In some parts this gets super sciency, so I'll try to pout a more easily worded sentence or two of summation under paragraphs that I think got really complicated.
There's a large section of just biology, which is necesary to understand what's being discussed, but I promise it's going to get into psychology in the second half.
If all the biology bores you, and you want to just take my word for it that even biological sex is not a strict dichotomy, skip it and go straight to the psychology section at the bottom.
Table of contents:
Bio section:
intro, internal development, external development, pseudohermaphroditism
Psych section:
intro, gender identity (including a case study and the implications), sexual orientation
INTRO
As you all know, sperm meets egg, egg carries x, sperm carries either x or y, then you get a zygote which is either xx or xy and will become either female or male, respectively... as long as all goes according to plan. Disregarding those instances of true hermaphrodites, when a person is xxy, this genetic gender is more or less a dichotomy; you are one or the other. After that, however, things get complicated.
The default gender is female. A fetus, whether xx or xy, will develop female genitalia and female brain structures unless exposed to testosterone in the womb. Female fetuses do not require estrogen to become feminized, that's what happens without interference. The male fetus requires testosterone to interfere with development in order to become masculinized.
What usually happens is that females develop normally, and for males a gene on the y chromosome codes for the release of testosterone and other male hormones, making them develop normally as well. However, all normal women's bodies produce some testosterone, including pregnant women. In fact, female testosterone levels have been linked to the strength of the female sex drive, and having sex causes women's bodies to produce testosterone, so the more you have the more you want, as seen here:
This means that even female fetuses become slightly masculinized, to whatever degree they and their mother and producing testosterone. Similarly, some males produce less testosterone and will not be as masculinized as their testosterone filled peers. Moral of this story: genetic gender is more or less a dichotomy, but the expression of gender in both genitalia, secondary sex characteristics, and behavior (related to brain structures) is a sliding scale, with super macho on one end, super feminine on the other, and everyone falling somewhere in between, usually towards the side of their genetic gender.
This is where things can get screwed up. This is also where I'm going to add some photography from the textbook to explain some things. If you're creeped out by diagrams and pictures of genitals stop here lol.
DEVELOPMENT OF THE INTERNAL SEXUAL ORGANS
A fetus which has not yet differentiated contains two gonads, which can become ovaries, or testes if the SRY gene is present (which is carried on the y chromosome). The undifferentiated fetus also contains Mullerian ducts, which will become the uterus, fallopian tubes, and inner vagina, unless the Mullerian inhibiting hormone is present. Once again, female is the default gender. There is also a set of Wollfian ducts, which will wither and be absorbed. In the male, a Mullerian inhibiting hormone is produced, which withers the Mullerian ducts. Then, testosterone causes the Wolffian ducts to develop into the seminal vesicles and vas deferens.
This is called organizing effects, and occurs relatively early in fetal development. Activation effects, on the other hand, come and go, like puberty.
Basically, one set of ducts becomes the internal female genitalia, and the other becomes in the internal male genitalia.
Here's a lovely diagram for you:
DEVELOPMENT OF THE EXTERNAL GENITALIA
As for external genitalia, once again the female is the default. This development, however, does not occur until far into the pregnancy. As you will see in the diagram, at the second trimester the differences are barely distinguishable (don't ask me how the doctors figure it out, I have no idea). The glans becomes a clitoris, or the glans of the penis if dihydrotestosterone is present. Another lump of flesh becomes either the labia minora, or the shaft of the penis, and the labioscrotal swelling becomes either the labia majora or the scrotum. These structures come from the same tissue, meaning, for example, that you could not have both a labia majora and a scrotum.
PSEUDOHERMAPHRODITISM
What you may have caught in that paragraph is that all that is required is the presence of dihydrotestosterone, not that the baby's body is producing it, and that this is undifferentiated for the majority of fetal development. This means the baby is highly susceptible to changes in hormone levels in the mother. Get a little too stressed and produce some extra testosterone while you're pregnant, and your baby girl might come out with a very large clitoris. Take a medication which mimics testosterone while you are pregnant, and you might get something like this:
Take testosterone shots throughout the pregnancy, and your baby girl might have a full grown penis.
This is called female pseudohermaphroditism. In pseudohermaphroditism, the person's gonads match their chromosomes (as opposed to a true hermaphrodite, which has both ovaries and testes), but the external genitalia does not match. The female pseudohermaphrodite has a uterus, ovaries, fallopian tubes, and an inner vagina, but the external genitalia looks masculine to some degree. Depending on how masculine the genitals look, the parents tend to ask for surgery to make the child look more feminine or masculine, and then raise the child accordingly. We will return to this when I (finally) get into the psychology.
Similarly, if the male baby does not produce testosterone, or it lacks testosterone receptors, it will have labia minor and majora, a clitoris, and the outer vagina. It will be totally indistinguishable from a girl without a DNA test or an MRI. A male baby which produces the Mullerian inhibiting hormone, but cannot produce or use testosterone, will not have a uterus or fallopian tubes, but it will have external female genitalia.
This is called male pseudohermaphroditism. It looks something like this:
Yup, that's an xy male. He is androgen insensitive, one of the many possible causes of male pseudohermaphroditism, meaning you could shoot him up with all the testosterone you want and he'll still look as feminine as ever. he has no receptors for testosterone. The Mullerian inhibiting hormone is still present, meaning no ovaries, fallopian tubes or inner vagina is present. Instead, the outer vagina ends in a blind pouch.
They often do not know they are male until they fail to get their period. Those who are very thin attribute this lack of menstruation to being underweight or exercising a lot (which is known to cause amenorrhea) and don't realize they're not female until they cannot get pregnant and a thorough medical examination revelas their undecended testicles up in thier body.
Because they are still sensitive to estrogen, these individuals, tend to have well developed breasts and a flawless complexion (no testosterone to cause acne). This is often combined with long slender legs, leading these individuals to repeatedly appear as fashion models. Yeah, that episode of House was for real.
Okay, no more biology now, I promise.
ONWARD TO PSYCHOLOGY!!!
INTRO
The question now becomes 'What is gender?'
It's pretty obvious what genetic gender is, but clearly the external genitalia are not telling the whole story. What's even more interesting is the different brain structures which are effected by hormones.
Some definitions:
Sex describes the biological characteristics of the person.
Gender describes the person's behaviors.
A gender role is the set of behaviors society lists as acceptable for a particular gender.
Gender identity is the subjective feeling of being male or female.
GENDER IDENTITY
The influences of testosterone, or lack there of, are felt in the brain as well as in the body.
Here we can see normal male and female rat brains, as well as female rat brains treated with two testosterone replacements:
The Sexually Dimorphic Nuclei (SDN) is clearly largest in the male and the female treated with testosterone, slightly smaller in the female treated with a teststerone replacement, and tiny in the normal female. In this case testosterone had a much larger effect on brain structure, and therefore on behavior, than chromosomal sex did.
This means that, regardless of your genetic sex, if you are exposed to testosterone your brain will be more masculine.
It was thought in the 50's, during the behaviorist movement which was all nurture and no nature, that gender identity was totally the product of upbringing. They cited the fact that many individuals born with ambiguous genitalia tended to accept the gender they were raised in. It has been contested by those who believe in the gender-at-birth hypothesis that this data actually supports their claims, because individuals with ambiguous genitalia tend to be raised as the gender their genitalia most resembled. This means they were probably raised as the gender their hormones most reflected. They cite the fact that individuals with totally unambiguous genitalia have never been successfully reassigned to another gender.
CASE STUDY
Case and point: David Reimer.
This case study was a psychologists dream, with all variables naturally controlled for. Bruce was born in 1965 with a twin brother. His brother's circumcision was successful, but Bruce's was botched, causing his entire penis to need to be removed. This was before the days when surgery could create a penis, so his parents decided to just raise him as a girl. Surgery made Bruce's genitals look feminine, and his parents gave him estrogen supplements when he reached puberty, and he was renamed Brenda.
While Brenda's psychologists happily reported that she was totally accepting her feminine role, things were far less peachy. Brenda was a tomboy, constantly teased by her classmates, who called her a caveman. She wanted to urinate standing, and began using the boy's bathroom. She preferred her brother's toy trucks to her own toy dolls. At the age of 11 she had decided that she was definitely a boy. At age 14 she decided to make the transition to being a male, and her father told her the truth.
By age 25 Brenda had been renamed David, had his breasts removed (which developed under estrogen treatment), had a penis constructed, and married a woman with 3 children.
Unfortunately, after 14 years of marriage, his wife asked for a divorce, unable to deal with the depression he suffered. He took his own life in 2004.
IMPLICATIONS
Cases like this have been used by many psychologists to advocate that a child with ambiguous genitalia be allowed to choose on their own what gender they are. Often times parents of children with ambiguous genitalia elect for surgery at birth and pick a gender to assign to their child, but should they choose the wrong one they often subject their child to a lifetime or troubles. Many children with ambiguous genitalia who are not assigned to a sex choose their own at some point, while others choose to remain intersex. Concern arises as to whether a child raised intersex might be tormented by their classmates for their ambiguous appearance.
SEXUAL ORIENTATION
Similarly to the SDN, the MPOA has an increased density in males, and the AH has an increased density in females.
However, when you stimulate the males AH and the female's MPOA, you can get something like this:
That's the female rat on top. Stimulate the male rat's AH and he will engage in the female version of rat flirting when a male walks by.
Super cuteness tidbit: A female rat flirts by jumping up and down and twitching her ears. Awwwww
The male rat will also engage in lordosis, the receptive sexual posture of the female rat, in which the rat arches it's back and moves it's tail to the side.
Essentially, you can induce homosexual behavior if you stimulate the proper areas of the brain.
Similarly, the brain structures of homosexuals and heterosexuals differ. Here we see the INAH3 in a heterosexual vs. a homosexual male:
The INAH3 of a female, while not pictured here, most resembles that of the homosexual male. INAH3 regulates sexual behavior in animals.
Additionally, the suprachiasmatic nucleus (SCN) is large in homosexual males than in heterosexual males, and the anterior commisure (AC) is larger in homosexual males and heterosexual females than in heterosexual males.
The SCN regulates the reproductive cycle. The AC is involved in communication between the two hemispheres. Women tend to have much greater communication between the two sides of the brain, and homosexual males display this tendency as well.
And here we have the response of a female, a homosexual male and a heterosexual male to a male pheromone (note that pheromones cannot be consciously detected):
Similar results have been found with homosexual females (the respond to female pheromones like their male counterparts and unlike their heterosexual female counterparts).
As far as genetics go, here's the frequency of homosexuality in people with a close relative who is gay:
And here's the location of the potential "gay gene":
The gene, like all genes, is not to be an all or nothing gene. Having it only conveys a certain percent chance of being gay, not a guarantee. Evidence suggests that it is inherited from the mother's side of the family, on the x chromosome she donates to her child.
An evolutionary explanation for this has been offered: while males with this gene tend to be gay, women with this gene tend to have more offspring. This is worth the evolutionary risk of producing a son who will not reproduce, because many daughters can be produced who will produce even more daughters.
Homosexuals also tend to perform in tests the way their opposite sex peers perform, as seen here:
Research shows that about 44% of gays and 54% of lesbians tend to be feminine or masculine, respectively. It should be understood that sexual orientation and gender identity are two separate issues. A woman may feel that she is actually a man and still be attracted to men. Similarly a woman may be attracted to other women but not feel that she should have been born male. They are two separate issues.
There is no evidence for the Freudian claim that a dominant mother and a weak father cause male homosexuality.
Overall the evidence shows that there is some potential influence of personal experience on sexuality, particularly in bisexuality, though it cannot be completely attributed to environment.
Most homosexuals report feeling "different" as early as 4 years of age, and they display a high level of gender non-conformity, or a breaking of gender roles.
An interesting note is that administering testosterone to homosexuals males does not make them heterosexual, it merely increases their sexual activity levels, as it would in heterosexual males. This suggests that changes in the brain occurred prenatally and cannot be reversed.
A common hypothesis suggests that homosexuality is caused by a woman having many sons, at which point she excretes too much testosterone, over-masculinizing the baby, because many gay men have significantly more older brothers than do heterosexual men. this makes sense because many homosexuals tend to exhibit characteristics of higher testosterone levels, such as larger genitalia, left-handedness, and longer ring fingers. This runs into a problem as males who have CAH (a disorder which produces excess testosterone in-utero) tend not to be gay, and there's no reason why a woman should produce more testosterone after successive male births.
A better suggestion is that the mother produces an antibody which prevents masculinization of the brain. This excess testosterone may not be the cause of homosexuality as much as a coincidence, that when the baby's brain is being demasculinized by the mother's antibodies the baby's body tries to compensate by producing excess testosterone. This makes the brain remain demasculinized but the body over-masculinized.
Some have hypothesized that the number of older brothers is an environmental influence, not a hormonal one, but this was discounted when it was shown that having many older brothers who are not genetically related has no effect on homosexuality.
Female homosexuality has been far less researched.
Sources:
Dr. G's lectures in biopsychology
Brain & Behavior: An Introduction to Biopsychology (2009). Second edition.
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