history or gene- Orgasmic dysfunction in females is commonly tic factors—which might indicate or refute a biological reported in the general population with little or evolutionary basis for the variation. We therefore consensus on its aetiology. We performed a performed a classical twin study comparing the simi- classical twin study to explore whether there larities in identical and non-identical twins to explore were observable genetic influences on femaleorgasmic dysfunction. Adult females from the whether there were observable genetic influences on TwinsUK register were sent a confidential survey variation in female orgasmic function.
including questions on sexual problems. Com-plete responses to the questions on orgasmicdysfunction were obtained from 4037 women consisting of 683 monozygotic and 714 dizygotic Subjects for this study were monozygotic (MZ) and dizygotic (DZ) pairs of female twins aged between 19 and 83 twins enlisted from the TwinsUK registry—the largest adult twin years. One in three women (32%) reported never or infrequently achieving orgasm during inter- population has been involved in a wide range of studies on commontraits and diseases () ranging from catar- course, with a corresponding figure of 21% acts and blood clotting to musical ability ( during masturbation. A significant genetic influ- ence was seen with an estimated heritability for this registry have been shown to be comparable to the age-matched difficulty reaching orgasm during intercourse of general population for a broad variety of medical and behavioural 34% (95% confidence interval 27–40%) and 45% traits After obtaining ethical approval from (95% confidence interval 38–52%) for orgasm the St Thomas’ Hospital ethics committee, a postal self-completionquestionnaire was sent to 3654 pairs of female twins aged between during masturbation. These results show that 19 and 83 years including questions relating to sexual behaviour.
the wide variation in orgasmic dysfunction in Complete confidentiality was assured and the twins were unaware females has a genetic basis and cannot be of any specific hypotheses. Zygosity was established by using attributed solely to cultural influences. These standardized questions that have over 95% accuracy results should stimulate further research into and further confirmed in 54% of the pairs by multiplex the biological and perhaps evolutionary pro- Orgasmic dysfunction was defined primarily using two questions cesses governing female sexual function.
on frequency of orgasm during intercourse and masturbation. The Keywords: twin studies; sexual dysfunction; questions were, ‘Overall, how frequently do you experience anorgasm during intercourse?’ and ‘Overall, how frequently do you experience an orgasm during masturbation by yourself or apartner?’ Responses were on a seven-point scale labelled as in An individual’s phenotype is the result of the effects of both genotype and environment. To study the sources of individualdifferences (i.e. the variance) in a phenotype, genetically related Until recently, little research has explored female subjects are required, and twins are ideal. MZ twin pairs share all of their genes and DZ twin pairs share, on average, 50% of their commonly report sexual dysfunction. For example, segregating genes. Assuming that both types of twins share equallysimilar family environments, one can assert that any greater over 50% of women in the UK report at least one similarity between MZ twins than between DZ twins is a result of sexual problem lasting a month or more during the genetic influences. The similarities are estimated using the intra- class correlation coefficient (ICC). Standard variance componentsmodel fitting was used to quantify the influences, and is based on quarter have never or rarely achieved orgasm during comparison of the covariance (or correlation) of a trait between MZ and DZ twins ). This permits separation is controversy over whether the problem has a real of the observed phenotypic variance into additive and dominant medical basis, or whether it has partly been created genetic effects, and shared and unique environmental effects. Thesignificance of genetic and environmental factors as components of by the media, pharmaceutical advertising and cultural the observed variance is assessed by removing each sequentially from the full model and testing the deterioration in fit of the male sexual function is widely accepted (with a recent various sub-models by hierarchic c2-tests. A heritability estimatecan be generated, defined as the proportion of total variation increase following the release of drugs like Viagra), that can be explained by genetic variance. Details of the the female orgasm is less well studied. This lack of analytical methods used are discussed in more detail in a paper by Table 1. Details of twins studied by zygosity.
a p-values corrected for the relatedness of twins.
b Range.
by either method (12% and 13%, respectively; Table 2. Reported frequency (%) of experiencing orgasm The ICCs for frequency of orgasm during inter- course and masturbation were higher for MZ pairs (31% and 39%, respectively) than for DZ pairs (10% and 17%; pZ0.0001), suggesting a clear genetic influence for both. Variance components modelling revealed that the effects of the shared environment and dominant genetic effects could be dropped from the model without significant loss of fit, and provided a quantitative estimate of additive genetic variance (heritability) of 34% (95% confidence interval 27–40%) for orgasm during intercourse and 45% a Excludes those not sexually active and missing responses.
(95% confidence interval 38–52%) for orgasm during masturbation (see ). The remainder of thevariance was a result of random error and non-shared . Preliminary analyses were carried out with environmental factors. There was no significant effect STATA software, while all genetic modelling was carried out with the variance components program MX GUI v. 1.4.06 ( 4. DISCUSSIONThis study shows that there is a significant genetic component to variation in female orgasmic function The questionnaire was returned by 4037 women, that has not been reported previously. We believe our comprising 1697 complete pairs (and 643 whose co- results to be generalizable to singletons based on the twin did not reply). A subset of 683 MZ pairs and 714 DZ twin pairs responded fully to the orgasmic ) and previous comparison studies of twins and dysfunction questions, with no difference in respon- dents between the groups. Details of the sample are for complete pairs is comparable with the typical response for medically oriented twin surveys, and twin groups were similar in terms of divorce (26% there was no evidence for an age bias. Further and 25%, respectively), number of sexual partners evidence for the generalizability of the findings comes (means of between 4 and 5) and age (49 and 50 from the similarities in prevalence figures reported years, respectively). The sample of respondents did not appear to be biased towards younger subjects; the average age of respondents was 50 years, the same as variation in ability to orgasm can be explained by for all the twins to whom the questionnaire was sent.
underlying genetic variation, with little or no role for Of the sample, 98% sample reported being heterosex- the shared environment (e.g. family environment, ual and sexually active at some point.
religion, social class or early education). These herit- The reported frequencies of experiencing orgasm ability findings are in a similar range (35–60%) to through intercourse and masturbation are shown in other behavioural and complex traits such as migraine, blood pressure, anxiety or depression and unable to achieve orgasm during intercourse more than a quarter of the time, and half of these women ). This is despite the greater difficulty in obtain- (16% overall) never reached orgasm during inter- ing information on aspects of sexual relationships course. Conversely, 14% of women always experi- compared with obtaining data, for example, on enced orgasms during intercourse. More women were able to orgasm during masturbation, with 34% always ). Our data lend support to the idea that reaching orgasm. However, 21% were still unable to variation in female orgasmic ability has a biological orgasm more than a quarter of the time, two-thirds of whom never achieved orgasm during masturbation.
As in all studies of this nature, there are limitations There was no difference in the proportion of MZ and to our study. There is potential for recall bias—but DZ twins who never or infrequently reached orgasm this would only have affected the results if there were Table 3. Genetic modelling and estimates of heritabilities (given as additive genetic effects with 95% CI).
(A, additive genetic effects; C, common environmental effects; E, unique environmental effects; D, dominant genetic effects.
Bold indicates best-fitting model and principal result.) a systematic bias; for example, if MZ twins who particularly its role in increasing fertility ( replied were more likely to show similar direction of ) and even in selecting sexually proficient mates recall bias to their co-twin than DZ pairs. There was no evidence of this, nor was there evidence of a showing that genetic factors are the predominant response difference in MZ or DZ twins, and the measurable influence on female orgasmic function subjects were not aware of any specific hypotheses.
will perhaps stimulate more research into understand- Ideally, we would have used a more standardized ing the underlying biological basis of the female detailed questionnaire such as the GRISS orgasm and sexual dysfunction. This will become increasingly important, particularly as hormonal intercourse frequency and sexual satisfaction. Use of therapies for women (such as testosterone patches) a questionnaire like this may have provided more may be licensed shortly for a condition we know little specific answers. However, such instruments and investigations are lengthy, and inclusion may havereduced the response and hence the number of The TwinsUK cohort collection is supported by the Well- come Trust, the Chronic Disease Research Foundation and errors resulting from the method of classification in the ARC. We thank the twins for their invaluable help andAnn Johnson for help with the questionnaire and Dr Jannini this study are unlikely to be different among MZ and DZ twins. Such errors would result in an under-estimate of the genetic component, and the heritablecomponent may have been higher with more preciseinstruments.
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