Sexual development is never straight: biological and psychological approach

Sexual development is never straight: biological and psychological approach

Sexual orientation has been under a glaring spotlight during recent years, with every right so, due to being neglected and mislabelled for the longest time. Trying to rectify this mistake, it has produced an abundance of information in a short time, often invoking similar, but not identical meanings under the same name. The first step to better understand the differences in sexual development is accurate labelling and clear nomenclature.

In scientific language

To fully deter any and all misconceptions, terms used in this article are to be looked at only in context with the text itself. Some of them might have a slightly different meaning outside of this article or could have a different name overall.


Sex, defining a person’s biological function describes the biological role given to us at the time a foetus is conceived. It is defined by our sexual chromosomes – XX for a female and XY for male, present throughout the whole body and persistent through life. It is the biological role which defines if a person will be born male or female, with a small number of exceptions, and (un)fortunately, it is permanent. To clarify, a surgical or chemical sex change will change the appearance of a person together with some of their traits, but on a biological level, their cells will still have the same chromosomal imprint they had before the surgical or chemical procedure. To address sexes, male and female will be used in this article.


Gender is a cultural and a sociological construct assigned to each sex encouraged by the sexual evolution of humanity and strengths and weaknesses of sexes, and greatly varies in different cultures. It is the behaviour associated with a sex, and the behaviour taught, both actively and passively to a person on the premise of their sex and the cultural and sociological aspect they find themselves in. Both gender and its roles can be changed due to a person’s efforts, or as culture and society evolve. To address genders, men and women will be used in this article.


Sexual orientation is the person’s appeal to the members of a particular sex (and gender). If a person is attracted to a member of the same sex their sexual orientation is homosexual or gay, and if it is attracted to the opposite sex, their orientation is heterosexual or straight. If the person is attracted to both sexes, they are bisexual, and if they are attracted to neither, they are asexual.


Cisgender or cis, and transgender or trans are indicators of sex-to-gender translations. A female woman is considered a cis-female (or cisgender), and a male woman is considered a trans-female (or transgender). The same goes for men.


Messengers of sexual orientation

Development of human foetus in sexual terms

To fully understand causes and effects of biological aspects on human sexual development, we need to visualize the timetable of events that lead to the onset of a particular sex. First, a sperm cell and an egg cell merge, each carrying half of their respective parents’ chromosomes. This is the moment at which sex is determined. Depending on which half of the chromosomes the newly formed embryo gets, its sexual chromosomes will either be XX or XY – meaning female or male, respectively. The egg cell, coming from a female, will always give the X chromosome, so it is up to the sperm cell to determine the sex of the embryo, as it can either deliver the X chromosome, making the embryo a female, or the Y chromosome, making it male.

There is a difference in size, as well as in function between X and Y chromosome. How specifically they give rise to either a female or a male sex is still not fully understood, but, because they contain different DNA, it is easy to see that some differences have to exist.

In accordance with their respective chromosomal pair, female or male gonads (sexual organs) are developed. They develop based on presence or absence of androgen hormones, mainly testosterone, dihydrotestosterone and androsterone, which are present or absent based on the chromosomal pair available. Production of testosterone and conversion into dihydrotestosterone in weeks 6 to 12 of pregnancy are key factors in the production of a male foetus’s genitals. Absence of those hormones leads to development of female genitals.


Sculpting the brain


Once the genitals are formed, they release their own set of hormones, together with other chemical factors that shape further development of the foetus. Sexual differentiation of the brain occurs, among other factors, under the watch of sex hormones derived from gonads, with which they exert organizational effects that will activate in puberty.

The degree of genital femininity or masculinity doesn’t necessary relate to their counterpart in the brain, as those processes occur separately. Organizational differences between sexes have been found throughout the brain, most notably in the hypothalamus and amygdala. Differences in brain structure present due to chemical influences (hormones and other chemical messengers) are believed to be the basis of different sexual orientation and gender preferences.

Having in mind both the influence of chemical messengers and the different time points at which gonads and brain sexually develop, it is easy to understand that exposure to different hormones and different concentrations of hormones can affect sexual orientation and gender preferences in adults.


Orienting the brain


As with almost everything else in biology, foetal hormones are not the only influencers in sexual orientation, but are probably the main ones. Together with genes and environmental conditions, as well as external chemical factors, they influence sexual orientation.

One of the evidences for such claim comes in form of a condition called Congenital Adrenal Hyperplasia, or CAH. It is a genetic disorder that results in exposure to high levels of androgen hormones in early pregnancy due to hyperactivation of adrenal glands. Females with CAH are born with masculinized genitalia, and are more likely to be homosexual, in comparison to the overall population.

Several other factors, observed in higher percentages in homosexual population, can influence sexual orientation. Some aspects of prenatal stress levels, as well as several drugs can change the outcome of sexual orientation and/or gender preference. Severe psychical traumas, such as war, severe mental shock and death of a loved ones have been linked to having an impact on sexual orientation. Psychostimulants abuse has been debated as well as to having an impact, but both cases need further research.


Fraternal birth order effect


It has been established that gay males have on average more older brothers. This is due to the increasingly higher level of androgen hormones foetuses are exposed to with every male pregnancy. Additionally, this phenomenon is explained by the immunological response of the mother to a product of Y chromosome of her sons, which also increases with every male pregnancy. A gene of the Rhesus factor (Rh), a protein found on the red blood cells, has been linked to this effect. Rh factor can be inherited from either parent, with a slight chance of a mother’s immune response to a father’s Rh factor in the foetus. Each consecutive male pregnancy slightly increases the mother’s immune response to the Rh factor, with cumulative effect.

Genetical background also shapes sexual orientation. Additionally, several studies found chromosomal differences between homosexual and heterosexual people, especially on chromosome 11, tied to olfactory functions and visual perception. Prior studies have determined those regions to be linked with perception of pleasure in both males and females.

All of these factors are either a cause or an effect of an immunological, endocrine or genetic mechanism observed over the years as small, albeit statistically significant changes.


Sliding scales of sexual orientation


One of the obstacles in research of sexuality was misconception and mislabelling of research materials, as well as sexual groups. For the longest time sexual orientation was considered as an either-or scenario, with odd cases of both or neither in between. In recent times, sexual orientation has been classified as a sliding scale, greatly easing both research and nomenclature.

A person can identify as straight, have only partners of the opposite sex, and still express some form of sexual attraction, be it periodical or continual, to the members of the same sex. That person might still consider themselves as purely heterosexual and never have a homosexual relation, whether by choice or by not having the opportunity to express these tendencies, or simply because the attraction isn’t strong enough. It can be seen as a percentage of being heterosexual or homosexual.

Same sliding scale can be used to determine gender, as it is a sociocultural construct, rather than a biological one. It is that same sociological and cultural environment that needs to be taken into account when considering the re-emergence of both problems around sexual orientation and gender identity.


An history of culture and society

Cultural and sociological determination of an idea is the individual’s choice to accept a higher power’s interpretation of that idea. For a long time that power was religion, with its hold still strongly affecting the individuals’ comfort level and awareness regarding sexual orientation. Such powers, both directly - through teachings, laws or other means, or indirectly - through social appreciation, human interaction and example, have pushed the sexuality towards one end of the sliding scale while marginalizing the rest. As expected, apart from social ignorance, this can lead to several medical conditions.

Sexual orientation is in part also taught, with the most influential teachers being the ones closest to us.


Direct influences


Apart from religion, country laws are the second biggest power that actively forms social and cultural ideas. In recent times, European union has a rising, albeit slow tendency towards actively enforcing laws for the betterment of gay populations. While there is no more laws directly against being gay, the nonexistence of laws for their benefit also influences their choices, to a degree. A bisexual person might opt for a heterosexual relationship because of marriage privileges, or similar benefits such a relationship can bring, in contrast to a homosexual one.


Indirect influences


Appreciation for the same sex, if not very prominent, can be stifled by the environment. The person might never know they have a tendency towards the same sex, or they could find it out much later in life. This is especially true for developing children, who are most susceptible to learn by examples. Subtle signs of heterosexual relations are all around us, from wedding rings, couples holding hands and children amidst parents. Homosexual relations are still a much rarer public occurrence.


Exceptions to every rule

Seeing how delicate the sex-to-gender transition can be, and how many factors actively and passively take part in shaping a person’s sexuality, it is no wonder that sometimes things go awry.


Biological inbetweeners: chromosomal aberrations

Disorders of sex chromosomes are rare conditions where an individual doesn’t have the usual XX or XY chromosomes. One problem that can happen is the deletion of a part of such a chromosome. Effects of such deletions are proportional to their severity, but also correspond to the part of the chromosome that has been deleted.

Similar, but more severe type of this condition is so called Turner syndrome, also known as 45,X or 45,X0 syndrome. It is a genetic condition in which a female is missing a part or a whole X chromosome. They are characterized by a short and webbed neck, low-set ears and short statue. Typically, they are unable to reproduce.

Opposite of the Turner syndrome is the Triple X syndrome, also known as trisomy x. It is characterized with an extra X chromosome in female’s cells that ensues when a sperm cell fertilizes an egg cell that has two X chromosomes instead of one. Such females are usually taller than average, with occasional mental and physical difficulties.

Klinefelter syndrome is a similar occurrence, where a sperm cell containing Y chromosome fertilizes an egg cell containing two X chromosomes, or a sperm cell containing X and Y chromosome fertilizes a normal egg cell. The primary features are infertility and small poorly functional testicles, often not even noticed.

XX gonadal dysgenesis is a rare condition where no functional ovaries are present to induce puberty in an otherwise normal female.  Such persons usually have a demonstrable abnormality in one of the sex chromosomes. It is inherited as an autosomal disease, and is, to an extent, treated with hormonal therapy.

XY gonadal dysgenesis, also known as Swyer syndrome, is a similar condition in a person who has the male sex chromosomes, but due to their aberrations hasn’t developed male genitalia, and in term, due to reasons discussed above, has developed female external genitalia and secondary female features.




Dysphoria is a term meaning “a state of unease or generalized dissatisfaction”, and comes with a stigma attached to it. It has been renamed from gender identity disorder in 2013. Both gender and sexual orientation dysphoria appear when a person is expected (or forced) to behave as one side of the sliding scale, while they feel as belonging to another.

Sex and gender noncomformity is a state similar to dysphoria, but without “clinically significant distress”.

Sexual and gender dysphoria have been linked to prenatal androgen exposure and discrepancies between genital and cerebral femininity or masculinity, as discussed above. Gender dysphoria can also occur when a male is brought up as a woman, or a female as a man, in otherwise healthy individuals. Etymology of such dysphorias remain mostly unknown, as they can (and usually are) the interplay between biological, chemical, environmental and culturological factors.

Criteria for gender dysphoria include incongruence between a person’s experienced or expressed gender and their primary sex characteristics, a desire to be rid of existing primary or secondary sex characteristics or the desire for those characteristics of the opposite sex.

The International Classification of Diseases (IDC-11) aims to rename gender dysphoria to gender incongruence, which would be described as “a marked and persistent incongruence between an individual’s experienced gender and the assigned sex”.


A great body of research has been done in recent years that has improved the way we think about sex, gender and sexual orientation. With it, improved nomenclature and unambiguous terms have emerged easing further research. That being said, any term used to either entitle or marginalize a person above or below another person should be shunned and frowned upon. It is not the words and titles that are good or bad, it is the person behind them and their intentions. And all we need for a thriving community are good intentions, understanding and some love.


1. Ashlyn Swift-Gallant, Lindsay A. Coome, Madison Aitken, D. Ashley Monks, Doug P. VanderLaan. Evidence for distinct biodevelopmental influences on male sexual orientation. Proceedings of the National Academy of Sciences Jun 2019, 116 (26) 12787-12792; DOI: 10.1073/pnas.1809920116

2.       Berenbaum, Sheri A.; Adriene M. Beltz (April 2011). "Sexual differentiation of human behavior: Effects of prenatal and pubertal organizational hormones". Frontiers in Endocrinology. 32 (2): 183–200. doi:10.1016/j.yfrne.2011.03.001. PMID 21397624. S2CID 205776417.

3.       Bogaert, A. F., Skorska, M. N. A short review of biological research on the development of sexual orientation, Hormones and Behavior. Volume 119, 2020, 104659, ISSN 0018-506X,

4.       Garcia-Falgueras, A; Swaab, DF (2010). "Sexual hormones and the brain: an essential alliance for sexual identity and sexual orientation". Endocr Dev. Endocrine Development. 17: 22–35. doi:10.1159/000262525. ISBN 978-3-8055-9302-1. PMID 19955753.

5.       Gooren, Louis (November 2006). "The biology of human psychosexual differentiation". Hormones and Behavior. 50 (4): 589–601. doi:10.1016/j.yhbeh.2006.06.011. PMID 16870186. S2CID 21060826.

6.       Hines, Melissa (October 2010). "Sex-related variation in human behavior and the brain". Trends in Cognitive Sciences. 14 (10): 448–456. doi:10.1016/j.tics.2010.07.005. PMC 2951011. PMID 20724210.

7.       Namiki M, Koh E. [Disorders of sex chromosome]. Nihon Rinsho. 1997 Nov;55(11):2963-8. Japanese. PMID: 9396296.

8.       Pasterski V. Fetal Androgens and Human Sexual Orientation: Searching for the Elusive Link. Arch Sex Behav. 2017;46(6):1615-1619. doi:10.1007/s10508-017-1021-6

9.       Saewyc EM. Research on adolescent sexual orientation: development, health disparities, stigma and resilience. J Res Adolesc. 2011;21(1):256-272. doi:10.1111/j.1532-7795.2010.00727.x



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