Human Sexual Development

Transsexualism (TS) is one of the many different natural biological variations that can occur in human sexual formation.

The process of sexual differentiation in a human being takes place in the following steps:

  1. First the chromosomal configuration is established (XX or XY),
  2. next gonadal differentiation (ovaries or testes),
  3. next differentiation of the internal and external genitalia (physical genital formation) and
  4. finally the differentiation of the brain into male or female (brain-sex).

Normally all these steps in the process of sexual differentiation follow a predictable pattern. The final step occurs only after birth and up until four years of age.

In an XY foetus (usually male) – an XY chromosomal pattern (chromosomal configuration), testis (gonadal development), male internal and external genitalia and finally a male brain differentiation being the substrate of male type behaviour.

In an XX foetus (usually female) – an XX chromosomal pattern (chromosomal configuration), ovary (gonadal development), female internal and external genitalia and lastly a female brain differentiation being the substrate of female-type behaviour.

Most of this physical development takes place in the developing foetus stage before birth. The last stages of this process, that of brain sexual differentiation takes place after birth. These are brain structures that only become sex-dimorphic (differentiated) between the ages of two and four years, well after birth and long after legal assignment to the male or female sex has taken place.

gooren-expertevidenceTS is a disorder of sexual differentiation: the process of becoming male or female as we conventionally understand it. It is a condition where the sexual form and structure (phenotype) of the appearance of a person’s body and the genetic construction (genotype) of a person’s body are opposite that of their brain.(1)

There are many people for whom not all traditional criteria of physical sex development are in harmony. For a variety of reasons, one in 80 or so babies is born with some kind of sex or gender identity anomaly.(2) These are many intersex conditions which result in a birth with some biological characteristics of one sex and some of the others. Most intersex conditions are observable at birth, others are not observable at birth and still others are only discovered much later at puberty.

The predicament of TS means the sexual differentiation of the brain has not followed the pattern predicted by their earlier steps in the sexual differentiation process (such as chromosomes, gonad, genitalia) but has followed a pattern typical of the opposite sex in the final stage of brain differentiation process.

Like other people born with disorders in this process of sex differentiation, men and women with TS seek medical rehabilitation for increased harmony with that of their brain. This decision is similar to the one made in cases of intersexed children where legal assignment takes place to the sex in which they in all likelihood will function best.(3)

The decision to recommend hormonal and surgical treatment for men and women with this condition takes place much later in life and is based on the conclusion of a thorough psycho-diagnostic process that concludes that a disorder has occurred in the process of sexual differentiation and that the man or woman will benefit from hormonal and surgical sex assignment.


The current medical viewpoint, based on the most up-to-date scientific research, is that TS, is strongly associated with a neuro-developmental condition of the brain(4)and this theory of physical development has held over almost fifty years.(5)

There is now evidence to consider that for men and women with TS, the differentiation process of the brain which occurs in the first years after birth has not followed the expected course of the preceding criteria of sex (chromosomal, gonadal, and genital).(6)

Medical professionals working in these fields for over half a century agree that there is not one cause for TS, but that ‘genetic, prenatal (before birth) hormonal, postnatal (after birth) social, and post pubertal (after puberty) hormonal determinants’ are all mutually responsible.(7)

Hormonal and surgical treatment for this condition is highly successful under medical supervision, with up to a 97% success rate(8) in the case of individuals identified ‘female’ at birth.

See also


  1. Gooren LJ, (2000) University Hospital, Vrije Universiteit of Amsterdam, affidavit inBellinger v Bellinger, TLR 22-11-2000.
  2. Gender Dysphoria (2004) The Gender Identity Research & Education Society UK.
  3. Gooren LJ, (2000) University Hospital, Vrije Universiteit of Amsterdam, affidavit inBellinger v Bellinger, TLR 22-11-2000.
  4. Benjamin, H (1953) ‘Transvestism and Transsexualism’, Journal of Sex Research, 5:2, p. 13; Hoenig, J (1985) ‘The Origin of Gender Identity’ Gender Dysphoria, ed. Steiner, B W, New York: Plenum Press; and Docter, R F (1988) Transvestites and Transsexuals, Towards a Theory of Cross-Gender Behaviour, New York: Plenum Press, p. 63. Hoenig follows Benjamin in ultimately depending on a biological force or forces to account for transsexualism . Summarising and commenting on this and other medical viewpoints Docter indicates that the overall weight of evidence is that there is “the formation of some kind of gender system within the brain that is fundamental to ultimate gender identity and gender-role development”; Reiner, WG. (1997) To Be Male or Female – That is the Question, Arch Pediatric Adol. Medicine151:225 “the organ that appears to be critical to psychosexual development and adaptation is not the external genitalia, but the brain”.
  5. Playdon, ZJ (2000) Transsexualism as an Intersex Condition, Correspondence with Dr Stephen Whittle, UK.
  6. Gooren L G J (1993) ‘Biological Aspects of Transsexualism and their relevance to its legal aspects’, Proceedings of the XXIIIrd Colloquy on European Law: Transsexualism, Medicine and the Law, Strasbourg; Council of Europe.
  7. Money, J (1994) ‘The Concept of Gender Identity Disorder in Childhood and Adolescence After 39 Years’, Journal of Sex and Marital Therapy, 20(3), 163-177.
  8. Green R & Fleming DT (2000) ‘Transsexual Surgery Follow-up: Status in the 1990s’,Annual Review of Sex Research, ed. J Bancroft, 1:163-174.