I. The Biology
When we see a man or a woman we can usually tell the
difference between the genders pretty easily. But sexual differentiation is not
as simple as it seems at first glance.
There are multiple levels of sexual differentiation. The most basic and
molecular is genetic, i.e., at the level of the sex chromosomes; the X and the Y. Humans have 23 pairs of chromosomes for a total of 46. The 23rd pair is the sex chromosomes. The Y chromosome is the chromosome that determines "maleness." Men have a combination of the X the Y. Women have two X chromosomes. Therefore, women can only contribute an ovum with an X
chromosome. Men can contribute a sperm with either an X or a Y chromosome. So, it is the male's sperm that will determine if the offspring will be male (XY) or female (XX). Then there is differentiation at the
level of the gonadal tissue. The same tissue in the developing
embryo becomes either male gonads (testes) or female gonads (ovaries). There
are no true hermaphrodites in humans, despite what people may tell you on the "street corner." True hermaphroditism describes having
both male and female functional gonadal systems which is typically seen only in
invertebrates such as earthworms and slugs. At the level of the internal sex organs, each
embryo has both male and female internal sexual organs but during development
one system develops and the other system deteriorates and disappears. The male system is the Wolffian
system which consists of epididymis,
vas deferens, and seminal vesicles. The female system is the Mullerian system which consists of the uterus, fallopian tubes, and inner one third of the vaginal canal. At the
level of the external sex organs, the same tissue will become either male or
female genitalia (penis and scrotum or clitoris and labia). Secondary sex traits like body hair,
breast development, muscle mass, fat deposition, proceed later in life during
puberty. However, during embryological and fetal development the brain is also differentiated
by sex hormones, which is manifested at the gross structural level by
differences in the size of the corpus callosum as well as the size of some
hypothalamic nuclei.
Sex Hormones
The sex hormones are steroid hormones and are structurally related to other steroid
hormones produced by the body. All
the steroid hormones have a similar structure because they are all made from cholesterol, hence the derivation of the term steroid
from cholesterol. Cholesterol
is normally a constituent of the cell membrane and helps the waxy oils of the
membrane become more fluid. Because of this, all the steroid hormones can cross
cell membranes relatively easily. In fact, the receptors for steroid hormones
are located inside the cell. So steroid hormones must ÒdissolveÓ across
the cell membrane and enter the cell to have their effects.
While the testes are the main source of the primary
male sex hormone, testosterone, and the
ovaries are the main source of the primary female sex hormone, estrogen (or estradiol) in women, each gonad produces small trace amounts of the other sex
hormone during synthesis because of their chemical similarity. So, males also have a small amount of
estrogen and females also produce a small amount of testosterone. However, even in women these small
amounts of testosterone are important.
For instance, in both genders, armpit and pubic hair are dependent upon
testosterone. The other steroids
are produced by the adrenal glands. They include the stress hormone, cortisol
(a glucocorticoid), and the salt/water
balance regulating hormone, aldosterone (a mineralocorticoid). In
both genders the adrenals also produce trace amounts of testosterone and
estrogen because of their chemical similarity to cortisol and aldosterone.
After Fertilization
There is a dynamic interplay of factors which lead to
a cascade of events that differentiate a male from a female that occur after a
sperm fertilizes an egg. One thing
to keep in mind is that the male pattern of development requires things to
occur. It is, in a sense, a much more active process that the female pattern of development. The female pattern is basically the default pattern for an embryo or fetus. At the cell level, whenever there is not an explicit "order" issued, development progresses according to the female pattern.
When an ovum is fertilized by a sperm with a Y
chromosome, a gene called SRY (for sex-determining region Y) on the Y
chromosome will be activated and begin production of a hormone called testes
determining factor (TDF). TDF causes the primordial
gonad, the undifferentiated gonadal tissue,
to become testes. Without TDF, the gonad would have become ovaries by
default. Once the testes are
formed they produce two hormones, testosterone and Mullerian inhibiting factor (MIF). Testosterone nourishes and promotes the development of the Wolffian system while MIF actively causes the Mullerian system to deteriorate. The
external genitalia are then masculinized by testosterone or more readily by its
metabolite, dihydrotestosterone
(DHT), which is a much more potent binder of the testosterone receptor. The enzyme 5-alpha-reductase converts testosterone to DHT. Ironically, however, the brain is
masculinized by testosterone only after it is converted to estrogen within the
cells of the brain by the enzyme aromatase. To prevent inadvertent masculinization of the brain by maternal
estrogen, all embryos and fetuses produce an estrogen binding protein called alpha-fetoprotein which captures any estrogens it comes in contact
with. Testosterone is unaffected by alpha-fetoprotein. It enters the brain
cells and is converted into estradiol within the neurons. As far as the body is concerned simple
sexual differentiation is basically complete at this point. The secondary sex characteristics will
be finalized during the sex hormone surges during puberty.
When Things Go Wrong
There are numerous things that can go wrong with sexual differentiation. There can be problems at any of the levels: genetic, gonadal, external genitalia and internal genitalia. WeÕll examine some of the more notable cases.
In Klinefelter's syndrome, the individual is an XXY male (sometimes called 47-XXY). Now, there are cases of XXY males without KlinefelterÕs (about 20% of them seem to escape the disorder) but all Klinefelter's patients are XXY males. As children, Klinefelter's males may have reduced muscle strength, but they grow they catch up though they may have reduced muscular coordination. As they mature past puberty and into adulthood, they are often taller than average, but often thin, less muscular sometimes show breast development and are infertile. While they may seem to have slightly below normal intelligence this is likely due to an increased rate of language problems (both generating and processing language as well as reading problems) which make testing more difficult.
In Turner's syndrome, the individual has only one sex chromosome, an X (sometimes referred to as a 45-X0) and since there is no Y chromosome the individual is a female. Turner's women tend to be shorter than average and are usually infertile because of a lack of ovary function. These individuals also often have extra skin on the neck (webbed neck), a broad chest with widely spaced nipples, puffiness/swelling of the hands and feet, skeletal abnormalities, heart defects and kidney problems. They are of normal intelligence with good verbal skills and reading skills; however, many have problems with math, memory skills and fine-skilled finger movements.
Androgen Insensitivity syndrome (AIS; sometimes referred to by the older term, testicular feminization) refers to a condition where a genetic XY male develops phenotypically (mostly) as a female due to inoperative testosterone receptors. However, the Y chromosome led to the development of testes which produce testosterone and MIF. While the body tissues do not respond to testosterone and develop as female by default, the MIF causes the Mullerian system to deteriorate and these individuals have no uterus, fallopian tubes and deepest third of the vaginal canal. Hence these individuals, while appearing externally to be normal females will be infertile. This disorder is typically not apparent until puberty because of a lack of menstruation and underarm and pubic hair.
5-alpha-reductase deficiency is a very rare condition
(only reported in isolated populations in the Dominican Republic, New Guinea
and Turkey) of genetic XY males with a poorly functioning enzyme which converts
testosterone into the far more potent DHT. Individuals are often born with external genitals that look
female and are raised as females.
However, the massive surge of testosterone at puberty then triggers the
final masculinization of the external genitalia and the descent of the testes.
Hence, the common name given to the condition in the Dominican Republic, heuvodoces, which is Spanish for "eggs at twelve." These individuals are typically
infertile and may need surgery to relocate the urethra.
II. Neural Development
Questions of sexual differentiation aside, before
birth and during early life the brain continues its development. Our experience shapes our neural
architecture as our neurons process the sensory qualities of our experiences
and respond to them.
Neuronal Survival
Before birth the brain has more neurons than it will
have after birth. The survival of neurons is determined by their activity. It literally is a case of "use it or lose it." Neurons that make enough
functional connections with other neurons are provided with growth factors, hormones that are trophic (sustaining health and
vitality), by their target neurons. Neurons that do not make enough functional
connections with other neurons go through a programmed cell death process
called apoptosis.
Early Experience & Fine
Tuning Synapses
After birth the size of the brain will increase but
that increase is due to increases in the size of the surviving neurons as well
as increases in the number of their connections and the dendrites and terminal
buttons where they are made. There are also increases in the number and size of
glial cells. The laboratories of
Mark Rosenzweig in the 1960s and William Greenough in the 1970s showed that
rats raised from puphood in enriched environments (large complex environments with playmates, a changing variety of toys
and structures allowing for climbing and running) had larger, heavier brains
rats raised in typical laboratory cages with limited opportunities for play,
exploration, physical activity and social interaction. The primary source of
the increase was in the outer layer of the cortex, the molecular
layer, where connections are made between
different distant regions of the cortex.
These enriched environment animals also learn problems quicker than rats
raised in non-enriched environments.
As in the case of Òuse it or lose itÓ with regard to neuronal number,
the enhanced connections also are dependent on sustained use. If enriched
environment rats are removed from that environment their brains do lose
connections but they still wind up with more than rats that were always raised
in the typical cage. Also the
effects of enhanced connections are greatest when the enriched environment is
experienced from birth. However, modest increases are still possible when a
typical non-enriched environment adult rat is placed in an enriched environment.
Complex early experience is not only necessary for
enhanced cognition but it is also necessary for the proper wiring of the
sensory processing parts of the brain.
Not only is it necessary, but there are critical periods, especially sensitive time frames where the
experience must be had or else the proper wiring of the sensory areas will never
be accomplished. In the 1960s and 1970s, the work of Nobel Prize winners David Hubel and Torsten Wiesel in cats showed that in the first six weeks of a cat's life if the visual cortex is deprived of the proper sensory stimulation from the eyes, the cortex will never be able to properly process visual information. The brain requires early experience to properly wire the cortex and there's a bit of a time deadline to do the job. In humans, critical periods seem to exist not only for sensory processing but also for such things as language, math, music and other cognitive skills and traits.
However, the critical periods are not narrowly or well-defined as in
cats. However, the ability to
flexibly wire the brain seems to decrease after puberty starts.
III. The Psychology
In contrast and in concert with the biological basis
of development, psychological development also seems to proceed in stages. Jean
Piaget (1896-1980) was the originator of the stage theory of childhood
cognitive development. While not complete
in its original form, like Freud he started a field that owed much to his
raising basic theoretical questions.
For Freud, it was the role of the unconscious. For Piaget, it was stages
of cognitive development; that children were not really learning like little
adults. His conceptual
breakthrough came while he was working for Binet and Simon on the development
and refinement of intelligence testing. While grading them he noticed that
young children consistently gave wrong answers to certain types of
questions. They kept making kept
making the same pattern of mistakes that were different than those of older
children and those of adults.
Cognitive Development
Piaget's theory proposes four stages and the processes by which children develop through them. The four stages are:
Sensorimotor stage (birth - 2 years old): The child interactions with the world and builds a set of concepts about it and how it works. This is the stage where a child does not know that physical objects remain in existence even when out of sight (object permanence).
Preoperational stage (ages 2-7): The child is not yet able to think abstractly and needs concrete examples and physical situations. The child also has difficulty taking the view point of others. Usually fails at conservation (the realization that the core properties objects or sets of objects stay the same even when they are superficially changed about or made to appear different).
Concrete operations (ages 7-11): As their experience grows, the child starts to conceptualize, creating logical structures that explain his or her world more completely. Conservation appears. Abstract problem solving is now possible at this stage.
Formal operations (beginning at age 11 and up): By this point, the child's cognitive structures are like those of an adult and include advanced conceptual reasoning. Abstract thinking is commonplace.
Piaget outlined several principles for building cognitive structures. During all developmental stages, the child experiences his or her environment using whatever mental constructs he or she has developed. If the experience is a familiar one or similar to a previous experience, it fits easily (is assimilated) into the child's cognitive framework without changing it. If the experience is different or new, the child alters his or her cognitive structural framework to accommodate the new information. This way, the child constructs more and more complex and accurate cognitive structures to explain and organize their experience of the world around them.
Personality & Social Development
During development, a childÕs personality and social skills also progress just as their brains and cognitive skills. Temperament is the emotional traits that organize the child's view of the world. They are critical in the development of the child's personality. These traits also determine how the child goes about learning about the world around him. Temperament appears to be relatively stable from birth. These characteristics are themselves never "good" or "bad." How they are received by others determines whether they are perceived by the child as being a bad or good thing. When there is an understanding of a childÕs temperament, then parenting can be more effective. For instance, some children are noisier than others, some may be more easily startled than others. When parents understand how their child innately responds under certain conditions, difficulties for the child can be anticipated and the child can prepared to cope or deal with the situation. While temperament is stable during childhood, especially the first five years of life, temperament changes do occur by adulthood. Childhood and adult temperament are positively correlated, but not highly (r=0.3). However, some adults may regress to their childhood temperaments under times of prolonged stress in their lives.
Humans are social creatures and that is evident from early infancy. All children, (even the blind) begin to smile by 2 months of age and they do so initially to potentially everybody. However, by 7-8 months, children start to exhibit wariness, especially to strangers. This wariness increases gradually peaking at 14-18 months of age then it begins to gradually decrease. By the age of 3 years most children feel secure enough to interact with other children and adults even in the absence of their parents.
These social behaviors indicate the human need for attachment in the development of the normal personality. The first indication of its importance came from studies in primates by Harry Harlow. In his first experiments infant monkeys were taken from their mothers at six to twelve hours after birth and were raised alone with access to substitute mothers made either of heavy wire or of wood covered in terry cloth. In one experiment both types of substitute moms were available, but only one was equipped with a nipple for nursing. Even when the wire mother was the source of food, the infant monkey spent a much more time clinging to the cloth mother, indicating that the infant had an innate preference for the softness and the warmth of contact with the cloth which would have been more similar to that of their real mother. When they matured and were reintroduced to other monkeys their actions were extremely unusual. They often clutched themselves and rocked constantly back and forth and they often exhibited excessive and misdirected aggression to other monkeys. Sexual behavior was seriously impaired. Usually when a female raised with inanimate substitute mothers was approached by a normal male, she would sit unmoving, squatting upon the floor. A substitute mom male might approach a sexually receptive female, but he might grab the head instead of the hind legs, engaging in pelvic thrusts. Others grabbed the females sideways, rendering their reproductive efforts in vain. The substitute mom females also failed as mothers themselves, being either indifferent or abusive toward their offspring. The indifferent ones did not nurse, comfort, or protect their young, but did not harm them. The abusive mothers violently bit or otherwise injured their infants, sometimes to the point of death. HarlowÕs studies indicated that maternal care and contact are important for normal social skills to develop as well as the ability to form emotional bonds with others. This ability to form emotional bonds is attachment. And learning how to do this in the parent-child relationship seems to serve as the emotional blueprint for forming general emotional attachments in all our other relationships
When a human infant (or monkey infant) is separated from its mother, the infant goes through three emotional reactions in stages. First is protest, in which the infant cries and refuses to be consoled by others. Second is despair, in which the infant is sad and unresponsive. Third is detachment, in which the infant intentionally disregards and avoids the parent if the parent returns. Attachment research on human infants suggests that sensitive responding by the parents to the infant's needs results in an infant who demonstrates secure attachment by the infant to the parents, which is usually demonstrated by the childÕs efforts to draw close to the parents or contact or at least greet the parent at a distance with a smile or wave. A lack of sensitive responding results in an insecure attachment by the infant. With insecure attachment, there seems to be a split into two varieties, termed avoidant (in which the child avoids or ignores the parent) and resistant/ambivalent (in which the child either passively or actively show hostility toward the parent). The attachment style is often assessed in the procedure called the strange situation:
As
assessed by this procedure 60-65% of American children are securely attached, approximately 20% are avoidant (~20%) and
approximately 10% are resistant/ambivalent. However, a new category called disorganized (10-15% of children) has been proposed which includes children many of whom were mistreated and often had parents in psychiatric treatment. These children seem to lack any coherent or consistent coping style due to the erratic parenting they've received.