it. Some of these processes are similar in both male and female; others are different. Genetic and developmental factors play a role, not least in the regulatory functions of the brain and hormonal system. These biological processes are also found in nonhuman mammalian species, so that generalisations from animal research have some justification, although non-human species are unaffected by the overlay of political, sociocultural, moral, and gender issues.68 The combined activity of the biochemical processes activating and inhibiting sexual motivation at any particular time forms the basis of the prevailing BDP, and this I now outline.
A study of brain regions associated with sexual desire and arousal or inhibition gives insights into the biology of the sexual response. However, it is difficult to separate the brain’s activity as a biological source motivating sexual behaviour, from its role as a processing mediator creating a physiological response to the ideas and perceptions generated by the subjective self. The neuroendocrine system is a key player in sexual motivation and sexual behaviour (and in reward systems more generally), producing various neurochemicals including hormones, peptides and neurotransmitters.69 We find that although neuro-biological systems are involved in both priming sexual interest and in the expression of sexual behaviour, it plays a lesser role in the reason, focus and object of this interest and behaviour (why and with whom I might want to have sex), the latter being mediated by subjective attributions relating to the sexual object. This drive source is also independent of the perceived meanings of behaviour: at this level, it is the sensory experiences that become the stimulus for sexual interest or arousal.
Genetics and sexual predispositions
Biological systems find their origin in genetics. However, the interplay of environment and physiological development can make it difficult to disentangle those aspects of our biological makeup that are fully determined by genetics, and those which are only partially so determined. It is now recognised that many characteristics with a genetic base nevertheless require environmental events to activate their expression. Wieten (2001) observes: ‘the impact of genetic makeup depends on the environment, and the impact of the environment depends on genetic makeup’ (p. 86). While there is no doubt some genetic contribution to a person’s hormonal profile,70 what we do know is that the environmental contribution to that profile is significant. Twin studies are routinely used on the basis that identical twins share identical genetics, while non-identical twins share the same uterine and family environment, but not identical genetics. Strictly speaking, if something is governed by genetics alone, we would expect a 100% correspondence for identical twins, and some smaller percentage for non-identical twins. Typically, when a less than 100% correspondence is found, we talk of genetic predisposition. But this leaves open the question of the nature and extent of such genetic contribution.
When it comes to sexual behaviour, twin research has largely centred on the question of sexual orientation. Questions relating to interpersonal variation in hormonal profiles and temperament and personality factors in sexual attraction and desire have received little attention. Although the nature/nurture debate has long featured in personality research, the extent of the role of genetics in these individual differences remains unclear. Nevertheless, whatever the extent of genetic involvement, research has linked certain personality and temperament characteristics to certain sexual behaviours. For example, extraversion has been linked to having more sexual partners71 and to sexual risk-taking,72 while interpersonal assertiveness and dominance have also been linked to having more sexual partners.73 Also people who are sensation-seeking, that is, who tend to pursue thrilling and risky activities (perhaps to compensate for lower levels of dopamine74) are more likely to be unfaithful75 and to engage in risky sexual behaviours with larger numbers of sexual partners.76
Returning to the question of the genetic contribution to sexual orientation, we find relatively low percentages of such contribution have been found in twin studies. Rosario and Schrimshaw suggest that there is a lack of significant findings in more representative samples of the population when it comes to large familial (sibling versus identical versus non-identical twin) studies. They conclude that up to 50% of homosexual orientation may be attributed to genetic factors ‘of some kind’.77 In fact, feminine behaviour in boys, which the intra-uterine hormonal environment contributes to, appears to be a much stronger predictor of male homosexuality.78 Either way, there remains a significant proportion of men experiencing same-sex attraction for reasons other than genetic predisposition and feminine inclinations. This is certainly also true of women, where a greater incidence of sexual fluidity is reported. 79
Neurochemicals and the sexual response
The biological mechanisms underlying sexual desire, arousal, and expression involve many different elements. Not least of these is the timely release of various neurochemicals, some of which activate sexual priming and some of which inhibit it. The continually changing composition of these neurochemicals forms an important component of the BDP.
Let us first consider the neurochemicals associated with sexual arousal. Dopamine and melanocortins are released in the hypothalamus and limbic regions when a person is exposed to various sexual cues, heightening attention and desire: these associate with generating sexual interest. Dopamine is typically associated with expectation of reward, and in its link to encoding expectation of reward, it plays a role in the conditioning process — but also in addiction. This alerts us to the addictive potential of sexual behaviour.80 Dopamine release has a role in ‘sexual wanting’. It links with anticipation, excitement, desire, concentration, memory, and learning, as well as enhancing the feeling of meaningfulness in things. It has been linked with sociability, but also to social anxiety.81 It is not surprising then, that those with a predisposition to high levels of dopamine are more likely to be involved in sexual activity early, and to have many sexual partners.82 Dopamine interacts with the hormonal profile. Oestradiol and testosterone can facilitate dopamine activation,83 while dopamine can also stimulate hormone release mediating the body’s fight-and-flight response — which may not be so good for the relationship itself. Critical as dopamine activation is to the sexual reward system, it needs to act in concert with other neurochemical and subjective factors to find expression in any particular sexual impulse.
The neurochemicals noradrenaline, oxytocin and vasopressin are also associated with sexual arousal. Noradrenaline (or epinephrine) helps regulate sexual arousal and motivation.84 Meanwhile, oxytocin and vasopressin are released just before orgasm in both men and women, playing a role in pleasure (motivating future sexual activity), but also in emotional attachment to the person with whom the orgasm is experienced.85 This mechanism (along with the role of dopamine) can be understood within the classical conditioning paradigm. Lehmiller (2014) notes that the release of oxytocin plays ‘a vital role in developing bonds between romantic and sexual partners because it is released during physical intimacy’ (p. 99),86 while Toates (2014) suggests that the raised oxytocin levels in women following orgasm could ‘consolidate the incentive value of the partner and sense of belonging with this person’ (p. 108). We see here that belonging is not only created by the sense of shared personal space and experience, but is also supported by a biological link between the sexual event and bonding.
Of course, oxytocin is not only featured in the sexual encounter. It is also released in parent-child bonding which should be anything but sexual. But there is other hormone activity present in the adult sexual profile to distinguish these events — the release of vasopressin and testosterone, for example.87 We see here the finely tuned balance of a neuroendocrine system that lays the biological basis for relational experiences; but also the interplay between the bonding qualities of oxytocin and vasopressin, and the territorial and protective aggressiveness with which testosterone release might be associated. Here the source of the bonding experience which associates with belonging
