Human Differentiation

information the basics human differentiation

It has been assumed that sexual differentiation (development into a male or a female phenotype) is completed at birth.

However, research shows the physical sex of the child (genitals) and the sexual differentiation of the brain (neuro-biological) are still in development at the point of birth and continue on until four years after birth (Gooren, 2000; Besser et al., 2004; Lavranos, Angelopoulou, Manolakou & Balla, 2006).

Sex and gender development has been identified to occur within four main processes of sexual differentiation in human development.

The first three steps occur between the time of conception up until birth (indicated by the middle line in the above diagram), and the last process after birth until around four years of age (Swaab & Hofman, 1988; Clarke, Kraftsik, Van der Loos & Innocenti, 1989; Gooren, 1993; Kawata, 1995; Swaab, et al., 2001).

First, the karyotype (or the appearance of the chromosomal configuration) is established (usually XX or XY);
next, gonadal differentiation (internal reproductive organs);
next, differentiation of the internal and external genitalia (physical genital formation); and
finally, the differentiation of the brain into male or female (brain-sex).

Ideally, all stages correlate or 'fit with' each other, resulting in the formation of what we conventionally understand to be a sexed human being (male or female) with a matching congruent gender identity (girl or boy).

Before birth, the chromosomal configuration (such as XY for male or XX for female) usually gives rise to different gonadal development (ovarian or testicular), resulting in internal and external genitalia upon which legal-sex is eventually assigned. Physical phenotype is assigned by genetic and hormonal forces before birth and occurs sometime during the first three stages of development.

After birth, up until around four years of age, is the final process of brain differentiation. This is the starting point for specific sex-based behaviours and the development of the newborn's gender identity. Gender identity develops as a result of an interaction between the developing brain and sex hormones (Allen & Gorski, 1990; Zhou, et al., 1995; Kruijver, et al., 2000).

Brain-sex
"Since the brain is the organ determining or scripting male or female behaviors, the term brain sex is short hand to reflect on how an individual thinks and organizes the world; whether in stereotypical male or female ways. It is certainly true that the brain is the most sexual organ of the body and the term brain sex reflects its male or female disposition. It directs the individual to think and act more like a stereotypic male or more like a female."
- (Diamond, 2002, p36)

For most people, all four stages link together to create a harmonious physical sex development. For about one in 80 babies, there is some disharmony. These infants are born with some kind of sex or gender identity anomaly.

The most common sex anomalies are due to chromosomal configurations in human beings such 47XXY, 47YYX, 47XXX, 47XYY, 45XO or mosaicism, which shows a variety of chromosomal configurations within the same individual. For these infants, the anomaly is easy to spot. A newborn with genitals not clearly male or clearly female is termed an 'intersex birth'.

There are times when the anomaly is discovered much later in adulthood. Sometimes menstruation doesn't occur as expected or after seeking medical help to conceive a child, medical tests show reproductive infertility or another anomaly.

Men and women affected by transsexualism are properly considered to have an intersex variation. The effects of transsexualism are not visible in external physical sex characteristics (like some other intersex births), individuals with transsexualism are always assigned the sex opposite to their actual-sex at birth.

The anomaly is only becomes evident when the child emphatically corrects a parent or other caregiver ("I'm not a girl! I'm a boy!"). Other children don't necessarily voice their concerns, but they have a strong sense that something isn't quite right.

Based on the accumulation of research after over 40 years of research, Professor Milton Diamond concludes:

"Transsexuals, who I believe are intersexed, have the body and genitals of one sex and the brain of the other".
- Diamond, 2000

As far back as 1957, it was suggested transsexualism be considered an intersex condition: "…the definition of hermaphroditism should not be confined to those rare individuals with proved testes and ovaries but extended to include all those with indefinite sex attitudes." (Giles and Millard); Gooren, 1993; Diamond, 2000, 2002; In 2000, Playdon advised the UK Government, transsexualism be thought of as an intersex condition.; ReKevin, 2001;
For a thorough discussion on Transsexualism as an intersex condition see Gurney, 2004.

Human Differentiation

Further reading