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Sexual Development

A tiny fetus growing in its mother's womb starts out sexually undifferentiated, neither male nor female in outward appearance. Male and female sexual anatomies develop from the same starting point.

Up until the age of about 5 or 6 weeks, male and female embryos cannot be distinguished. Then, around the 8th week, the sex glands begin to differentiate (become different).

Normally, sexual differentiation is trig­gered by the sex chromosomes. If the fetus has two X chromosomes, it is genetically female. The sex glands of a genetic female will develop into ovaries, full of tiny egg cells that start to mature during puberty.

What are details of the sexual differentiation process?

If the fetus has an XY sex chromosome pair, making it genetically male, then a chemical called H-Y antigen will cause the sex glands to develop into testes. As the testes develop in the womb, they begin to secrete two substances, testosterone and Mullerian-inhibiting substance (MIS) triggering development of male reproductive structures.

What if the fetus is genetically male but there are no male hormones?

Alterations in the complex process of hormone regulation can alter a person's sexual identity biologically or psycholog­ically. The presence of XX or XY genes, in themselves, do not make a person male or female.

A genetic female can develop as a male or vice versa. If there are no male sex hormones present, the fetus naturally develops as a female, even if the embryo has an XY (male) chromosome pattern.

This first phase of sexual differ­entiation is called the gonadal phase. During this time the gonads (hormone-releasing sex organs) develop, and they guide the rest of sexual differentiation.

Scientists once assumed that all sexual differentiation was driven by hormones such as testosterone. However, there are some direct genetic effects due to the X and Y chromosomes that do not require the mediation of testosterone.

Rats, like humans, have a surge of testosterone during embryological development. It occurs around day 17 or 18. However, neurons removed from dopaminergic areas of the rat brain on day 14 already show profound differences between male and female rats (Reisert and Pilgrim, 1991).

What is evidence that sexual differences are not due only to hormone effects?

Those differences must come from the X and Y chromosomes, because on day 14 the rat embryo has not yet been exposed to hormones. This discovery by itself shows that sexual dimorphism (difference between the sexes) is due to more than just hormones bathing the fetus. Part of sexual dimorphism is due to direct genetic effects from the sex-linked chromosomes.

The next phase is development of other internal sex structures. Finally, in a third phase, the external sexual structures develop.

The fact that each phase occurs separ­ately explains how people can have mismatched inner feelings, sexual attractions, or outer physiology. Hormonal or epigenetic disturbances can produce many variations of disorders of sexual development (DSD).

What accounts for mismatches between inner feelings and outer physiology?

Researchers caution that the mechan­isms of sexual development are quite complex. That is especially clear now that scientists know to look for epigenetic influences as well as hormonal influences.

Forger (2016) wrote, "The evidence suggests there is unlikely to be a simple formula for masculine or feminine devel­opment of the brain and behaviour; instead, underlying epigenetic mechan­isms may vary by brain region..." More­over, "males and females may use diff­erent combinations of epigenetic modifi­cations to control gene expression."

The complexity of epigenetic effects comes up on the page about causes of homosexualty. There is some evidence that homosexual females are influenced directly by testosterone levels in the womb but males might be more influ­enced by epigenetic changes altering responsiveness to testosterone.

Normal Sexual Development in Childhood

In the adult human, the ovaries and testes are homologous organs, meaning they have common origins. The clitoris and penis are homologs, representing a different course of development from the same beginning structures.

What are "homologous" organs?

After a baby is born, it is effectively sex neutral for a few years. People cannot tell, just by looking, whether a newborn baby is male or female.

Experiments show that a male baby dressed in a pink dress is rated feminine, while a girl baby dressed in male clothes is treated like a boy. People take their cue from clothing and hair arrangements because the faces of baby boys and girls look much alike.

Until they are about 4 or 5, most children are without shame or self-consciousness about their bodies. In most cultures, parents must teach the value of modesty.

Freud said that children around the age of 5 entered a latency stage that lasts until adolescence. During this stage, Freud said, children do not take an active interest in sex.

However, present-day researchers disagree with Freud. Hyde said flatly there is "no latency stage" (Hyde, 1982, p.262). During the entire pre-adolescent phase of life children have about the same reactions to sex.

Children sometimes engage in explor­ation and "show me" games before puberty. Such play may occur between same-sex children, although this does not predict homosexual orientation later, unless accompanied by crushes and actual sex. Marlowe (2010) reported typical early same-sex interaction among the Hadza, a hunter-gatherer tribe in Tanzania:

Hadza girls and boys begin 'playing house' literally, building litle huts, around the age of 7 or 8. There is some sex play when they enter the huts... Once, several Hadza and I watched two girls about 8 years old hugging and rolling around on the ground, clearly enjoying themselves in a sexual way. (p.168)

Is early sexual experience harmful? Probably not, in that form. However, if "games" proceed to masturbation or partner sex early in life, that does corre­late with later problems.

What sort of early sexual experiences correlate with later problems?

Early sexual experience is part of a cluster of traits that produces obsessive interest in sex, or a repulsion from it, in children from some families. Griffee et al. (2014) reported that "both sexual addic­tion and a low interest in sex as an adult have their origins during childhood."

Other researchers found the same thing. Adults were more likely to engage in risky sexual behaviors, or be uninter­ested in sex at all, if their first sexual experience (either masturbation with human images or partner sex) occurred early in life.

Puberty

Around the age of 9-10 in girls, later in boys, appear early signs of secondary sex characteristics appear. These are associated with hormones secreted in adolescence.

In girls, the first pre-pubertal changes are the beginnings of breast development and pubic hair. In boys, the genitals increase in size and pubic hair appears, appearing coarse and pigmented by 13-16 years of age.

Other secondary sex characteristics in males, emerging between the ages of 10 and 16, are body growth and a deepening voice. The prostate glands also mature during this time so the ejaculation of fluids may accompany orgasm, although in young adolescents the sperm may be immature and infertile.

What are secondary sex characteristics?

The average age of menarche (onset of menstruation) is about 12.5 years old. There are great variations; some girls begin to menstruate when they are 10, others when 16 or older.

Menstruation may be delayed in girls who are thin and exercise vigorously. For example, in a study of 89 ballerinas averaging 16 years old, 20 had not yet menstruated. Among those who had menstruated, the average age of menarche was 13.5, a year later than average (Frisch, 1980).

What can delay onset of menstruation?

Once puberty occurs, the human body is biologically ready for reproduction. In many tribal societies, the age of puberty is the age of marriage.

The age of first marriage, in developed countries, has been going steadily upward for decades. This correlates with access to reliable contraceptives and the empowerment of women. The trend is less visible in male-dominated (patriarchal) and undeveloped societies.

What are correlates of earlier sexual activity, in developed countries?

In developed countries, earlier sex is correlated with lower socioeconomic levels (less education, more povery) and by family dysfunction (fights, incest, child abuse). Reitz et al. (2014) found that teenagers in Holland were more likely to engage in early sex if they were impulsive and had poor relations with parents.

A meta-analysis of studies published between 1980 and 2012 showed that earlier puberty correlated with earlier sexual behavior and riskier sexual behavior, especially in girls (Baams, Dubas, Overbeek, and Aken, 2015). The cause-effect basis of this relationship is difficult to determine, because other research suggests that early sexual experience can trigger earlier puberty (Booker, 2013).

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References:

Baams, L., Dubas, J. S., Overbeek, G., & Aken, M. A. G. v. (2015) Transitions in body and behavior: Meta-analytic study on the relationships between pubertal development and adolescent sexual behavior. Journal of Adolescent Health, 56, 586-598.

Booker, K. (2013, November 27) Sex abuse triggers early puberty and its problems. Cornell Chronicle. [blog] Retrieved from: http://www.news.cornell.edu/stories/2013/11/sex-abuse-triggers-early-puberty-and-its-problems

Griffee, K., O'Keefe, S. L., Beard, K. W., Young, D. H., Kommor, M .J., Linz, T.D., Swindell, S., & Stroebel, S. S. (2014) Human sexual development is subject to critical period learning: Implications for sexual addiction, sexual therapy, and for child rearing. Sexual Addiction & Compulsivity, 21, 114-169. http://dx.doi.org/10.1080/10720162.2014.906012

Hyde, J. S. (1982) Understanding Human Sexuality. (2nd ed.) New York: McGraw-Hill, 1982.

Marlowe, F. (2010). The Hadza: Hunter-gatherers of Tanzania. Berkeley, CA: University of California.

Reisert, I., & Pilgrim, C. (1991) Sexual differentiation of monoaminergic neurons–genetic or epigenetic? Trends in Neurosciences, 14, 468-473.

Reitz, E., Bongardt, D. van de., Baamas, L., Doornwaard, S., Dalenberg, W., Dubas, J., Aken, M. van., Overbeek, G., Bogt, T. ter., Eijnden, R. van der., Vanwesenbeeck, I., Kunnen, S., Timmerman, G., Geert, P. van., & Dekovic, M. (2015) Project STARS: A longitudinal, multi-domain study on sexual development of Dutch adolescents. European Journal Of Developmental Psychology, 12, Issue 5. http://dx.doi.org/10.1080/17405629.2015.1018173


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