Empirical support for an explanation of samesex preferences in incentive motivational terms

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When discussing human sexual incentives in Chapter 4, I mentioned that sexual incentive stimuli in the form of pornographic movies are efficient for enhancing genital blood flow, the classical indicator of sexual arousal. The genital response is an example of the visceral reactions produced by incentive stimuli through their actions on the central motive state. Provided that the responsiveness of the central motive state is kept constant, it can be assumed that the magnitude of the genital response is determined by the incentive stimulus' efficiency. In all the studies that were mentioned in Chapter 4, the incentive stimuli depicted either individuals of the sex opposite to the experimental subject, or heterosexual activities. Furthermore, these studies included only subjects that declared themselves to be heterosexual. Please excuse me for continuing to use a term that I dismissed myself, but I am using the terminology employed by the authors of the studies I have mentioned and that I will mention. Rather than using incentive stimuli of the sex opposite to the subjects, some scientists have varied the incentives and included depictions of same-sex activities (two individuals of the same sex engaged in copulatory behaviors and of the same sex as the experimental subject (samesame-sex) or of the sex opposite to the experimental subject (opposite same-sex) in addition to heterosexual activities. Furthermore, some studies included not only heterosexuals but also male and female homosexuals, vulgarly called gays and lesbians, as experimental subjects.

I will describe one of the early studies (Wincze and Qualls, 1984) in some detail. The subjects' sexual preferences were determined through an interview and ratings on the famous Kinsey Scale and only individuals whose current sexual behavior was exclusively homosexual were included. The italics on 'current' are not there by accident. It is most important to notice that no assumptions as to stability of same-sex preference are made here. Of importance are only these preferences at the moment of test. The female subjects' genital arousal was assessed by vaginal photoplethysmography and the male subjects' arousal by measurement of penile circumference. All subjects were exposed to a series of film fragments. One film illustrated the coastline of Nova Scotia, which was considered as a neutral stimulus. Another film fragment showed a man and a woman engaged in copulatory activities, a third film fragment depicted two men displaying copulatory behaviors, a fourth film showed two women in the midst of sexual activities and the fifth and last film depicted copulatory activities within a group of men and women. These films were presented in counterbalanced order to the subjects. When the changes in vaginal blood flow in response to the various film fragments were analyzed in the homosexual women, it was found that the film fragment depicting sex within a group of men and women produced the largest response. Then followed the film fragment with sex between two women, the fragment showing sex between a man and a woman and, finally, the film illustrating sex between two men, in decreasing order of magnitude. The difference between the group sex, heterosexual sex and lesbian sex movies was not large. The gay movie had a much inferior capacity to enhance vaginal blood flow. These very interesting data show that the presence of a woman (group sex, heterosexual sex, lesbian sex) is the critical variable determining the vaginal response. The sex of that woman's sexual partner has only a minor influence. Here, the obvious conclusion is that visual stimuli produced by and emitted from the female body are powerful sexual incentives for the lesbians. Visual stimuli produced by and emitted from a man's body are less efficient. With regard to the gay men, the film fragment producing the largest increase in penile circumference was the one depicting sexual activities between two men. That film was followed by the group sex movie, which in turn was followed by the heterosexual movie. The film showing two women engaged in sexual activities produced a penile response that was not much different from baseline. Again, film fragments where a man is present are efficient enhancers of genital responses in gay men, while a film showing only women having sex is inefficient for activating sexual arousal. The conclusion we can draw from the gay men's responses is quite similar to that drawn from the lesbian's responses: visual stimuli produced by and emitted from a man have sexual incentive properties for gay men, while stimuli produced by and emitted from the female body have no or very weak sexual incentive properties.

The kind of results obtained in the study described above has been reported in several other studies (e.g. Freund, 1963; Mavissakalian et al., 1975; Sakheim et al., 1985; Freund et al, 1989; Chivers et al., 2004; Rieger et al, 2005). It seems that stimuli emitted by men are powerful sexual incentives for homosexual men and heterosexual women, but not for heterosexual men or lesbians. On the contrary, stimuli emitted by women are powerful sexual incentives for lesbians and heterosexual men, but not for heterosexual women or homosexual men. Thus, exactly as has been proposed many times, sexual preference is determined by the relative incentive value of the available incentives. Measurements of the sexual arousal response of enhanced genital blood flow give an objective indicator of a stimulus' sexual incentive value, making it possible to base the preceding conclusion on experimental facts rather than on speculations.

The mechanisms determining the incentive value of a specific stimulus remain obscure. One possibility that immediately comes to mind is that the central nervous system is predisposed to respond to certain stimuli with enhanced genital arousal. This kind of expression is quite popular in some areas of psychology. The predisposition may be based on structure or on functional organization. All those suggesting a 'biological' basis for same-sex preferences can argue that the central nervous predispositions differ between same-sex preferring and opposite-sex preferring individuals. This is certainly a possibility that must be considered seriously. In fact, it seemingly explains the causes underlying the results of experiments like those mentioned above. There are, however, many serious problems with the notion of predispositions and one of them is that of stability. If 'predisposition' were understood as the momentaneous central nervous responsivity to certain stimuli, it would pose no problem. On the contrary, if it were understood as a constant response tendency in the central nervous system, it would become highly problematic. Most, if not all, of the biological explanations of same-sex preferences share the belief that such preferences are stable throughout the adult lifespan. Consequently, the predispositions must also be stable. We have already seen abundant evidence to the contrary, making the notion of constant predisposition untenable when faced with experimental and other data. By abandoning the notion of stable predisposition and replacing it with momentaneous predispositions, we can avoid one crucial problem with the notion of predispositions. However, other problems remain.

Although it may be convenient to say that the impact of an incentive is determined by momentaneous predispositions of the central nervous system, the explanatory force of this statement is not impressive. Indeed, the very notion of predisposition offers little in terms of cause-effect relationships. To propose that something was caused by a predisposition is nothing more than pronouncing a string of words without any empirical content until that predisposition has been materialized by a neurobiological fundament. So far, this fundament has not been found, neither for opposite- nor for same-sex preferences. On the other hand, if we propose that the impact of an incentive is determined by prior experience with it, we have provided a piece of useful information. We know that experience affects behavior through several mechanisms of learning and negative alliesthesia. Learning mechanisms, with the possible exception of some aspects of social learning, are well understood. What we need to know in order to explain the value of a sexual incentive is simply the individual's reinforcement history with that incentive and all social learning of relevance to that incentive.

We can advance one step more and maintain that we do not care about the individual's learning history because the result of that history is manifest in the individual's response to the incentive stimulus. The sum of history of reinforcement plus social learning regarding the incentive is the determinant of the present response and if we already have the sum we do not get more illuminated by figuring out its components. This is exactly equivalent to one of the examples of scientific explanation I provided in Chapter 1. I am referring to Einstein's second law of gravity, which can be used to describe perfectly the path of a planet, including predictions of its position in a distant future. However, there is no mention of how the planet got in motion. This is entirely irrelevant, because the result of that motion is what we can observe and what we should be concerned about. Speculations about the origin of motion are futile. Regarding the possibility of prediction of the individual's future response to an incentive, it is evident that this can be done as long as we can predict future reinforcement contingencies and the future social learning related to that incentive.

Even though we can consider that an individual's reinforcement history and social learning with regard to a particular incentive is expressed in the present response to the incentive and that the history therefore is irrelevant, a curious mind may persist in asking what this history may look like. A critical mind may go so far as to ask for some empirical data suggesting or showing that reinforcement history and social learning indeed can determine the sexual incentive value of a stimulus. The answer to the critical mind was partly given in Chapter 7, where I mentioned some studies showing that a neutral stimulus may be transformed into a sexual incentive through classical conditioning. The operation of social learning is less understood, as pointed out in Chapter 7, but the insightful analysis presented by Gagnon and Simon (2002) gives some hints. An answer to the demand made by the curious mind requires some more caution. We have no recorded complete history of sexual reinforcement starting in childhood and ending at some arbitrary point in adult life and we have no complete description of all social learning that may have affected the impact of a particular stimulus. I doubt that such a history or description ever will be possible to obtain.

A state of ignorance is not typical for potential sexual incentives. It is typical for most of the incentives of importance for human behavior. For example, the complex incentive of the performance of a Wagner opera at the Bayreuther Festspiele attracts people from all over the world. Most of those succeeding in getting a ticket have been on the waiting list for many years, while some others have paid an exorbitant price on the black market. This shows that the incentive value of these opera performances is high. Despite its considerable importance for human well-being and the European opera culture, we do not know much about how these performances acquired this high incentive value. Likewise, there are people following a bullfighter all over Spain throughout the long fighting season, at great monetary expense and with a considerable investment of time and effort. We do not know how this bullfighter has acquired his most respectable incentive value. Some pages ago, I mentioned the mysteries surrounding the incentive value of stimuli essential for survival, like food or drink. We do not know how a particular taste acquires its incentive value and, far less, its relative incentive value. How come, for example, that for some people the incentive value of the taste of a Laphroig 20-year old single malt whisky is far superior to that of a 30-year old Tallisker?

Having made clear that our knowledge of how a stimulus acquires sexual incentive properties of a particular intensity is not more limited than our knowledge of most other potential incentives, I will summarize the few facts and observations we have concerning the issue.

An extensive study (van Wyk and Geist, 1984) of the role of early sexual experiences for future sexual preferences was performed on a subset of the interview data obtained by Kinsey and collaborators. A very large number of variables were entered into many regression analyses and their capacity to predict adult homosexuality as evaluated by the Kinsey scale was determined. Among the significant predictors for homosexuality in men we find 'being masturbated by the same sex' and 'observing masturbation' when these men were prepubertal. Other important events occurring prior to puberty and predicting adult homosexuality was sexual interactions with adult men. The number of such experiences, the variety of sexual techniques involved, the attainment of orgasm and the total duration of sexual relations with adult men were all predictors of adult homosexuality. On the contrary, post-pubertal heterosexual activities were all negatively related to adult homosexuality. These extremely interesting data show that the amount of sexual interaction with other young boys or adult men, including the attainment of orgasm, before puberty predicts future homosexuality positively. Likewise, the amount of sexual interaction with women after puberty predicts future homosexuality in a negative way.

The frequent sexual interactions with other boys and men allow stimuli emitted by these boys and men to become conditioned sexual incentives. The only requisite for that is that the sexual activities engaged in were reinforcing and there is no reason to doubt that this was the case. Regarding data obtained with future female homosexuals, it was again found that being masturbated by another girl when prepubertal predicts adult homosexuality. On the contrary, intense prepubertal sexual activity, including oral, anal and vaginal penetration, with boys was negatively related to adult homosexuality. Thus, sexual interaction with boys seems to favor heterosexuality. On the contrary, if the first contact with adult male genitals occurred during a prepubertal sexual encounter, then it was more likely that the girl would become homosexual. Likewise, the larger the difference in age between the girl and the male partner, the more likely it was that the girl would turn into a homosexual. After puberty, the amount of heterosexual activity as well as the absence of orgasm when having such activity were related to future homosexuality.

When interpreting these data in terms of conditioning of sexual incentives, we immediately find out that when potentially reinforcing same-sex events, such as masturbation, were performed by another girl, the likelihood for future homosexuality increased. On the contrary, reinforcing sexual interactions with boys reduced that likelihood. An interesting result is the positive relationship between pre-pubertal exposure to male genitals, in the course of physical sexual interaction on one hand, the age difference between the girl and the owner of these genitals on the other and future homosexuality. This fact suggests that the male genitals had become a negative incentive when the age difference was large. It is quite likely that physical sexual interaction with a much older man provokes a strong reaction of fear or guilt or some other aversive state in a prepubertal girl. That aversive experience can easily become associated to a salient stimulus in the situation, the man's genitals. They would thereby turn into a negative incentive, activating the response of withdrawal. Considering that the data employed in this study were collected between 1938 and 1963, a time when sexual encounters between a young girl and a much older man must have been regarded as still more outrageous than it is today, the proposal of a fear or guilt reaction does not seem entirely unwarranted. Independently of this, the data reported in the van Wyk and Geist (1984) study are easily interpreted in terms of classical conditioning of initially neutral stimuli to sexual incentives. Interpretations in other terms are quite difficult, if not impossible.

The results reported in the van Wyk and Geist (1984) study were, as I already mentioned, based on regression analyses. A significant regression means in essence that the predictor variable covaries with the predicted variable. Covariation may be a result of accidental events, or it may be an expression of a cause-effect relationship. Whether one or the other of these alternatives applies to a particular regression analysis is impossible to say. Moreover, the data used for the regression analyses come from interviews where the subjects orally reported their past and present experiences. The veracity of these reports is completely unknown. It is not necessary to suppose that false information was obtained only because the interviewees consciously lied. Nevertheless, in addition to the danger of consciously lying subjects, it is well known that memories are a most unreliable source of information. Their correspondence to facts is frequently only remote. The double uncertainty, caused by the origin of the data on one hand and the regression analyses on the other, means that all the arguments exposed in my discussion of the van Wyk and Geist (1984) paper are no more than informed speculations, at best. I must admit that I included a discussion of this paper under substantial doubt. The reason convincing me to mention it was merely that there are so few data available concerning early sexual experience and future sexual preference that the luxury of choice is almost unavailable.

In addition to the van Wyk and Geist (1984) report based on the Kinsey et al. data, there is an extensive interview study of about 700 homosexual men and almost 300 homosexual women living in the San Francisco bay area (Bell et al., 1981). The explicit purpose of these interviews was to gather information relevant for the development of sexual preference, from childhood until late adolescence. In addition to interviewing the homosexual men and women, identical interviews were made with more than 300 heterosexual men and with 140 heterosexual women. The inclusion of a heterosexual 'control' group is quite unusual in studies of homosexuality and this fact makes the Bell et al. (1981) study stand out. A comparison of interview responses between homosexual and heterosexual individuals gives a lot more valuable information than a simple description of responses within groups of homosexuals, which unfortunately is the commonly used procedure.

Before summarizing the observations of particular relevance to the issues of how neutral stimuli may be transformed into sexual incentives and how the incentive value is determined by experience, a few caveats need to be made. The sexual preference of the interviewees in the Bell et al. (1981) study was determined on the basis of a Kinsey scale score obtained in connection with the interview. Those with a score of 2 or more were classified as homosexuals and those with a score of less than 2 were classified as heterosexuals. The classification as homosexual or heterosexual was probably correct at the moment of the interview, but we cannot know if it had been stable over any extended period of time. This is problematic, as it has been in all studies employing the concept of homosexuality. Another factor limiting the value of all information based on interviewees' recollections of events in a distant past is the uncertain relation between what they remember and what actually occurred in this distant past. Most of the subjects in the Bell et al. (1981) study were about 35 years old at the moment of the interview. This means that they were asked to remember events that had passed somewhere between 25 and 15 years before. As I pointed out only a few paragraphs ago, it is not impossible that unconscious modifications of memory give rise to recollections of events that are quite different from the original event. It is also possible that these unconscious modifications of memory somehow are influenced by our current sexual preferences. We all have a tendency of trying to make our memories of even the remote past coherent with our present life circumstances. Finally, the elegant statistical methods employed by Bell and collaborators are most adequate for detecting coincidences in a data set, but they say nothing about causal relationships. With these caveats in mind, we can turn to the data.

When looking at the recollections of childhood and adolescent experiences in currently homosexual men, we find a few differences between these and heterosexual men that might support the incentive motivational approach. Before the age of 19, only 26% of the currently homosexual men had either masturbated a woman or been masturbated by a woman, while the corresponding figure for heterosexual men was 58%. Likewise, only 18% of homosexual men had achieved orgasm by rubbing their body against a woman's body. The figure was 54% for heterosexual men. Many heterosexual men, 34%, engaged very often in heavy petting with women, while few, 12%, of the homosexuals did so. While 32% of the homosexual men had performed penile-vaginal intercourse before the age of 19 years, 62% of the heterosexuals had done so. Thirty-seven per cent of heterosexuals had experienced cunnilingus or fellatio against 17% of homosexuals.

When comparing the incidence of homosexual behaviors before 19 years of age between homo- and heterosexual men, it was found that 84% of homosexuals had masturbated or been masturbated by another man, while only 21% of heterosexuals had had this experience. About 70% of homosexual men had fellated or been fellated by another man. Only 15% of heterosexuals had been so. Finally, 39% of homosexuals had obtained orgasm by rubbing their body against another man, but only 5% of heterosexuals had experienced the same. These data clearly show that homosexual men had experienced far less sexual interactions with females than heterosexual men had. An obvious consequence is that the possibility of associating sexual reward with female stimuli was much lower for homosexuals than for heterosexuals. On the contrary, homosexual men had experienced far more experiences with other men than heterosexual men had. Thus, the possibility for homosexuals to associate sexual reward with male stimuli was far above that of the heterosexuals. These data, then, coincide perfectly with the hypothesis outlined some pages ago. Learning theory has established that the number of reinforcements is a critical determinant of the amount of learning, or habit strength. The stronger the association between a rewarding event and a stimulus is, the larger is the conditioned incentive value of that stimulus. Consequently, the larger the number of sexual rewards obtained in the presence of male stimuli, the larger the incentive value these stimuli acquire. The differential reinforcement history of male versus female stimuli can explain how the homosexual became a homosexual and how the heterosexual became a heterosexual. This latter question is often ignored, but it is just as intriguing as the question of how homosexuality arises.

Rather than choosing the parsimonious and elegant explanation in terms of incentive learning proposed above, Bell et al. (1981) interpret their data as suggesting that the current male homosexuals as children and adolescents already lacked interest in women. Thus, an incipient homosexuality was already present and caused them to avoid women and approach men. To my humble judgment, this reasoning is to beg the question and enter into a circular argument. The current state, which they wish to explain as a result of childhood and adolescent experiences, suddenly becomes the cause of these childhood and adolescent experiences.

The data obtained in the interviews with homo- and heterosexual women were not identical to those obtained in men. Homosexual women had generally more sexual experiences with other women, but there was not much difference between homo- and heterosexual women with regard to their experiences with men.

However, homosexual women rated the reward value of heterosexual encounters as lower than that of homosexual encounters. The enhanced number of sexual experiences with other women, and their larger reward value, can easily explain how stimuli emitted by women have higher sexual incentive value than stimuli emitted by men. As was the case with male homosexuals, Bell et al. (1981) prefer to interpret their data in terms of incipient homosexuality. This appears to be equally absurd for women as it was for men.

The results of the Bell et al. (1981) study prompt me to propose a hypothesis accounting for the acquisition of same-sex preferences as well as for opposite-sex preferences in terms of differential numbers of reinforcements. We already know that sexual activity ending in orgasm is reinforcing and rewarding. It is also likely that sexual activities ending short of orgasm are rewarding. Thus, most of the sexual experiences an individual has had can be supposed to have been rewarding and reinforcing. Even though the number of sexual experiences is not necessarily identical to the number of sexual rewards and reinforcements, we can assume that the larger the number of experiences, the larger the number of rewards. It does not seem unreasonable to assume that the number of sexual experiences with the opposite sex is normally distributed in a given population. We can also assume that the number of sexual experiences with the same sex is normally distributed. The mean for the distribution of same-sex experiences is probably much lower than the mean of opposite-sex experiences because social pressure, or social learning, favors the latter. Nevertheless, the individuals on the far right of the normal distribution of same-sex experiences would have a larger number of such experiences than a part of those to the left on the normal distribution of opposite-sex experiences. This group of individuals would then constitute those attributing a larger sexual incentive value to members of the same sex than to members of the opposite sex. The fact that the mean differs in the two normal distributions explains how the incidence of same-sex preference is lower than the incidence of opposite-sex preference.

There are certainly events that may modify the simple scheme outlined in the preceding paragraph. Some sexual activities are more likely than others to lead to orgasm and those leading to orgasm have probably a larger reward value than those not doing so. This means that the number of sexual experiences does not correlate exactly with the number and/or intensity of sexual reward and reinforcement. However, in a large population this should be only of minor importance. Furthermore, some individuals may have a strongly aversive sexual experience, dramatically reducing the incentive value of stimuli present during that experience. Although Bell et al. (1981) did not find any evidence for a role of such experiences in the development of homosexuality, they could have some influence in some individuals. Nevertheless, this kind of experience would not be a major factor in a large population.

To end this section, I will present some anecdotal data from a most exotic place, New Guinea, which might seem to challenge the hypothesis presented in the preceding paragraphs. I have not abandoned my skeptical attitude to anecdotes, but

Number of same-sex reinforcements

Number of opposite-sex reinforcements

FIGURE 9.1 Distribution of the number of same-sex and opposite-sex reinforcements in an imaginary population of humans. The distribution to the right shows the number of opposite-sex sexual reinforcements. The mean of that distribution is much superior to that of the distribution to the left, showing the number of same-sex sexual reinforcements. This difference is due to social learning, which strongly favors the obtention of opposite-sex sexual reinforcements. Those within the dotted area have received a larger number of same-sex than opposite-sex reinforcements and will therefore display a preference for their own sex. All others will display preference for the opposite sex.

Number of same-sex reinforcements

Number of opposite-sex reinforcements

FIGURE 9.1 Distribution of the number of same-sex and opposite-sex reinforcements in an imaginary population of humans. The distribution to the right shows the number of opposite-sex sexual reinforcements. The mean of that distribution is much superior to that of the distribution to the left, showing the number of same-sex sexual reinforcements. This difference is due to social learning, which strongly favors the obtention of opposite-sex sexual reinforcements. Those within the dotted area have received a larger number of same-sex than opposite-sex reinforcements and will therefore display a preference for their own sex. All others will display preference for the opposite sex.

this is the only example I have been able to discover where someone has tried explicitly to analyze the acquisition of sexual preferences in terms of social learning. This is interesting, because one of the basic principles of social learning is that the individual does not need to experience reinforcement in order to learn. However, I will use the following account of sexual behavior in New Guinea for illustrating that an interpretation of the anecdotal evidence in terms of reinforcement is equally possible as an interpretation in terms of social learning. It is important to keep in mind that the following paragraphs are an account of something that may have occurred. My summary is entirely based on a paper by Baldwin and Baldwin (1989). Most of the original anthropological accounts were published in a book some years ago (Herdt, 1987).

Sambia is a rather unknown tribe, at least to me, living somewhere on New Guinea. A peculiarity of that tribe is that all boys are isolated from women and housed in a kind of clubhouse from the age of 7-10 until marriage. Before that age, the boys have interacted with their parents and siblings in a way similar to other boys throughout the world. In the male clubhouse, the boys are told that they need to ingest sperm in order to mature and become men capable of impregnating women. The most efficient way to ingest fresh sperm is to fellate other boys or adult men. It appears that not all boys engage in fellatio voluntarily. In that case, they are coerced. The boys have to persist with frequent fellatio for a few years. When puberty arrives, their role changes from fellator to recipients of fellatio. The preceding fellator period does not seem to be associated with any sexual reinforcement since the boys never experience orgasm or even genital stimulation while fel-lating. On the contrary, being fellated to ejaculation should be associated with reinforcement and reward. In fact, it is said that many boys experience pleasure while being fellated. This period approaches an end in the late teens or early twenties, when the boys get married, always to a very young girl. Sambian marriage is somewhat unusual, though, since the husband and wife do not start living together immediately. In fact, they have to wait until the girl has had her first menstruation. Although not living together, wife and husband may interact in several ways during the waiting period. It seems that one typical interaction consists of the man demanding his wife to fellate him. He can also be fellated by younger boys, whenever he should feel a need for that. At menarche, husband and wife start to live together. From now on, the boy is expected to show only heterosexual behavior with his wife and avoid all sexual activities with other men. This seems to occur, since the incidence of adult homosexuality among the Sambia has been estimated to be similar to the one found in Western societies. This assertion must be most uncertain, though, since the anthropologists cannot have any objective data to substantiate it.

The rather long period the boys spent in the clubhouse and interacted sexually with other boys and adult men instead of with women should have offered many opportunities for conditioning of sexual incentives. When experiencing the reinforcing event of being fellated, for example, the boys should associate stimuli emitted by the other boy or the man with the pleasure of fellatio. Since there were no women available for the boys, no association between sexual activities and stimuli emitted by females could have been established. When adult, then, the conditioning in boyhood should manifest itself as a preference for men. So far as the anthropologists have ascertained, this is not the case. There may be two reasons for this. First, the boys had spent years being the fellator, sometimes with coercion, and always without reinforcement and reward. That should be enough for forming associations between male genitals and an aversive event, the act of fellatio. When assuming the role of the fellated and experiencing reinforcement and reward, the aversive nature of the act should sooner or later be lost and eventually be replaced by an experience of positive affect, making it possible to form associations between that affect and the stimuli provided by the fellating boy. This may well be the case. However, after entering marriage the boy is also fellated by his wife, making it possible for him to form associations between stimuli emitted by a female and sexual reward. The result should be that both male and female stimuli are sexual incentives. Nevertheless, the boy now being man will be heterosexual, with rare exceptions. This can easily be explained by social learning. During early childhood he saw his parents in a heterosexual relationship, and in the clubhouse he was told that he should eat sperm in order to become able to impregnate women. Indeed, Sambian society is based on a rather traditional heterosexual couple. The virtues of this social structure are inculcated into the children during early childhood and during the years in the clubhouse, despite the boys' engagement in frequent homosexual activities. Furthermore, these activities are explicitly understood as a step on the road to adulthood and heterosexual marriage. Another important aspect is that the boys are told many stories about the pleasures of sex with women.

At the same time, they are taught that heterosexual activities are strictly forbidden before and outside of marriage. These two messages do not seem to be much different from what most Western boys will hear. The socially transmitted information regarding the pleasures of sex with women may allow the boys to form mental representations of heterosexual activities and these mental representations may function as incentives. At the same time, the teachings about homosexual activities as something typical of maturing boys but unworthy of adult men may cause a devaluation of the possible incentive value of such activities and consequently of sexual incentive stimuli emitted by men.

The combined result of sexual reinforcement and the socially determined mental representations of homo- and heterosexual activities is apparently enough for assuring that the vast majority of adult Sambia men will be predominantly heterosexual. Despite the social encouragement of homosexual activities during part of childhood and adolescence, it appears that the social construction of sexuality is based on the same principles as that occurring in Western societies, as eloquently analyzed by Gagnon and Simon (2002). All this is based on the supposition that the anthropologists' accounts of the behavior of the Sambians are true. The veracity of that supposition is unknown, particularly since the source of the anthropological information is one single research group.

The studies I have presented above do not include any direct, observational data of relevance to sexual learning in the human. As already suggested, such data will perhaps never be available. The uncertainties with regard to the empirical foundations of the accounts given above imply that we should regard these accounts as possible descriptions of how sexual learning may take place. Two kinds of learning, fundamental with regard to the processes by which neutral stimuli acquire sexual incentive properties, were illustrated. The first was classical conditioning. Although this process may be highly important, it is not enough for understanding how external stimuli or mental representations of such stimuli act on the central motive state. The actions of both external and internally generated incentive stimuli are modified by social learning. Some stimuli may get the incentive value, as established by conditioning, enhanced through social learning. Other stimuli may get their incentive value, as established by classical conditioning, reduced because of social learning. Still other stimuli may acquire sexual incentive properties entirely by social learning. Despite these apparent complexities, the mechanisms of classical conditioning and social learning determine the sexual incentive value of a stimulus. Since preference is no more than relative incentive value, these processes also determine sexual preferences in the human.

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