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Article Excerpt
In a special issue of the Journal of Sex Research, Weis (1998) pointed out that the majority of sex research appeared to be atheoretical and that, although various theoretical models of relevance existed in the literature, they were seldom used. Fifty years earlier, Kinsey, although not explicitly theoretical in his sex research, had recognized the phylogenetic mammalian origins of much of human sexuality. The guiding theme in both his earlier entomological research and his sex research was individual variability, and, for the latter, he developed an exceptionally long and detailed interview to document this variability (Kinsey, Pomeroy, & Martin, 1948; Kinsey, Pomeroy, Martin, & Gebhard, 1953). More recently, Kinsey Institute researchers introduced a theoretical model of sexual response, the Dual Control Model (Bancroft & Janssen, 2000), based on the interaction of sexual excitation and inhibition in the brain. With this model, we aim to conceptualize individual variability in sexual responsiveness in ways that can be systematically measured in men and women, thus allowing the formulation and testing of a range of hypotheses relevant to human sexual behavior. Although the Dual Control Model has cross-species relevance, the focus of this article is on the human; in particular, we present the development of measures to assess individual variability in propensities for sexual excitation and sexual inhibition and the use of such measures in research over the past 8 years. (For a recent review of the underlying mechanisms, see Bancroft, 2009). We conclude with some suggestions for further development.
The Dual Control Model is an example of "a conceptual nervous system," a phrase introduced by Hebb (1949) to describe a theoretical model of brain function that accounts for observed behavior and precedes a conclusive neurophysiological explanation. Gray's (1987) model of Behavioral Activation and Behavioral Inhibition, which led to a rich body of research on relevant brain mechanisms in the rat, is another good example and of considerable relevance to our Dual Control Model of sexual response. Theoretical models of this kind have two principal purposes. First, they provide a conceptual framework that helps organize thinking about the complexities of human behavior, the underlying psychological and neurophysiological mechanisms, and the way in which those mechanisms interact with social and cultural factors. Second, they allow formulation of testable hypotheses. In these ways, such models may prove to have heuristic value and are likely either to be modified as a result of their use or abandoned for new and better models. The crucial point is that they are models rather than precise descriptions of reality.
It is generally accepted that most brain functions involve both excitatory and inhibitory processes. To understand how this dual process leads to specific behaviors relevant to sexuality, it is useful to distinguish between these processes at a systems level. Bancroft (1999) reviewed the available neurophysiological evidence for the existence of such systems in the area of sexual functioning and behavior. In animal research, more attention has been paid to the excitatory system, reflecting the fact that it involves relatively discrete anatomic structures and pathways that can be studied by lesion experiments, whereas inhibition results from more diffuse and less easily manipulated structures and mechanisms. In research involving humans, particularly in psychophysiological studies of information processing and sexual arousal, attention has also focused on the excitation process and the various ways that excitation may be impaired (e.g., by distraction). However, for various reasons reviewed by Bancroft (1999), it has become apparent that, in addition to excitation, active mechanisms of central inhibition also needed to be considered, leading to our Dual Control concept (Janssen & Bancroft, 1996). Subsequently, with the use of functional brain imaging in studying sexual arousal, strong evidence of inhibitory brain mechanisms relevant to sexual response has emerged (reviewed by Stoleru & Mouras, 2007). This evidence is considered further later in this article.
A key characteristic of our Dual Control Model is its focus on individual variability. We make three basic assumptions:
1. Neurobiological inhibition of sexual response is an adaptive pattern relevant across species, which reduces the likelihood of sexual response and recognizes the distracting effects of sexual arousal occurring in situations when sexual activity would be disadvantageous or dangerous, or would distract the individual from dealing appropriately with other demands of the situation.
2. Individuals vary in their propensity for both sexual excitation and sexual inhibition. Whereas for the majority, these propensities are adaptive and non-problematic, individuals with an unusually high propensity for excitation or a low propensity for inhibition are more likely to engage in high-risk or otherwise problematic sexual behavior. Conversely, individuals with a low propensity for sexual excitation or a high propensity for sexual inhibition are more likely to experience problems with impairment of sexual response (i.e., sexual dysfunctions).
3. Although sexual arousal typically occurs in interactions between two or more individuals, and the context and cultural meanings or scripts attributed to these interactions are important sources of stimuli, both excitatory and inhibitory, the effects of such stimuli are mediated by psychological and neurophysiological characteristics of the individuals involved, influenced by both genetic factors and early learning.
The Concept of Sexual Inhibition
The focus on inhibitory mechanisms per se, and in particular on the concept that these mechanisms are in most cases adaptive, opens up substantial new opportunities for understanding "normal" sexuality, individual variability, and problematic sexuality; it also has considerable relevance to the clinical assessment and management of sexual problems, as well as interventions to reduce high-risk sexual behaviors. Whereas the function of sexual excitation is relatively apparent, the function of inhibition, with its possible underlying mechanisms, warrants further consideration. The following five adaptive functions of inhibition of sexual response have been postulated for men (Bancroft, 1999):
1. When sexual activity in a specific situation is potentially dangerous or disadvantageous (this would include not only physical threats but also the threat of negative emotional or interpersonal consequences).
2. When a nonsexual challenge occurs and suppression of otherwise distracting response patterns, including sexual, is necessary for focusing on the appropriate coping response.
3. When excessive involvement in the pursuit of sexual pleasure distracts from other important adaptive functions.
4. When social or environmental pressures result in suppression of reproductive behavior and reduction of population density.
5. When the consequences of continued excessive sexual behavior includes, in men, reduction of fertility due to excessive ejaculation.
These five functions have cross-species relevance. However, the fourth function, although of potential importance in humans, is less clearly relevant to humans. There is no evidence that either reproductive behavior or fertility is reduced in conditions of overcrowding or poverty.
Bjorklund and Kipp (1996) made a convincing case for inhibitory mechanisms being more crucial and, hence, better developed in women. Of the prior five male functions, the first three are likely to be relevant to women. The first is of particular importance because of the risks of pregnancy in disadvantageous circumstances. The fourth, as with men, is relevant as a negative impact of social or environmental pressures, although not apparently relevant to women. The fifth function may also not be relevant to women. A further function, the inhibition of female sexual responsiveness to restrict sexual activity to the fertile phase of the reproductive cycle, occurs across most species, but not with most primate or human females. Gender differences, particularly in the human, may reflect sociocultural, as well as biopsychological, factors. Thus, if women have more enhanced sexual inhibitory mechanisms than men, they may be more susceptible to sociocultural suppression of their sexuality (Bancroft, 2009).
Given this range of potential functions, it is not surprising that evidence of more than one type of inhibition is emerging. Recent investigators of functional brain imaging in response to sexual stimulation have revealed a number of different relevant mechanisms. This is a new area of research, as yet limited and predominantly focused on the response to visual erotic stimuli. The conclusions at this stage should be considered preliminary and of less certain relevance to overt sexual behavior. However, on the basis of this evidence, Redoute et al. (2005) postulated three components of sexual inhibition, along with their neurological origin:
1. Inhibitory processes operating in the resting state and imposed by the temporal lobes are evident in brain imaging by predictable areas of deactivation in the temporal lobes that precedes or accompanies sexual response.
2. Processes that limit the development of sexual excitation once it has been initiated, particularly in terms of its active expression, are mediated, they suggested, by the caudate nucleus and the putamen.
3. Cognitive processes relevant to problems of low sexual desire, which involve devaluation of potential sexual partners, result from a lack of deactivation of the medial orbito-frontal cortex.
The first component can be conceptualized as inhibitory tone that needs to be lowered for sexual response or arousal to occur. What is not yet clear is whether this temporal lobe-based inhibitory tone relates to what has been called inhibitory tone in the periphery (Bancroft & Janssen, 2000). In men, for example, tonic constriction of the smooth muscles of the erectile tissues needs to be reduced to allow erection to occur. It is also unclear what relevance this form of peripheral inhibitory tone has to the sexual response of women. Although it is less likely to be involved in vaginal response, a uniquely female function involving increased vaginal blood flow to enhance vaginal lubrication, it may be important for clitoral response, which is homologous to penile tumescence (a comparison considered more closely in Bancroft, 2009).
The second component may be relevant to reactive inhibition, and reflects what some women have described as "putting the brakes on" in situations when becoming sexually aroused could be disadvantageous or risky (Graham, Sanders, Milhausen, & McBride, 2004). It is noteworthy that this particular pattern was observed in brain imaging studies involving men, when sexual arousal occurred in a laboratory context, resulting in some restraint in its expression (Redoute et al., 2005).
The third component is interesting given its focus on devaluation of potential sexual partners. This response does not fit with conventional concepts of reactive inhibition to a sexual threat; it may prove to be an example of how the advance in knowledge of brain activity based on brain imaging studies requires reconceptualization (and possible revision of models) of how the brain works.
The lack of sexual responsiveness that affects some people when they are stressed may reflect increased inhibitory tone, but it may also involve an impairment of excitation. This lack of responsiveness may also apply to the post-ejaculatory refractory state in men, the mechanisms of which are not well understood. The limited evidence of the neurophysiological basis of "sexual satiation" in the rat suggests a complex pattern (reviewed in Bancroft, 1999).
Overall, it is important to keep in mind that our theoretical model of inhibition, even allowing for a distinction between inhibitory tone and reactive inhibition, is probably an oversimplification.
Development of Measures of the Propensities for Sexual Excitation and Sexual Inhibition
Having formulated our theoretical model, the next requirement was to develop instruments for measuring the postulated individual variability in propensities for sexual excitation and sexual inhibition. This process has been carried out in two stages, as we first developed a questionnaire for men and then one for women.
The Sexual Inhibition/Sexual Excitation Scales (SIS/ SES; Janssen, Vorst, Finn, & Bancroft, 2002a)
The method of developing the men's questionnaire involved formulating a range of sexual stimuli and situations, some potentially exciting without any obvious threat involved, others threatening (i.e., involving risk, danger, or likelihood of some negative consequence), as well as potentially sexually exciting. The items were written in an "if--then" format, with ratings on a 4-point scale ranging from 1 (strongly agree) to 4 (strongly disagree). For items relevant to excitation, the "if" statement described a potential sexual stimulus or situation (e.g., visual, tactile, imaginary, social), and the "then" statement described the occurrence of a sexual response. The majority of the inhibition items were written to reflect situations in which existing sexual arousal is lost due to the introduction of some intrapersonal or interpersonal threat (e.g., negative consequences of having sex, performance-related concerns, norms and values, and physical and psychological harm). Instructions included asking participants to respond based on how they would "most likely" respond in a particular situation. Feedback on the initial questionnaire was obtained from both laypersons and sex researchers.
The first version of this measure had 73 items. Factor analysis of the results from a sample of 408 male undergraduate psychology students (mean age = 22.8 years) identified 10 factors, involving 45 items. Further factor analysis of the 10 subscale scores identified a single excitation factor (SES) and two sexual inhibition factors that, based on the items involved, were called Inhibition Due to Threat of Performance Failure (SIS1) and Inhibition Due to Threat of Performance Consequences (SIS2; see Appendix A). Confirmatory factor analysis (CFA) of data from two further samples of men, one consisting of undergraduate psychology students (N=459; mean age=20.9 years) and the other a random sample of university employees and men from the local community (N = 313; mean age = 46.2 years), was carried out. This showed the 10-factor model to be best, but only marginally better than the nested 3-in-10 model. Therefore, further research focused on the 3-factor structure. Correlations between the SES and the two SIS scores were low and nonsignificant, indicating that the excitation and inhibition factors were relatively independent. A significant but low correlation (+.28) between SIS1 and SIS2 showed only modest overlap between these two factors.
The SIS/SES questionnaire has now been used in a number of large convenience samples, some of which are reported later in this review. To date, only one study has used the questionnaire in a representative sample (Varjonen et al., 2007). From a large population-based twin sample, 1,289 male, 33- to 43-year-old Finnish twins were recruited (a 36% response rate). The findings relevant to genetic effects are considered later. The authors randomly split the twin sample in two and conducted exploratory and confirmatory analyses in each subsample. A three-factor structure, comparable to the original one, was reported, although there were some differences in the factor structure and in the extent to which specific items loaded on the factors. Out of the original 45 items, 7 items were dropped because of low factor loadings (<.35) in one of the two subsamples (1 SES item and 1 SIS2 item), complex loadings (2 SIS1 items and 1 SIS2 item), or a skewed response distribution (2 SIS1 items). In addition, one item was excluded from the study due to a technical error. The CFA showed that the best-fitting model included SIS1, SIS2, and SES--the first two as main factors and the last as consisting of three subfactors. The majority of model-fit criteria were met for this factor structure.
In the original psychometric validation of these scales, reasonable test retest reliability was demonstrated (SES: +.76, SIS1: +.67, SIS2: +.74; Janssen et al., 2002a). To assess to what extent our questionnaire assessed distinctly sexual rather than general inhibition or excitation tendencies, scores were correlated with the Behavioral Inhibition/Behavioral Activation Scales (BIS/BAS; Carver & White, 1994). The three subscales of the BAS all correlated with SES (+.31 to +.22); BIS correlated with SIS2 (+.22); and, unexpectedly, BIS correlated positively with SES (+.21). Modest correlations were found between SIS1 and SIS2 and the Harm Avoidance Scale from the Minnesota Personality Scale (+.22 and +.28, respectively; Tellegen & Waller, 1994) and SES and neuroticism (-.22; Eysenck & Eysenck, 1975). In a later dataset, involving 880 heterosexual men, the trait measure of anxiety on the StateTrait Anxiety Inventory (STAI; Spielberger, Gorsuch & Lushene, 1970) correlated with SIS1 and weakly with SIS2 (+.25 and +.11, respectively; Janssen, 2008).
Somewhat higher correlations were found with established measures of sexuality (Janssen et al., 2002a). The Sexual Opinion Survey (SOS; Fisher, Byrne, White, & Kelley, 1988), which assesses erotophilia-erotophobia, correlated +.45 with SES and -.29 with SIS2. The Sociosexual Orientation Inventory (SO1; Simpson & Gangestad, 1991),...
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