Myla's notebook

Why are trans people trans? A biological perspective

This is a compilation of the current scientific knowledge regarding trans people. It was done with the Deep Research option from ChatGPT. All sources are included.

Biological and Neurological Explanations for Gender Incongruence

Gender incongruence (often called gender dysphoria in clinical contexts) refers to the experience of a mismatch between one’s assigned sex at birth and one’s internal sense of gender. Research indicates this phenomenon has multifactorial causes, with significant biological underpinnings. Below is a detailed overview of scientific findings across genetics, prenatal hormones, brain structure, endocrinology, psychology, and environmental factors that help explain why transgender individuals experience gender incongruence.

Genetic Factors in Gender Identity

Evidence from twin and family studies suggests a heritable component to gender identity. Multiple independent twin studies have found much higher concordance for transgender identity in monozygotic (identical) twins than in fraternal twins, indicating moderate genetic influence. For example, one large twin study (n=1891) reported gender dysphoria heritability estimates around 50% in assigned-male individuals and 30% in assigned-female individuals ( Gender Dysphoria: A Review Investigating the Relationship Between Genetic Influences and Brain Development - PMC ). Other studies similarly show significant genetic contributions – one found that 62% of variance in gender dysphoria could be attributed to genetics ( Gender Dysphoria: A Review Investigating the Relationship Between Genetic Influences and Brain Development - PMC ). In contrast, concordance in non-identical twins is often near zero (), underscoring that shared genes (not just shared environment) play an important role. Overall, these twin studies consistently point to heritability in the moderate range for transgender identity ( Gender Dysphoria: A Review Investigating the Relationship Between Genetic Influences and Brain Development - PMC ) ().

Beyond heritability, researchers are investigating specific gene variants that might influence gender identity development. While no single “gender identity gene” has been found, several candidate genes involved in sex hormone signaling have shown intriguing associations. Notably, transgender women (AMAB, assigned male at birth) have been reported to more frequently possess a longer repeat variant of the androgen receptor (AR) gene compared to cisgender men ( Gender Dysphoria: A Review Investigating the Relationship Between Genetic Influences and Brain Development - PMC ). A longer AR gene CAG-repeat can reduce the effectiveness of androgen signaling, potentially leading to less “masculinization” of the brain during development. Indeed, studies have identified an overrepresentation of long AR gene repeats in transgender women (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). Likewise, certain variants in genes related to estrogen signaling have been linked to transgender identities: for example, one study found transgender men (AFAB, assigned female at birth) more often carry a particular SNP in the CYP17 gene (affecting sex-hormone synthesis), as well as specific repeat polymorphisms in the estrogen receptor genes ERβ and ERα (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). These findings suggest that multiple genes each make small contributions – in other words, gender identity may be polygenic or “oligogenic” in origin (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic) (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). It is hypothesized that such gene variants could alter how the developing brain responds to sex hormones, leading to atypical sexual differentiation of brain structures that underlie gender identity (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). It’s important to note that results of gene studies have been mixed (some studies find associations, others do not), likely due to small sample sizes and the complexity of the trait (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). Nonetheless, the accumulating genetic evidence supports a biological basis: specific genetic profiles may increase the likelihood of a person identifying as transgender (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). Ongoing research (including genome-wide studies) is aiming to clarify these genetic links on a larger scale.

Prenatal Hormonal Influences on Brain and Gender Identity

Sex hormones in the womb play a pivotal role in brain sexual differentiation, and research indicates that atypical hormone exposures during prenatal development can contribute to gender incongruence. During typical fetal development, a testosterone surge in mid-gestation “masculinizes” certain brain structures in genetic males, whereas lower testosterone in genetic females leads to “feminization” of brain pathways ( Neurobiology of gender identity and sexual orientation - PMC ) ( Neurobiology of gender identity and sexual orientation - PMC ). If this hormone-driven process diverges from the development of the genital sex, it can create the neurological foundation for transgender identity (A sex difference in the human brain and its relation to transsexuality - PubMed). Several lines of evidence support the influence of in utero hormones:

In sum, prenatal endocrine factors appear to set the stage for gender identity. Elevated or reduced exposure to androgens at critical periods can lead the developing brain to organize itself in a way that is misaligned with the individual’s gonadal sex. Indeed, clinical data show higher prevalence of transgender identity among those with atypical prenatal hormone environments (e.g. CAH) (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). While exceptions exist and not every transgender person has a known prenatal hormone variation, these studies demonstrate that the hormonal milieu in utero is a key factor in brain-based identity development.

Neurological and Brain Structure Differences

Another robust area of research examines the brains of transgender individuals for structural and functional differences. A growing body of evidence shows that certain brain characteristics of transgender people resemble those of their identified gender (and differ from their birth-assigned sex). These studies provide a biological explanation: the brain itself shows features aligned with one’s experienced gender, which would naturally give rise to an internal sense of that gender.

Brain Structure (Morphology): Some of the earliest evidence came from postmortem neuroanatomy. In a landmark study, Zhou et al. (1995) measured a tiny region in the hypothalamus called the BSTc (central bed nucleus of the stria terminalis), which is known to be sexually dimorphic (larger and with more neurons in men than women). They discovered that transgender women’s BSTc volumes were similar to cisgender women’s (female-sized), despite the trans women having male chromosomes and upbringing (A sex difference in the human brain and its relation to transsexuality - PubMed). Importantly, the BSTc size in these trans women did not correlate with adult hormone levels or sexual orientation, suggesting it was a stable trait likely set during development (A sex difference in the human brain and its relation to transsexuality - PubMed). This was “the first study to show a female brain structure in genetically male transsexuals,” supporting the idea that gender identity reflects brain anatomy shaped by prenatal hormones (A sex difference in the human brain and its relation to transsexuality - PubMed). Subsequent research reinforced this finding by counting neuron numbers in the BSTc and another sex-dimorphic hypothalamic nucleus (the INAH3), again showing transgender individuals’ counts aligning with their gender identity and not their birth sex (sex difference in the hypothalamic uncinate nucleus: relationship to ...).

More recently, noninvasive MRI studies have compared brain anatomy (gray matter and white matter characteristics) in transgender vs. cisgender populations. Overall, these neuroimaging studies find that transgender people’s brains often exhibit a “shift” toward the pattern of their identified gender () (). For example, structural MRI and diffusion tensor imaging have shown that transgender women (before hormone treatment) have certain cortical thickness patterns and white matter connectivity profiles closer to cisgender female norms than male norms, and transgender men show the reverse trend (). A recent review concluded that virtually all structural MRI classifier studies support this shift away from birth sex and toward experienced gender in the brain (). In essence, aspects of brain organization (from region sizes to neural connectivity) in transgender individuals fall between the typical male and female ranges or lean toward the identity-consistent sex, indicating a distinct neurodevelopmental path () (). These differences are often subtle rather than absolute, and individual brains are a mosaic of traits (). However, patterns in group studies are clear enough to distinguish transgender vs. cisgender brain scans above chance level in some analyses (). This provides compelling neurobiological evidence that brain development diverges in transgender people, giving them an internal map of gender that differs from their anatomy.

Brain Function and Activation: Beyond structure, studies of brain function show parallels between transgender individuals and their experienced gender. A notable example involves the hypothalamus’s response to pheromone-like odors. Certain steroidal odors (AND and EST) selectively activate the hypothalamus in men vs. women. In tests, transgender women showed a female-typical pattern of hypothalamic activation when smelling these odors, meaning their brain response matched their identity, not their birth sex (Male-to-female transsexuals show sex-atypical hypothalamus ...) (Odor ratings for AND and EST. The vertical axis indicates a visual ...). Likewise, transgender men tend to have male-typical hypothalamic responses. Another innovative study examined brain activity during a body self-image task. Transgender and cisgender participants were shown images morphed to resemble their own body or the opposite sex’s body while in an fMRI scanner. The findings were striking: transgender individuals identified most with (and had the strongest neural activation for) images of the gender they feel themselves to be, whereas cisgender people identified with images of their own sex ( Neural Systems for Own-body Processing Align with Gender Identity Rather Than Birth-assigned Sex - PMC ). Moreover, the underlying brain networks involved in self-body recognition were activated according to identified gender in both groups ( Neural Systems for Own-body Processing Align with Gender Identity Rather Than Birth-assigned Sex - PMC ). In other words, a transgender man’s brain responded to a masculinized image of “himself” similarly to how a cisgender man’s brain responds to seeing his body – aligning with gender identity rather than birth anatomy. Such studies confirm that the brain’s self-perception and sensory processing are tuned to one’s experienced gender.

Overall, neurological research strongly supports that transgender individuals have brain differences that correlate with their gender identity. From the level of specific nuclei in the hypothalamus to global patterns of connectivity and activation, their brains show features more typical of the gender with which they identify () (). These innate brain characteristics likely give rise to the persistent knowledge of “being male or female” that characterizes gender identity. Thus, gender incongruence has a clear neurobiological signature, reinforcing that it is rooted in biology rather than being “all in one’s mind” in the colloquial sense – although of course it manifests as one’s innermost mental identity.

Endocrinological Factors Beyond Prenatal Development

In addition to prenatal hormone effects, scientists have explored whether postnatal hormonal factors or endocrine differences later in life might contribute to gender incongruence. The development of gender identity is largely thought to occur in early childhood, but hormonal influences do continue through puberty and beyond, potentially interacting with preexisting traits.

One consideration is the effect of pubertal hormones on the brain. Some sex-dependent brain differences only emerge at puberty, when the gonads produce a surge of estrogen or testosterone (). For example, connectivity patterns between brain regions become more male-typical or female-typical during adolescence under the influence of puberty hormones () (). If an individual’s brain was prenatally structured towards one gender, the changes of puberty could either reinforce that identity or, if incongruent, intensify the distress. The case of 5α-reductase deficiency mentioned earlier illustrates this: an XY child with that condition often adopts a male gender role at puberty when testosterone effects kick in, even if raised female prior, because the pubertal hormone changes “awaken” the masculinized brain identity (5-Alpha-Reductase Deficiency - StatPearls - NCBI Bookshelf). This suggests that the timing of hormone exposure (neonatal mini-puberty, adolescent puberty) can shape or reveal aspects of gender identity formed earlier.

There is also interest in whether some transgender individuals might have had atypical hormone levels or sensitivities during childhood or adolescence (outside the womb). Generally, hormone profiles in transgender people without DSDs are not pathologically different from cisgender peers. However, some studies have noted correlations that could indicate an endocrine influence. For instance, research has found a high prevalence of polycystic ovary syndrome (PCOS) – a condition causing elevated testosterone – in transgender men (AFAB) prior to any hormone therapy. One study reported that 58% of female-assigned trans patients met PCOS criteria, significantly higher than expected (Association between polycystic ovary syndrome and female-to-male ...). This has led to speculation that naturally high androgen levels in adolescence might contribute to or facilitate a male gender identity in some AFAB individuals (Association between polycystic ovary syndrome and female-to-male ...). While causation isn’t proven (it’s equally possible that undiagnosed PCOS was simply more likely to be noticed in this group), it underlines a potential link between postnatal endocrine milieu and gender identity. Similarly, rare cases of endocrine tumors or exposures that alter hormone levels have anecdotally been associated with shifts in gender feelings, though data are very limited.

It’s important to stress that postnatal hormonal factors are considered secondary compared to prenatal influences. By the time of puberty, most people (including transgender youth) have a well-established core gender identity (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). Thus, while ongoing hormone production in childhood and puberty certainly affects one’s body and may modulate certain gendered behaviors or cognitive traits () (), it is not thought to “create” a transgender identity out of nowhere. The current view is that endocrinological factors beyond the womb might interact with an underlying predisposition. For example, they might accentuate discomfort (as the body develops in an incongruent way) or, conversely, partially alleviate dysphoria if an individual happens to have hormone levels closer to what their brain expects. In summary, apart from the critical prenatal window, hormonal effects in later development (such as puberty) can influence the expression of gender incongruence, but they work on a foundation that is likely laid down earlier by genetics and prenatal hormone organization.

Psychological and Cognitive Studies

From a psychological perspective, gender identity is a deeply ingrained aspect of self. Transgender individuals usually become aware of their gender incongruence at an early age, often in childhood. Research in developmental psychology shows that most children can identify themselves as “boy” or “girl” (or another identity) by around age 2–3 (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). This identity is highly resistant to external push – for example, a child who consistently insists “I am a girl” or “I am a boy” cannot be talked out of this feeling without causing distress. In the case of transgender children, they often express a cross-gender identification despite being raised as their birth sex, indicating an internal certainty. Only a small minority of children experience this level of distress with their assigned gender (estimates of persistent gender dysphoria range from well under 1% of the population) (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). Those who do, however, typically describe that their sense of gender “does not match” their body and this realization can emerge as soon as they have the language to express it. These observations underscore that gender identity is not simply a matter of upbringing or choice, but rather an innate cognitive identity that forms early in life (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic).

Psychological studies have long tried to understand if any family dynamics or social factors might cause a person to become transgender. Earlier theories proposed that things like parenting style, childhood trauma, or abnormal psychosexual development might lead to gender dysphoria. However, no credible evidence has supported such theories (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). In fact, clinical observation and systematic studies have refuted the idea that you can “teach” a child to be trans or not trans. A famous case in the 1960s – the John/Joan case – involved a biologically male infant raised fully as a girl after a botched circumcision. That individual (David Reimer) never felt female and ultimately reverted to living as male, illustrating that core gender identity cannot be imposed purely by environment. Modern psychology affirms that being transgender is not a result of mental illness or faulty upbringing; rather, it is a natural variation of human identity development. The psychological distress that transgender people often experience (gender dysphoria) stems from the incongruence between brain identity and physical sex, and from social ostracism – not from the identity itself.

Cognitive neuroscience provides further insights by examining how transgender individuals’ brains handle tasks related to self-perception and cognition. As mentioned in the neurology section, trans people’s neural responses align with their identified gender during self-recognition tasks ( Neural Systems for Own-body Processing Align with Gender Identity Rather Than Birth-assigned Sex - PMC ). Another avenue of study is cognitive performance and preferences. Some research finds that, even before any medical transition, transgender people may exhibit cognitive patterns more similar to their identified gender. For example, visuospatial ability (on average slightly stronger in males) and verbal memory (on average slightly stronger in females) have been tested. In small studies, trans men often score higher on spatial tasks than cisgender women, and trans women score higher on verbal tasks than cisgender men, hinting at brain function parallels with identity. Moreover, when transgender individuals undergo hormone therapy, their cognitive profile often shifts further toward that of their affirmed gender, likely due to the influence of hormones on the brain. A review noted that trans men on testosterone showed improved visuospatial skills, while trans women on estrogen showed improved verbal memory, with no adverse effects on overall cognitive function (). These changes mirror what is known about how testosterone and estrogen respectively modulate certain cognitive skills in adults. The fact that such shifts occur confirms that their brains are responsive to sex hormones in expected ways, but crucially, hormones change performance ability – they do not create the identity. The identity was already firmly in place; hormones simply bring some secondary sex-related traits (like certain cognitive biases) in line with the individual’s gender.

In summary, psychological and cognitive research portrays gender identity as a fundamental aspect of the self, rooted in the brain. Transgender individuals experience their gender identity as real and persistent, often from a very young age, which aligns with biological evidence of early brain differentiation. Attempts to psychosocially “reassign” gender in the absence of biology have failed, whereas approaches that affirm one’s identity tend to improve mental health. Cognitive studies further demonstrate that the brains of transgender people function in accordance with their identity, reinforcing that their gender incongruence has a legitimate neurocognitive basis and is not a delusion or confusion.

Social and Environmental Factors

While biology plays a dominant role, social and environmental factors can influence the experience of gender incongruence – though not by causing someone to be transgender, but by shaping how comfortably it is expressed and how individuals cope. There is no evidence that upbringing, parenting, or social factors alone “make” a child transgender (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). In the past, some clinicians hypothesized that having certain parental dynamics (like an overbearing mother or absent father) or experiencing early trauma might lead to gender dysphoria. Extensive research has not substantiated these claims (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic). Children raised in all sorts of family environments have turned out cisgender, and likewise, transgender individuals come from every kind of social background. In fact, cross-cultural studies find consistent rates of transgender identification across very different societies, further indicating an intrinsic basis rather than a specific cultural cause.

However, the environment can affect transgender individuals in secondary ways. One important area is epigenetics – the chemical modifications to DNA that regulate gene expression. Environmental factors such as stress, endocrine-disrupting chemicals, or maternal conditions could potentially trigger epigenetic changes that influence brain sexual differentiation. Researchers have begun exploring whether epigenetic marks are associated with gender incongruence. A recent epigenome-wide study found distinct DNA methylation patterns in transgender people (prior to hormone treatment) compared to cisgender controls (). The differences were found in genes related to brain development and hormone signaling, suggesting that environmental influences on the genome (through methylation) might contribute to the development of a transgender identity (). This is still an emerging field, but it highlights a plausible mechanism by which external factors (e.g. maternal stress hormones or other womb environment aspects) could alter gene expression involved in gender identity without changing the genes themselves. In essence, epigenetics offers a bridge between nature and nurture: for example, the prenatal hormone effects we discussed could themselves be mediated by turning certain genes on/off at critical times () (). Ongoing studies are examining whether specific methylation signatures or other epigenetic markers consistently differ in transgender populations.

On a social level, cultural environment and upbringing primarily influence how gender variance is interpreted and whether a person can comfortably express their identity, rather than the core identity itself. A child who feels different in a very conservative or repressive environment might suppress their feelings longer, whereas in a more accepting environment they might articulate their identity earlier. Social factors also significantly impact the mental health outcomes for transgender individuals. High levels of family rejection, bullying, or societal stigma can lead to anxiety, depression, and other issues, whereas support and affirmation correspond with far better psychological well-being. These outcomes, though, are responses to the environment, not the origin of being transgender. It’s also worth noting that historical and cross-cultural records show the existence of transgender and gender-nonconforming people in many societies (often with specific social roles), indicating it is a consistent human phenomenon not caused by any modern trend.

In conclusion, gender incongruence arises from a complex interplay of biological factors (genes, hormones, brain development), with environmental factors potentially modulating these biological underpinnings to a degree () (). The evidence overwhelmingly points to significant innate components: genetic propensity, hormone-driven brain differentiation, and distinct neural characteristics all contribute to a transgender identity. Social and environmental factors do not appear to independently produce a transgender identity, but they can influence how one’s identity is expressed and how one fares. Ultimately, contemporary science supports that transgender individuals experience gender incongruence because their brain and identity development took a different path biologically, one that doesn’t align with their assigned sex. This understanding fosters a more compassionate and factual view: being transgender is a human variation rooted in biology, and not a result of choice or external persuasion () (Genetic Link Between Gender Dysphoria and Sex Hormone Signaling | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic).

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