Sexual orientation refers to relatively permanent pattern of emotional, romantic, and/or sexual attractions to men, women, or both sexes. Sexual orientation also refers to a person’s sense of identity based on those attractions, related behaviors, and membership in a community of others who share those attractions. Sexual orientation is usually discussed in terms of three categories: heterosexual, referring to those individuals having emotional, romantic, or sexual attractions to members of the other sex; homosexual, gay/lesbian, referring to those individuals having emotional, romantic, or sexual attractions to members of one’s own sex; and bisexual, for those individuals having emotional, romantic, or sexual attractions to both men and women (APA 2008).
Sexual orientation is defined in terms of relationships with others. Sexual orientation is distinct from other components of sex and gender, including biological sex which defines the anatomical, physiological, and genetic characteristics associated with being male or female, and gender identity that defines the psychological sense of being male or female and social gender role, as the cultural norms that define feminine and masculine behavior. People express their sexual orientation through behaviors with others, including emotion as desiring and loving, fantasies as dreaming about, and actions as holding hands or kissing. Sexual orientation is the intimate personal relationships that produces and generates love, attachment, and intimacy. In addition to sexual behaviors, hence, these bonds include nonsexual physical affection between partners, shared goals and values, mutual support, and ongoing commitment. Sexual orientation is not merely a personal characteristic within an individual. One’s sexual orientation also defines the group of people in which one is likely to find the satisfying and fulfilling romantic relationships that are an essential component of being an individual (APA 2008). Research over several decades has demonstrated that sexual orientation ranges with different frequencies, along a continuum, from exclusive attraction to the other sex to exclusive attraction to the same sex (APA 2008).
Development of Sexual Orientation
Prospective studies suggest that childhood cross-sex-typed behavior is strongly predictive of adult homosexual orientation for men and for women. Though methodologically somehow problematic, retrospective studies are useful in determining how many homosexual individuals displayed cross-sex behavior in childhood (Bailey and Zucker 1995; Burri et al. 2015). On this line, a sociological perspective proposes that biological variables, such as genes, prenatal hormones, and brain neuroanatomy, do not code for sexual orientation per se, but for childhood temperaments that influence a child’s preferences for sex-typical or sex-atypical activities and peers. These preferences might lead children to feel different from opposite or same-sex peers–to perceive them as dissimilar, unfamiliar, and exotic. This, in turn, according to certain researchers, could produce heightened nonspecific autonomic arousal that subsequently gets eroticized to that same class of dissimilar peers, in other words: Exotic becomes erotic (Bem 1996).
According to another perspective, sexual orientation, at least in males, could be influenced by prenatal influences on the mother over the fetus, according to the number of previous male pregnancies she has experienced. This hypothesis called fraternal birth order influences sexual orientation and could be interpreted as a sort of a progressive immune reaction of the mother to subsequent male pregnancies. According to this hypothesis, the probability to develop a homosexual orientation grows around 30% for each older brother the subject has (Blanchard 2001).
Explicit and Implicit Methods to Measure Sexual Orientation
The most common method to measure in adults sexual orientation is the Kinsey scale, which is composed by a series of question measuring self-evaluation on a numerical scale ranging from 0 to 6 on erotic attraction, fantasies, emotions about other or same sex and then averaged into a final score ranging from 0 complete heterosexuality to 6 complete homosexuality and intermediate values measure level of bisexuality (Kinsey et al. 1948). However, self-reported questionnaires on explicit measures of sexual orientation yield significant underestimations of nonheterosexuality. Recently, nonheterosexual preferences can be explored in the general population through a sexual preference implicit association test (sp-IAT) or other implicit or veiled methods (Camperio Ciani and Battaglia 2014). The Kinsey questionnaire strongly dichotomized the responses as either attracted to females or attracted to men, whereas the implicit methods as sp-IAT showed a wider distribution of responses from gynephilia to androphilia at least in males sexual orientation. These implicit methods could be a novel instrument useful to better ascertain the true sexual orientation distribution in natural human populations.
Frequency of Sexual Orientation
The prevalence of gay, lesbian, and bisexual orientation may largely vary over time and geographical region. Homosexual and bisexual orientation is not limited to Western developed counties, but is present in almost all human populations, always with relatively low frequency, or prevalence. However, not all researchers agree with a universal distribution of sexual orientation variants across cultures (Barthes et al. 2013). Population-based surveys have suggested prevalence of 2–9% for lesbians, compared with 0.5–15% for gays, with a more general agreement on 2–7% in most Western countries for both genders. The distribution of bisexuality is somehow less clear, in males is lower than male homosexual orientation, while in females it is higher than the frequency of lesbians (Diamond 2008). It should be noted that sampling problems affect surveys on sexual preference, as well as the location, whether urban or rural, and the method used, etc. The above figures should thus be considered as fairly uncertain estimates producing no final consensus. Further, prejudice, persecution, homophobia, and religious orthodoxy might induce individuals not to reveal, even in anonymous questionnaires, their own sexual orientation, creating an “obscure number” of individuals who will never explicitly disclose such information (Camperio Ciani and Battaglia 2014; Savin-Williams 2006).
Flexibility of Sexual Orientation
A noticeable sex difference in sexual orientation frequency is that the distribution of homosexuality and bisexuality is very different among genders. While it is polarized with a U-shaped distribution for males, with a relative higher frequency of complete or almost complete heterosexuals or homosexuals and relatively few bisexuals. In contrast, females show a different distribution progressively declining in frequency from complete heterosexuality toward complete homosexuality (Camperio Ciani et al. 2015). Further differences between female and male sexual orientation distribution point to a relatively higher shared (social, educational) environmental influence in female sexual orientation rather than males. Sexual orientation in females seems more flexible in response to shared-environmental factors compared with males. Females might express sexual preference for females also because of feminist, political, egalitarian reasons, in addition to being driven to homosexuality by erotic or emotional attraction. More females than males appear to have switched from heterosexual to homosexual preference, even after a satisfactory period of heterosexuality and vice versa. Females may maintain their sexual fluidity well into adulthood on the contrary the sexuality of males becomes relatively fixed by young adulthood (Diamond 2008).
Causes for Different Sexual Orientation
There is no consensus among scientists about the exact reasons why an individual develops a heterosexual, bisexual, gay, or lesbian orientation. Although extensive research has examined the possible genetic, hormonal, developmental, social, and cultural influences on sexual orientation. No conclusive findings have emerged that permit scientists to affirm that sexual orientation is determined by any particular factor or factors. In generating sexual orientation, as many other aspect of social behavior, nature and nurture both play complex roles. Further, most people experience little or no sense of choice about their sexual orientation (APA 2008). It should be noted that among the biological influences on sexual orientation a partial genetic influence is quite clear. Multiple independent causes, genetic and environmental, concur to establish same-sex preference in humans, with noticeable differences in the expression of this phenotype in males and females (Jannini et al. 2010).
Genetic Influence on Sexual Orientation
The observed population distribution of sexual orientation, such as frequency (prevalence), distributions, family clustering, pedigree asymmetries, and sibling concordances, point to genetic heritability of both male and female sexual orientation. These data point to a low but significant genetic influence on sexual orientation, with a low-penetrance and partial-expressivity, for homosexual orientation in both genders. A recent review reporting over two decades of evidence from family studies, comparing pairs of adoptive brothers, biological brothers, dizygotic and monozygotic twins, showing that the probability of homosexual sibling concordance in all such classes progressively increase if the proband has an homosexual orientation, which strongly suggests the presence of a genetic influence (Camperio Ciani et al. 2015). Further, the much lower rate of sexual orientation concordance between adopted sisters as compared with biological sisters is in line with the genetic heritability paradigm. Overall, twin studies, despite their large heterogeneity, do suggest human sexual orientation to be influenced by a significant genetic component, with a likely larger effect in males than females. After earlier work on the population genetics of sexual orientation, the more recent debate has considered diverse selection mechanisms, including direct selection modes in which the fitness is solely influenced by an individual’s genotype, based on overdominance, or sexually antagonistic selection, and other selection modes for male homosexuality including maternal effects and genomic imprinting, possibly involving epigenetic activity (Gavrilets and Rice 2006; Camperio Ciani 2014).
The possibility of a genetic influence on sexual orientation has posed a long-lasting Darwinian paradox. How genetic factors influencing relatively exclusive homosexuality can evolve and be maintained in the population if they are carried and expressed by individuals that reproduce significantly less then heterosexuals. For male homosexuality, about which there exist adequate population data, the systematic analysis of the evolutionary propagation mechanisms eliminates the possible Darwinian paradox associated with a genetic influence on homosexual orientation, resolving it within the framework of sexual conflict, suggesting that the same genetic factors influencing homosexuality in males produce in relative females an increased fecundity thus balancing fitness (Camperio Ciani et al. 2004).
Sexual antagonism for a multi-locus factor with at least an X-linked locus is the selection model providing closest adherence to all the empirically known patterns for both homosexual orientation of males (distribution, frequency, generational, persistence) and higher-than-average fecundity for females only in their maternal line. These findings point, with a particularly relevant example, to the occurrence of a first well-identified sexually antagonistic character in humans (Camperio Ciani et al. 2008). This perspective may help shift the focus away from male homosexuality preference per se. Rather than concentrating on the sole aspect of the reduced male fitness that it entails, this places male homosexual orientation within the more general sexual conflict framework of a genetic trait with gender specific benefits, which may have evolved by increasing the fecundity of females, and neither disappears nor completely invades the gene pool (Camperio Ciani et al. 2015). In females, sexual orientation is less clearly understood and further studies are necessary to understand the genetic influence on it (Rice et al. 2013). Asexuality is some time improperly defined as asexual orientation. Asexuality, as the absence of desire for sexual relationship pertains more to the intensity of sexual desire rather than a specific sexual orientation. Asexuality can be lifelong or more frequently interest only a specific life phase such as adolescence and senescence.
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