The ICD is also used to direct clinical care and research, allocate resources and monitor progress in achieving public health goals. The classification is organized into 21 chapters, each containing disease or health-related categories or both, including:. F categories include the F categories: psychological and behavioural disorders associated with sexual development and orientation. Z categories include the Z categories: counselling related to sexual attitude, behaviour and orientation.
Although F66 categories mention gender identity, historically the categories emerged from earlier classifications of sexual orientation. The Working Group recommends that the F66 categories should be deleted in their entirety. In this paper, the authors, who participated in the Working Group, summarize the rationale for this recommendation, with particular reference to concerns about sexual orientation. Sexual orientation is a contentious topic: internationally, homosexuality and other forms of expression of same-sex orientation are stigmatized.
Although ICD-6 classified homosexuality as a sexual deviation that was presumed to reflect an underlying personality disorder, subsequent research did not support this view. These changes reflect both emerging human rights standards and the lack of empirical evidence supporting the pathologization and medicalization of variations in sexual orientation expression. Here, we consider several issues raised by the presence of the F66 categories in ICD and how these issues have influenced the recommendations made by the Working Group. An overriding issue is whether the F66 categories capture unique mental disorders, which raises the core question: What is a mental disorder?
However, it may be so broad that it could also include clinically recognizable syndromes such as grief responses to bereavement or reactions to everyday problems — syndromes that were not intended to be viewed as mental disorders. If a disorder is present, an appropriate diagnosis i. In addition, ICD also recognizes that factors other than mental disorders may lead to behaviours or presenting complaints that could be misinterpreted as symptoms of disorders.
In addition, social or political disapproval has at times resulted in the abuse of diagnoses — especially psychiatric diagnoses — to harass, silence or imprison people whose behaviour violates social norms or challenges existing authority structures. Sexual orientation refers to a persistent tendency to experience sexual attractions, fantasies and desires and to engage in sexual behaviours with partners of a preferred sex.
When individuals categorize themselves on the basis of their own sexual attractions, desires and behaviours, they are described as adopting a sexual orientation identity: for example, gay, lesbian or heterosexual. The causes of sexual orientation are unknown but are likely to reflect some mixture of genetics, prenatal hormonal exposure, life experience and social contextual factors. Four important conclusions can be drawn from surveys of sexual behaviour in several countries. Second, patterns of reported sexual identity and behaviour vary with sociodemographic characteristics, such as sex, age and race or ethnicity.
For example, men are more likely to identify as gay rather than bisexual, whereas the reverse is the case for women. Third, there is evidence that inconsistent sexual orientation expression is associated with social and economic factors rather than psychopathology. Fourth, sexual orientation identity is not fixed for everyone and changes that occur throughout life do not always follow a linear pathway in or out of heterosexuality or homosexuality.
These studies also found substantial variability in patterns of sexual expression both between individuals and within individuals across time. The patterns observed in adolescents differ from those observed in adults and are consistent with the gradual acquisition of experience with sexuality and the formation of close relationships. Among individuals with same-sex behaviour, attractions, or identity, a variable pattern is the norm rather than the exception. Given this variability, it is difficult to identify a distinct pattern of abnormal sexual orientation expression.
Further, variation alone is an insufficient criterion for diagnosing a mental disorder. There is strong evidence that sexual orientation can be associated with substantial social stress. Consequently, the clause on the exclusion of social deviance in the ICD is particularly relevant in reviewing the F66 categories.
If a disease label is to be attached to a social condition, it is essential that it has a demonstrable clinical utility, for example, by identifying a legitimate mental health need, and its use should not exacerbate existing stigma, violence and discrimination. Presumed disruption of this hypothesized process is the conceptual basis for sexual maturation disorder. An immediate concern is whether sexual maturation disorder conflates developmental patterns within the normal range with pathological processes. Research repeatedly demonstrates that indicators of emerging same-sex sexual orientation are time-varying in their appearance, with the process beginning typically in late childhood or early adolescence.
In , even this diagnosis was removed. However, the concept was incorporated into ICD, which was approved in , as part of a set of changes parallel to those made in the Diagnostic and Statistical Manual of Mental Disorders more than a decade before. Evidence shows that lesbian, gay and bisexual individuals often report a higher level of distress than heterosexuals.
However, the elevated distress has been linked robustly to greater experiences of social rejection and discrimination. There are several socially stigmatized conditions, such as physical illness or poverty, 42 , 43 that are also likely to lead to distress. Sexual relationship disorder describes a clinical syndrome in which an abnormal sexual preference or gender identity makes it difficult to form or maintain a relationship with a sexual partner.
Generally ICD diagnoses reflect individual-level disturbances but the disturbance in sexual relationship disorder is dyadic by definition. Difficulties in relationships with sexual partners are commonplace and occur for many reasons. Moreover, ICD does not include a classification for relationship disorders due to other potentially contributory factors. In a given social science study, the concepts mentioned above will often each have its own particular operational definition for the purposes of research.
But they cannot all mean the same thing. Strong interest in finding a companion, for example, is clearly distinguishable from physical arousal. The philosopher Alexander Pruss provides a helpful summary of some of the difficulties involved in characterizing the related concept of sexual attraction:. Does it mean to have a tendency to be aroused in their presence? But surely it is possible to find someone sexually attractive without being aroused. Does it mean to form the belief that someone is sexually attractive to one?
Surely not, since a belief about who is sexually attractive to one might be wrong — for instance, one might confuse admiration of form with sexual attraction. Does it mean to have a noninstrumental desire for a sexual or romantic relationship with the person? Probably not: we can imagine a person who has no sexual attraction to anybody, but who has a noninstrumental desire for a romantic relationship because of a belief, based on the testimony of others, that romantic relationships have noninstrumental value. But if the concept of sexual attraction is a cluster of concepts, neither are there simply univocal concepts of heterosexuality, homosexuality, and bisexuality.
The problem is neither irresolvable nor unique to this subject matter. Other social science concepts — aggression and addiction, for example — may likewise be difficult to define and to operationalize and for this reason admit of various usages. It is also important to bracket any subjective associations with or uses of these terms that do not conform to well-defined scientific classifications and techniques. It would be a mistake, at any rate, to ignore the varied uses of this and related terms or to try to reduce the many and distinct experiences to which they might refer to a single concept or experience.
As we shall see, doing so could in some cases adversely affect the evaluation and treatment of patients. W e can further clarify the complex phenomenon of sexual desire if we examine what relationship it has to other aspects of our lives. To do so, we borrow some conceptual tools from a philosophical tradition known as phenomenology, which conceives of human experience as deriving its meaning from the whole context in which it appears. The whole set of inclinations that we generally associate with the experience of sexual desire — whether the impulse to engage in particular acts or to enjoy certain relationships — does not appear to be the sole product of any deliberate choice.
Our sexual appetites like other natural appetites are experienced as given, even if their expression is shaped in subtle ways by many factors, which might very well include volition. Indeed, far from appearing as a product of our will, sexual desire — however we define it — is often experienced as a powerful force, akin to hunger, that many struggle especially in adolescence to bring under direction and control.
What seems to be to some extent in our control is how we choose to live with this appetite, how we integrate it into the rest of our lives. But the question remains: What is sexual desire? What is this part of our lives that we consider to be given, prior even to our capacity to deliberate and make rational choices about it? We know that some sort of sexual appetite is present in non-human animals, as is evident in the mammalian estrous cycle; in most mammalian species sexual arousal and receptivity are linked to the phase of the ovulation cycle during which the female is reproductively receptive.
Whatever the explanation for the origins and biological functions of human sexuality, the lived experience of sexual desires is laden with significance that goes beyond the biological purposes that sexual desires and behaviors serve. This significance is not just a subjective add-on to the more basic physiological and functional realities, but something that pervades our lived experience of sexuality. Perhaps sexuality, like other human phenomena that gradually become part of our psychological constitution, has roots in these early meaning-making experiences.
If meaning-making is integral to human experience in general, it is likely to play a key role in sexual experience in particular. And given that volition is operative in these other aspects of our lives, it stands to reason that volition will be operative in our experience of sexuality too, if only as one of many other factors. This is not to suggest that sexuality — including sexual desire, attraction, and identity — is the result of any deliberate, rational decision calculus.
It might be more accurate to say that we gradually guide and give ourselves over to them over the course of our growth and development. This process of forming and reforming ourselves as human beings is similar to what Abraham Maslow calls self-actualization. In the picture we are offering, internal factors, such as our genetic make-up, and external environmental factors, such as past experiences, are only ingredients, however important, in the complex human experience of sexual desire.
These historical examples bring into relief the complexity that researchers still face today when attempting to arrive at clean categorizations of the richly varied affective and behavioral phenomena associated with sexual desire, in both same-sex and opposite-sex attractions. We may contrast such inherent complexity with a different phenomenon that can be delineated unambiguously, such as pregnancy.
With very few exceptions, a woman is or is not pregnant, which makes classification of research subjects for the purposes of study relatively easy: compare pregnant women with other, non-pregnant women. To increase precision, some researchers categorize concepts associated with human sexuality along a continuum or scale according to variations in pervasiveness, prominence, or intensity. Some scales focus on both intensity and the objects of sexual desire. Among the most familiar and widely used is the Kinsey scale, developed in the s to classify sexual desires and orientations using purportedly measurable criteria.
People are asked to choose one of the following options:. But there are considerable limitations to this approach. In principle, measurements of this sort are valuable for social science research. The ambiguity of the terms severely limits the use of the Kinsey scale as an ordinal measurement that gives a rank order to variables along a single, one-dimensional continuum. So it is not clear that this scale helps researchers to make even rudimentary classifications among the relevant groups using qualitative criteria, much less to rank-order variables or conduct controlled experiments.
In a critique of such approaches to social science, philosopher and neuropsychologist Daniel N. Another obstacle for research in this area may be the popular, but not well-supported, belief that romantic desires are sublimations of sexual desires. Romantic desires, following this line of thought, might not be as strongly correlated with sexual desires as is commonly thought.
All of this is to suggest that simple delineations of the concepts relating to human sexuality cannot be taken at face value and that ongoing empirical research sometimes changes or complicates the meanings of the concepts. Research over several decades has demonstrated that sexual orientation ranges along a continuum , from exclusive attraction to the other sex to exclusive attraction to the same sex.
Sociologist Edward O. Laumann and colleagues summarize this point clearly in a book:. While there is a core group about 2. In sum, homosexuality is fundamentally a multidimensional phenomenon that has manifold meanings and interpretations, depending on context and purpose. More recently, in a study, psychologists Lisa M. Diamond and Ritch C.
Savin-Williams make a similar point:. The more carefully researchers map these constellations — differentiating, for example, between gender identity and sexual identity , desire and behavior , sexual versus affectionate feelings, early-appearing versus late-appearing attractions and fantasies , or social identifications and sexual profiles — the more complicated the picture becomes because few individuals report uniform inter-correlations among these domains.
Some researchers acknowledge the difficulties with grouping these various components under a single rubric. For example, researchers John C. Gonsiorek and James D. At the very least, we should recognize that we do not yet possess a clear and well-established framework for research on these topics.
To that end, this part of our report considers research on sexual desire and sexual attraction, focusing on the empirical findings related to etiology and development, and highlighting the underlying complexities. K eeping in mind these reflections on the problems of definitions, we turn to the question of how sexual desires originate and develop.
Consider the different patterns of attraction between individuals who report experiencing predominant sexual or romantic attraction toward members of the same sex and those who report experiencing predominant sexual or romantic attraction toward members of the opposite sex. What are the causes of these two patterns of attraction? Are such attractions or preferences innate traits, perhaps determined by our genes or prenatal hormones; are they acquired by experiential, environmental, or volitional factors; or do they develop out of some combination of both kinds of causes?
What role, if any, does human agency play in the genesis of patterns of attraction? What role, if any, do cultural or social influences play? Research suggests that while genetic or innate factors may influence the emergence of same-sex attractions, these biological factors cannot provide a complete explanation, and environmental and experiential factors may also play an important role.
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However, as the following discussion of the relevant scientific literature shows, this is not a view that is well-supported by research. O ne powerful research design for assessing whether biological or psychological traits have a genetic basis is the study of identical twins. If the probability is high that both members in a pair of identical twins, who share the same genome, exhibit a trait when one of them does — this is known as the concordance rate — then one can infer that genetic factors are likely to be involved in the trait.
If, however, the concordance rate for identical twins is no higher than the concordance rate of the same trait in fraternal twins, who share on average only half their genes, this indicates that the shared environment may be a more important factor than shared genes.
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One of the pioneers of behavioral genetics and one of the first researchers to use twins to study the effect of genes on traits, including sexual orientation, was psychiatrist Franz Josef Kallmann. But the study was heavily criticized. Nevertheless, well-designed twin studies examining the genetics of homosexuality indicate that genetic factors likely play some role in determining sexual orientation. For example, in , psychologist J. On the basis of these findings, the researchers estimated that the heritability of homosexuality for men was 0.
The large confidence intervals in the study by Bailey and colleagues mean that we must be careful in assessing the substantive significance of these findings. While the concordance estimates seem somewhat high in the models used, the confidence intervals are so wide that it is difficult to judge the reliability, including the replicability, of these estimates.
Heritability is a measure of how much variation in a particular trait within a population can be attributed to variation in genes in that population. It is not, however, a measure of how much a trait is genetically determined. Traits that are almost entirely genetically determined can have very low heritability values, while traits that have almost no genetic basis can be found to be highly heritable. For instance, the number of fingers human beings have is almost completely genetically determined. But there is little variation in the number of fingers humans have, and most of the variation we do see is due to non-genetic factors such as accidents, which would lead to low heritability estimates for the trait.
Conversely, cultural traits can sometimes be found to be highly heritable. For instance, whether a given individual in mid-twentieth century America wore earrings would have been found to be highly heritable, because it was highly associated with being male or female, which is in turn associated with possessing XX or XY sex chromosomes, making variability in earring-wearing behavior highly associated with genetic differences, despite the fact that wearing earrings is a cultural rather than biological phenomenon.
Today, heritability estimates for earring-wearing behavior would be lower than they were in mid-twentieth century America, not because of any changes in the American gene pool, but because of the increased acceptance of men wearing earrings. So, a heritability estimate of 0. For men, these rates suggest an estimated heritability rate of 0.
For women, the heritability rate for having had at least one lifetime same-sex partner was 0. These values indicate that, while the genetic component of homosexual behavior is far from negligible, non-shared environmental factors play a critical, perhaps preponderant, role. Another large and nationally representative study of twins published by sociologists Peter S.
Overall, 8. While female opposite-sex twins in the study were the least likely of all the groups to report same-sex attractions 5. Note that this is different from the more general hypothesis that prenatal hormones influence the development of sexual orientation. In the study, the proportion of male opposite-sex twins reporting same-sex attraction was about twice as high for those without older brothers The authors argued that this finding was strong evidence against the hormone-transfer hypothesis, since the presence of older brothers should not decrease the likelihood of same-sex attraction if that attraction has a basis in prenatal hormonal transfers.
However, that conclusion seems premature: the observations are consistent with the possibility of both hormonal factors and the presence of an older brother having an effect especially if the latter influences the former.
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This study also found no correlation between experiencing same-sex attraction and having multiple older brothers, which had been reported in some earlier studies. Significant influence would require that identical twins have significantly higher concordance rates for same-sex attraction than fraternal twins or non-twin siblings. But in the study, the rates were statistically similar: identical twins were 6. The authors also argued that the higher concordance rates for same-sex attraction reported in previous studies may be unreliable due to methodological problems such as non-representative samples and small sample sizes.
This hypothesis is also suggested by findings, discussed below, that same-sex attraction may be more fluid in adolescence than in later stages of adulthood. In contrast to the studies just summarized, psychiatrist Kenneth S. Kendler and colleagues conducted a large twin study using a probability sample of twin pairs and 1, non-twin siblings.
Since some twin studies found higher concordance rates in identical twins than in fraternal twins or non-twin siblings, there may be genetic influences on sexual desire and behavioral preferences. One needs to bear in mind that identical twins typically have even more similar environments — early attachment experiences, peer relationships, and the like — than fraternal twins or non-twin siblings. Because of their similar appearances and temperaments, for example, identical twins may be more likely than fraternal twins or other siblings to be treated similarly. So some of the higher concordance rates may be attributable to environmental factors rather than genetic factors.
In any case, if genes do play a role in predisposing people toward certain sexual desires or behaviors, these studies make clear that genetic influences cannot be the whole story. But there is evidence that genes play a role in influencing sexual orientation. But there is some evidence from the twin studies that certain genetic profiles probably increase the likelihood the person later identifies as gay or engages in same-sex sexual behavior.
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Future twin studies on the heritability of sexual orientation should include analyses of larger samples or meta-analyses or other systematic reviews to overcome the limited sample size and statistical power of some of the existing studies, and analyses of heritability rates across different dimensions of sexuality such as attraction, behavior, and identity to overcome the imprecisions of the ambiguous concept of sexual orientation and the limits of studies that look at only one of these dimensions of sexuality.
I n examining the question whether, and perhaps to what extent, there may be genetic contributions to homosexuality, we have so far looked at studies that employ methods of classical genetics to estimate the heritability of a trait like sexual orientation but that do not identify particular genes that may be associated with the trait. One early attempt to identify a more specific genetic basis for homosexuality was a study by geneticist Dean Hamer and colleagues of 40 pairs of homosexual brothers.
Genetic linkage studies like the ones discussed above are able to identify particular regions of chromosomes that may be associated with a trait by looking at patterns of inheritance. Today, one of the chief methods for inferring which genetic variants are associated with a trait is the genome-wide association study, which uses DNA sequencing technologies to identify particular differences in DNA that may be associated with a trait. Scientists examine millions of genetic variants in large numbers of individuals who have a particular trait, as well as individuals who do not have the trait, and compare the frequency of genetic variants among those who do and do not have the trait.
Specific genetic variants that occur more frequently among those who have than those who do not have the trait are inferred to have some association with that trait. Genome-wide association studies have become popular in recent years, yet few such scientific studies have found significant associations of genetic variants with sexual orientation. The largest attempt to identify genetic variants associated with homosexuality, a study of over 23, individuals from the 23andMe database presented at the American Society of Human Genetics annual meeting in , found no linkages reaching genome-wide significance for same-sex sexual identity for males or females.
So, again, the evidence for a genetic basis for homosexuality is inconsistent and inconclusive, which suggests that, though genetic factors explain some of the variation in sexual orientation, the genetic contribution to this trait is not likely to be strong and even less likely to be decisive. As is often true of human behavioral tendencies, there may be genetic contributions to the tendency toward homosexual inclinations or behaviors.
Phenotypic expression of genes is usually influenced by environmental factors — different environments may lead to different phenotypes even for the same genes. Looking to developmental, environmental, experiential, social, or volitional factors will be necessary to arrive at a fuller picture of how sexual interests, attractions, and desires develop.
The growing field of epigenetics, for example, demonstrates that even for relatively simple traits, gene expression itself can be influenced by innumerable other external factors that can shape the functioning of genes. These gene-environment relationships are complex and multidimensional. Non-genetic developmental factors and environmental experiences may be sculpted, in part, by genetic factors working in subtle ways. While genes may in this way incline a person to certain behaviors, compelling behavior directly, independently of a wide range of other factors, seems less plausible.
They may influence behavior in more subtle ways, depending on external environmental stimuli for instance, peer pressure, suggestion, and behavioral rewards in conjunction with psychological factors and physical makeup. This oversimplified model, which underlies most current research in behavior genetics, ignores the critical importance of the brain, the environment, and gene expression networks. Genes constitute only one of the many key influences on behavior in addition to environmental influences, personal choices, and interpersonal experiences.
The weight of evidence to date strongly suggests that the contribution of genetic factors is modest. A nother area of research relevant to the hypothesis that people are born with dispositions toward different sexual orientations involves prenatal hormonal influences on physical development and subsequent male- or female-typical behaviors in early childhood. For ethical and practical reasons, the experimental work in this field is carried out in non-human mammals, which limits how this research can be generalized to human cases. However, children who are born with disorders of sexual development DSD serve as a population in which to examine the influence of genetic and hormonal abnormalities on the subsequent development of non-typical sexual identity and sexual orientation.
Periods of peak response to the hormonal environment are thought to occur during gestation. For example, testosterone is thought to influence the male fetus maximally between weeks 8 and 24, and then again at birth, until about three months of age. Specific hormones of interest in this area of research are testosterone, dihydrotestosterone a metabolite of testosterone, and more potent than testosterone , estradiol, progesterone, and cortisol.
The generally accepted pathways of normal hormonal influence of development in utero are as follows. The typical pattern of sex differentiation in human fetuses begins with the differentiation of the sex organs into testes or ovaries, a process that is largely genetically controlled. Once these organs have differentiated, they produce specific hormones that determine development of external genitalia.
This window of time in gestation is when hormones exert their phenotypic and neurological effects. Testosterone secreted by the testes contributes to the development of male external genitalia and affects neurological development in males;  it is the absence of testosterone in females which allows for the female pattern of external genitalia to develop. Genetic or environmental effects can also lead to disorders of sexual development. Stress may also play some role in influencing the way hormones shape gonadal development, neurodevelopment, and subsequent sex-typical behaviors in early childhood.
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It may originate from the mother, if she experiences severe stressors during her pregnancy, or from the fetus under stress. Hormone therapies are also often administered to mitigate the effects of excess androgen production. Likewise, there are disorders of sexual development in genetic males affected by androgen insensitivity.
In males with androgen insensitivity syndrome, the testes produce testosterone normally, but the receptors to testosterone are not functional. There are other disorders of sexual development affecting some genetic males i. During puberty, however, these individuals often experience physical virilization, and must then decide whether to live as men or women. Peggy T. The twin studies reviewed earlier may shed light on the role of maternal hormonal influences, since both identical and fraternal twins are exposed to similar maternal hormonal influences in utero.
The relatively weak concordance rates in the twin studies suggest that prenatal hormones, like genetic factors, do not play a strongly determinative role in sexual orientation. Other attempts at finding significant hormonal influences on sexual development have likewise been mixed, and the salience of the findings is not yet clear.
Since direct studies of prenatal hormonal influences on sexual development are methodologically difficult, some studies have tried to develop models whereby differences in prenatal hormonal exposure can be inferred indirectly — by measuring subtle morphological changes or by examining hormonal disorders that are present later during development. For women, the hypothesis for homosexuality that they have been hypermasculinized lower ratio, higher testosterone has also been proposed.
Several studies comparing this trait in homosexually versus heterosexually identified men and women have shown mixed results. A study published in Nature in found that in a sample of California adults, the right-hand 2D:4D ratio of homosexual women was significantly more masculine that is, the ratio was smaller than that of heterosexual women and did not differ significantly from that of heterosexual men.
Another study that year, which used a relatively small sample of homosexual and heterosexual men from the United Kingdom, reported a lower 2D:4D that is, more masculine ratio in homosexual men. A twin study compared seven female monozygotic twin pairs discordant for homosexuality one twin was lesbian and five female monozygotic twin pairs concordant for homosexuality both twins were lesbian. Much research has examined the effects of prenatal hormones on behavior and brain structure.
Again, these results come primarily from studies of non-human primates, but the study of disorders of sexual development has provided helpful insights into the effects of hormones on sexual development in humans. Since hormonal influences typically occur during time-sensitive periods of development, when their effects manifest physically, it is reasonable to assume that organizational effects of these early, time-linked hormonal patterns are likely to direct aspects of neural development. Neuroanatomical connectivity and neurochemical sensitivities may be among such influences.
Sexual orientation was assessed using the Kinsey scale. However, more recent studies have shown much smaller or no significant correlations. In summary, some forms of prenatal hormone exposure, particularly CAH in females, are associated with differences in sexual orientation, while other factors are often important in determining the physical and psychological effects of those exposures. Hormonal conditions that contribute to disorders of sex development may contribute to the development of non-heterosexual orientations in some individuals, but this does not demonstrate that such factors explain the development of sexual attractions, desires, and behaviors in the majority of cases.
T here have been several studies examining neurobiological differences between individuals who identify as heterosexual and those who identify as homosexual. In addition, we determined that the sex difference in volume was attributable to a sex difference in neuronal number and not in neuronal size or density.
Lasco and colleagues published a study examining a different part of the brain — the anterior commissure — and found that there were no significant differences in that area based either on sex or sexual orientation. Other studies have since been conducted to ascertain structural or functional differences between the brains of heterosexual and homosexual individuals using a variety of criteria to define these categories.
Findings from several of these studies are summarized in a commentary published in the Proceedings of the National Academy of Sciences. Due to inherent limitations, this research literature is fairly unremarkable. For example, in one study functional MRI was used to measure activity changes in the brain when pictures of men and women were shown to subjects, finding that viewing a female face produced stronger activity in the thalamus and orbitofrontal cortex of heterosexual men and homosexual women, whereas in homosexual men and heterosexual women these structures reacted more strongly to the face of a man.
In a similar vein, one study reported different responses to pheromones between homosexual and heterosexual men,  and a follow-up study showed a similar finding in homosexual compared to heterosexual women. While findings of this kind may suggest avenues for future investigation, they do not move us much closer to an understanding of the biological or environmental determinants of sexual attractions, interests, preferences, or behaviors.
We will say more about this below. For now, we will briefly illustrate a few of the inherent limitations in this area of research with the following hypothetical example. Suppose we were to study the brains of yoga teachers and compare them to the brains of bodybuilders. If we search long enough, we will eventually find statistically significant differences in some area of brain morphology or brain function between these two groups. But this would not imply that such differences determined the different life trajectories of the yoga teacher and the bodybuilder.
The brain differences could have been the result, rather than the cause, of distinctive patterns of behavior or interests. Suppose that gay men tend to have less body fat than straight men as indicated by lower average scores on body mass indices. Even though body mass is, in part, determined by genetics, we could not claim based on this finding that there is some innate, genetic cause of both body mass and homosexuality at work. It could be the case, for instance, that being gay is associated with a diet that lowers body mass. These examples illustrate one of the common problems encountered in the popular interpretation of such research: the suggestion that the neurobiological pattern determines a particular behavioral expression.
T here are some significant built-in limitations to what the kind of empirical research summarized in the preceding sections can show. Ignoring these limitations is one of the main reasons the research is routinely misinterpreted in the public sphere. It may be tempting to assume, as we just saw with the example of brain structure, that if a particular biological profile is associated with some behavioral or psychological trait, then that biological profile causes that trait.
This reasoning relies on a fallacy, and in this section we explain why, using concepts from the field of epidemiology. While some of these issues are rather technical in detail, we will try to explain them in a general way that is accessible to the non-specialist reader. Suppose for the sake of illustration that one or more differences in a biological trait are found between homosexual and heterosexual men.
That difference could be a discrete measure call this D such as presence of a genetic marker, or it could be a continuous measure call this C such as the average volume of a particular part of the brain. Showing that a risk factor significantly increases the chances of a particular health outcome or a behavior might give us a clue to development of that health outcome or that behavior, but it does not provide evidence of causation. Indeed, it may not provide evidence of anything but the weakest of correlations. The inference is sometimes made that if it can be shown that gay men and straight men differ significantly in the probability that D is present whether a gene, a hormonal factor, or something else , no matter how low that probability, then this finding suggests that being gay has a biological basis.
But this inference is unwarranted. Doubling or even tripling or quadrupling the probability of a relatively rare trait can have little value in terms of predicting who will or will not identify as gay. The same would be true for any continuous variable C. Showing a significant difference at the mean or average for a given trait such as the volume of a particular brain region between men who identify as heterosexual and men who identify as homosexual does not suffice to show that this average difference contributes to the probability of identifying as heterosexual or homosexual.
In addition to the reasons explained above, a significant difference at the means of two distributions can be consistent with a great deal of overlap between the distributions. That is, there may be virtually no separation in terms of distinguishing between some individual members of each group, and thus the measure would not provide much predictability for sexual orientation or preference. Some of these issues could, in part, be addressed by additional methodological approaches, such as the use of a training sample or cross-validation procedures.
A training sample is a small sample used to develop a model or hypothesis ; this model is then tested on a larger independent sample. This method avoids testing a hypothesis on the same data used to develop the hypothesis. Cross-validation includes procedures used to examine whether a statistically significant effect is really there or just due to chance.