How Sexual Desire Works- The Enigmatic Urge
Page 18
Going from rats to non-human primates and finally to humans, a progressive loosening of the link between hormones and behaviour occurs (Ford and Beach, 1951). Some women are able to experience desire even though they appear to be deficient in hormones. Ford and Beach suggest that a relaxing of hormonal controls is accompanied by an increasing role of so-called higher brain regions.10 These regions mediate learning so that desire reflects knowledge of what is happening, for example concerning contraception and pregnancy.
Monkeys permit controlled studies, which give pointers to the role of hormones in humans (Wallen, 2001). Observations suggest a cycle of female motivation that reflects her underlying hormonal cycle. Such a cycle appears when monkeys are housed in a large enclosure that provides for rich behavioural opportunities. Similarly, where the female is placed in a Skinner box and earns a limited period of access to a male by lever-pressing when she chooses to do so, there is clear evidence of peaks and troughs of motivation, corresponding to her hormonal cycle. Some women might envy the degree of freedom of the subjects in these experiments. When the female is placed in a small cage with a male, the observation that sexual activity occurs throughout the cycle might point to male rather than female choice.
Human sexual desire: general principles
In humans, there appear to be both general and human-specific factors in the link between hormones and sexual desire. This points to a complex interaction between biological and social factors in the determination of behaviour. The fact that humans possess an understanding of the implications of the menstrual cycle for pregnancy doubtless also plays a role.
The so-called ‘sex hormones’ (oestrogens and androgens in women and androgens in men) act on the brain regions concerned with sexual desire (Regan and Berscheid, 1999; Wallen and Lovejoy, 1993). One of their principal roles is to sensitize incentive pathways, thereby making it more likely that a given incentive will trigger approach. Androgens sensitize the dopaminergic input to the nucleus accumbens (Hermans et al., 2010).
Although hormones play a role in desire, there is not a one-to-one link. Thus, there are examples of very low desire in people with normal hormone levels. In keeping with incentive theory, Wallen and Lovejoy suggest that in humans (1993, p. 92) ‘hormones set the basic responsivity of individuals to their sexual environment’. In other words, the link between stimuli and the arousal of desire is modulated by sex hormones, which make sexual interaction more desired and more pleasurable when it happens. Sex hormones increase the frequency of sexual fantasies (Abdallah and Simon, 2007). Is sexual desire idiosyncratic and closed to scientific scrutiny or does it show universal properties rooted in biology? Clearly the answer is ‘both’. For example, fantasy takes many different forms but, whatever its content, the frequency is increased by the presence of hormones.
Human males
Testosterone is the hormone of most importance to human male sexuality. Its level in the blood shows a positive relationship to frequency of sexual interest (O’Carroll and Bancroft, 1984; Regan, 1999). The evidence suggests that testosterone acts on attentional processes such that sexual stimuli increase in salience (Alexander et al., 1997). For men with a hormone deficiency, supplementary testosterone increases sexual desire and frequency of sexual thoughts and fantasy (Jockenhövel et al., 2009). Development of androgen receptor blockers provides some hope of a treatment for sex offenders. These drugs tend to lower desire, but the ability to engage in sexual behaviour once in an appropriate sexual situation is less disrupted than is desire (Wallen and Lovejoy, 1993). However, even surgical removal of the testes does not invariably lead to a loss of all desire (Kinsey et al., 1953).
One study looked at the activity of the brain in men with a low testosterone level (Redouté et al. 2005). Relative to controls, a number of regions were underactive, including the orbitofrontal cortex,11 but their activity was restored to nearer normal after administration of testosterone.
For men, the sexual attractiveness of women’s body odours reaches a peak at the time of the women’s ovulation, which would presumably increase the chances of mating and insemination (Kuukasjärvi et al., 2004). This effect was not found when the odours were from women taking the contraceptive pill. So ovulation, often described as ‘concealed’ in humans, is not entirely concealed.
Human females
From an evolutionary perspective, one might expect cycles in desire corresponding to the female menstrual cycle. In reality, the results are confusing (Ågmo, 2007). Clearly, women are able to have intercourse at any point in the menstrual cycle. A question is whether, left to their own choice and ignoring complications, they desire it at the same intensity throughout.
As might be predicted from functional considerations, desire tends to peak at the time when the chances of fertility are at their maximum: ovulation (Stanislaw and Rice, 1988). Some, but not all, evidence points to women being more responsive to sexual stimuli at this time (Krug et al., 1994). In one experiment, very brief images were flashed on a screen. Images of a naked male were perceived more readily at the time of ovulation. Women made more errors then in ‘seeing’ sexual images that were not actually being presented.
At ovulation, women are most likely to masturbate or initiate sexual activity (Adams et al., 1978). Masturbation is presumably a good index of the hormonal contribution to desire, uncomplicated by issues of pregnancy. It appears that oestrogens, which peak at mid-cycle, are responsible for the effect. Women taking the contraceptive pill show relatively little such fluctuation over the menstrual cycle, this being further evidence of the normal role of hormones. Loss of oestrogens either through surgery or menopause is associated with some loss of desire. During pregnancy there is generally a decline in sexual desire (Regan and Berscheid, 1999), as would be expected from considerations of function. Of course, hormones might well be only one factor underlying this effect. The woman’s knowledge of her pregnancy and feelings of inappropriateness of desire could also contribute.
Gizewski et al. (2006) looked at changes in activation in brain regions concerned with emotional processing over the menstrual cycle.12 In response to viewing erotic films, greater activation was found at the time of maximum fertility and this correlated with subjective reports of the women’s desire.
Subjective reports on fluctuations in sexual desire over the menstrual cycle are not the only source of information and certain techniques allow objective measures. For example, the pupil tends to dilate in response to viewing sexually attractive images. Over the menstrual cycle, researchers investigated its dilation in response to such images consisting of the women’s boyfriends and favourite actors (Laeng and Falkenberg, 2007). The reaction was the greatest, indicating highest sexual interest, in the most fertile (‘ovulatory’) period of the cycle. The authors noted that the pupil reaction is automatic and unconscious. In keeping with a hierarchical understanding, the authors suggested that the pupil reaction gives a ‘raw’ measure, an automatic and unconscious index of sexual attraction. This is uncontaminated by any complex social interpretation, beliefs or inhibition that the conscious mind might link to desire.
Mass et al. (2009) investigated the facial reaction of women to the stimulus of a nude male over the menstrual cycle, specifically the muscular reaction in forming a smile.13 This measure was based upon the logic that a smile is an indicator of positive emotion, a necessary prior condition for sexual desire. The reaction of the muscles in forming a smile was stronger at the time of maximum fertility, as was the subjective measure by the women of their desire. Facial muscles might also present a spontaneous and automatic read-out of desire, unaffected by more consciously mediated ‘higher’ factors.
There is an important social consequence of this cycle of female desire. A significant number of young Americans are now taking pledges of sexual abstinence until marriage. Of course, these pledges sometimes break down and contraception is not being employed. Unfortunately, this is perhaps most likely to happen when the young woman is most fertile.
Intercourse frequency is probably a compromise between male and female wishes. For some women, desire tends to be high immediately pre- and post-menstruation (Ford and Beach, 1951), presumably reflecting the diminished fear of pregnancy at those times combined with a deprivation effect. This illustrates the peculiarly human role of conscious insight, that is knowledge about the chances of pregnancy, which would interact with any more basic effects of hormones on desire. Cultural factors like having Saturday and Sunday as a holiday also play an important role in determining sexual activity (Wallen, 1995). Lesbian couples allow the influence of desire to be seen, unaffected by a male presence and fear of pregnancy. A mid-cycle increase of desire is seen.
Surgical removal of the ovaries, and thereby loss of oestrogens, is not necessarily associated with reduced desire (Ford and Beach, 1951). It is sometimes followed by increased desire, probably resulting from removal of the fear of pregnancy. Where there is a loss of desire, this probably has as much to do with the psychological interpretation of the surgery rather than a hormonal effect. There can be a possible indirect effect of oestrogens on sexual desire. Since their reduction can be associated with reduced vaginal lubrication, the consequences of sexual desire can be less good. This might lower desire.
Testosterone also plays a role in women’s sexual desire and some argue that it is the dominant hormonal influence (Ågmo, 2007). Blood levels of testosterone show a positive link with desire and frequency of sexual thoughts (Regan, 1999). Surgical removal of the adrenal glands and thereby loss of the principal source of testosterone is followed by a reduction in sexual desire. For a group of women having had surgical removal of the ovaries, supplementing oestrogen replacement with testosterone greatly increased reports of desire and arousal (Sherwin and Gelfand, 1987).
Pheromones
Historically, a link between odours and sexual attraction has long been recognized and this link might be assumed to form an important foundation of the perfume industry (Bhutta, 2007). The term ‘pheromone’ refers to a class of chemical that is released into the air by one member of a species and which influences another of the same species. Pheromones have been most extensively studied in non-humans, particularly mice, where they play an important role in mating. Their influence is to trigger behaviour or to alter biological events (e.g. hormonal release) within the body of the recipient animal.
Jacob and McClintock (2000, p. 59) state: ‘We cannot expect humans to behave like moths flying up the concentration gradient toward the desirable source.’ Indeed, evidence suggests a role of pheromones in human sexual desire, albeit a more subtle one than for non-humans. For example, odours from lactating women increase sexual desire in other women (Spencer et al., 2004). For women with a partner, this was manifest by increased expressions of sexual desire towards their partner. For those without a partner, there was increased sexual fantasy. In other words, the pheromone exerts an effect that is not stereotyped. Rather, it varies according to circumstances; the pheromone appears to play a modulating (‘sensitizing’) role in sexual desire. The study suggested that participants had no conscious insight into the relationship between the pheromone and its effect on sexual desire. It is possible to make some functional sense of this phenomenon. The presence of lactating women could signal an environment that is currently favourable for reproduction.
Women prefer the odour of men with symmetrical facial features, pointing to a convergence of attractive qualities stimulating the visual and olfactory senses (Cornwell et al., 2004).
Evidence suggests that, comparing heterosexual men and women, the difference in the target of their desire is associated with differences in the activation pattern of particular brain region (Savic et al., 2001). The triggers are hormone-like substances secreted in the sweat and then vaporized, and so detected through the nose. A part of the hypothalamus is activated by an ‘androgen-like’ substance in women,14 whereas in men a different part is activated by an ‘oestrogen-like’ substance.15 The substances are secreted in the sweat of mainly men and women respectively and could contribute to the direction of heterosexual orientation of desire.
Approach motivation and differences between individuals
Some fortunate individuals experience actual levels of arousal and desire close to their wishes. Others find that their level of arousal is lower than desired and they seek various means to elevate the level, such as by drug-seeking or engaging in high-risk sexual activities (Alcaro et al., 2007). One suggestion is that dopamine is linked closely to arousal. It appears that low levels of dopaminergic activity are associated with low arousal and a search for ways of boosting arousal. This could be part of the process of escalation seen in some forms of sexual behaviour, whereby increasing intensity of activity, for example greater risks, are seen over time (Chapters 17 and 21).
At a general level, differences between individuals in the reactivity of dopaminergic systems appear to underlie differences in the strength of approach to incentives (King et al., 1986). However, it is also possible to discriminate between individual desires. Demos et al. (2012) examined activation of the nucleus accumbens in response to food, erotic or neutral images. The degree of activation in response to erotic images predicted sexual desire expressed in the subsequent months, whereas reactivity to food cues predicted weight gain. The effect was specific – food reactivity did not predict sexual desire/activity nor vice versa. Evidence obtained from people undergoing therapy for drug or food craving suggests that such interventions reduce the activation at the nucleus accumbens. So, craving appears to arise from a balance between activity in regions such as the nucleus accumbens and restraint arising from executive regions.16
Sexual taste
A study looked at two groups of people who had differing subjectively reported tastes in sadomasochistic sex (SM): (a) SM devotees and (b) ‘conventional heterosexuals’ who found the imagery to be disgusting (Stark et al., 2005). The reactions of the ‘conventional heterosexuals’ to conventional erotic images and SM folk to SM images were similar in a number of brain regions known to be implicated in approach motivation. By contrast, the reaction of the brain of ‘conventional folk’ to SM images was very different, being comparable to their reaction to unambiguous disgust-evoking images.17
Desire and disorders
Bipolar disorder
People with bipolar disorder alternate between phases of mania (heightened arousal, activity and goal-seeking) and low mood. In the manic phase, they sometimes exhibit hypersexuality. Krafft-Ebing (1978) described (p. 321):
maniacal exaltation in men, courting, frivolity, and lasciviousness in speech, and frequenting of brothels.
Whether simply a sign of the times, women exhibited more restrained ‘excesses’:
inclination for the society of men, personal adornment, perfumes, talk of marriage and scandals.
The manic phase is associated with excitation of a behavioural activation system and heightened dopamine activity. This is manifest as a range of exaggerated desires, excessive seeking of goals, elation and racing thoughts, relative to normal (Cousins et al., 2009). The elevated wanting is commonly associated with increased sexual desire, as indexed by an increased frequency of sexual thoughts, flirting and sexual behaviour, as in promiscuity. Features of this behaviour are described as ‘impulsive’ and ‘risky’, meaning triggered by a sexual situation without regard to long-term negative consequences (McCandless and Sladen, 2003). The pleasure of anticipation appears to be heightened in this state (Klein, 1987). Children sometimes show flirtatious behaviour towards medical staff (Geller and Tillman, 2004). Occasionally, hypersexuality is one of the first indicators of the appearance of mania (Tsuang, 1975). Treatments include reducing the dopaminergic activity by employing dopamine-blocking drugs. For some p
eople with mania, there is simply a shift between mania and normality. In other cases, phases of depression alternate with mania and the depressed phase is commonly associated with a loss of interest in sex.
For some, manic phases are also associated with increased drug use (Meade et al., 2008). Cocaine can exacerbate the hypersexuality, leading to unsafe sex practices and thereby to a still higher risk of HIV infection. Since cocaine boosts dopamine activity, this points to a double contribution through this route.
Encephalitis lethargica (‘sleeping sickness’)
Evidence from encephalitis lethargica (‘sleeping sickness’) points to the role of dopamine in wanting sex. In one study, patients were treated with the dopamine-boosting substance L-Dopa. Typical of the results are those of Mrs B and Miss R (Sacks, 1976, p. 96):
Previously indifferent, inattentive, and unresponsive to her surroundings, Mrs B became, with each week, more alert, more attentive, and more interested in what was taking place around her.
For Miss R, Sacks noted (p. 108)
captivation or enthralment of gaze…she had been forced to stare at one of her fellow patients, and had felt her eyes ‘drawn’ this way and that, following the movements of this patient around the ward.