Biological maturation throws a monkey wrench into our concepts of sex differences
The most outstanding sex differences, vaginas and penises for example, are unequivocally sex-specific from before
birth right on until death.
However, more subtle sex
differences may invert through development.
A few examples of these phenomena are the following. The body size (height, weight) of the human
male is greater, on the average, than that of the female. However,
there is a brief period, at
puberty, during which it is the average
girl who is taller and heavier than the average boy. We are accustomed to recognizing that girls
reach puberty before boys do. By
extension, we tend to generalize this
obvious biological precocity of girls to the ensemble of psychological function
and to think it applies to the whole of development up until puberty. In other words, we tend to think that girls
mature faster, psychologically and
biologically, than boys. However, there
are many exceptions to this rule. Boys
progress as fast as girls on many items of infant development scales, and there
are even a few examples of tissues that develop faster in boys. Even in terms of the development of
reproductive functions, in some
respects, it is the boy who matures
faster. An interesting such sex
difference prevails, in a sense, in the early development of endocrine
functions in the ovary and testis. The testis actively produces androgens
already in utero whereas the physiologically important steroid hormone
production of the ovary does not start until puberty. Likewise, several
components of the hypothalamic-pituitary-gonadal axis seem to mature earlier in
the male. The hormonal regulation of the fetal testes differs in many respects
from that of the adult, and these differences make it possible for the fetal
testes to function in the intrauterine endocrine milieu.
There are also twists and turns during development
(probably not entirely biologically determined)
having to do with sexual identity and sexual orientation. Girls manifest sexual orientation and
identity earlier than boys. But then
boys catch up. From adolescence to young
adulthood, females become slightly more
psychologically androgynous, while males
become less so. Then in mature adulthood
and senescence, there is an inversion of this sex difference. Men become slightly more psychologically
androgynous, and women slightly less so !
One investigation was based on observation of a group of between 84 and
91 rhesus monkeys (macaques) in captivity
for 18 months. Submission was most frequent in juveniles (aged 1.5-3.5
yrs), but aggression increased steadily with age. As infants (aged 0.5-1.5
yrs), males were more often involved in agonistic (friendly) behavior than were
females, but this sex difference reversed with age. By adolescence, the males manifested less «friendliness» and
more aggressiveness. In humans, male infants have most often been found to be
fussier, more irritable than female
infants. This difference is believed by
some to be wholly explainable by the greater incidence of inborn perinatal
complications affecting the male sex.
Male and female monkeys do not recover from brain
lesions in the same way. A researcher
named Patricia Goldman-Rakic lesioned the anterior area of the brain, the frontal lobe, of fetal monkeys. She found that the males manifested the
cognitive deficits earlier than the females. One study found that the ability
to perform on a concurrent visual discrimination task with 24 hour intertrial
intervals develops a few weeks earlier in female than in male infant monkeys.
To test whether this sex difference was related to the presence of perinatal
androgens, plasma testosterone levels were reduced in male infant monkeys by
neonatal orchiectomy (removal of testes) and increased in neonatally
ovariectomized (removal of ovaries) female infant monkeys by treatment with
either testosterone propionate or its reduced metabolite,
dihydrotestosterone. At 3 months of
age, the animals were tested on the task and their performance compared with
that of age-matched intact male and female monkeys. Orchiectomy which was
followed by a slight but visible atrophy of the external genitalia, hastened
performance of male infant monkeys to the level of intact infant females.
Conversely, androgenization of ovariectomized female infant monkeys given
dihydrotestosterone, which had only a slight virilizing effect on the external
genitalia, slowed the learning of these female infants to the rate of intact
male infant monkeys. Other experimenters
have found that adult rats from both sexes with frontal lesions were impaired
on many tasks, but the males were much less impaired than the females on tasks
which require animals to use multiple visual-spatial cues for a successful
solution (maze navigation). Another research
group found that relative rates of development of the prefrontal cortices in
either hemisphere inverse over time in rats,
and in some areas, these developmental inversions are opposite in males
and females. One team of researchers
looked at dendritic spining (this would correspond, say, approximately to the number of neuronal
synapses per cubic millimeter of brain tissue) of the hippocampal dentate gyrus
after environmental enrichment (lots of toys and play mates) or impoverishment
(no toys or play mates) of equally fed juvenile rats of either sex. They found that «cultural» enrichment in rats
favored the development of more dendritic spines in the females than
males. In the next section, we will see that there are numerous sex
differences in the size of various brain nuclei, and most of these sex differences remain
rather constant throughout life.
However, the development of any
particular brain tissue depends on another type of cell, namely glial cells. It has been observed that at certain phases
of prenatal brain development there are transient sex differences in glial cell
counts in certain of these brain nuclei before they become sexually
dimorphic, but not after. In fact,
these glial cell count differences probably reflect sex differences in
neuronal maturation.
All of these findings show that sex differences can
change with development. Changes in sex
differences in intellectual ability over the life span have also been reported
in humans. For example, the male advantage in spatial ability
undergoes some gain at puberty, not
because the pubertal boy improves but because the adolescent girl actually
undergoes a slight regression. The
interpretation of this particular phenomenon remains very controversial. Finally, we will see in detail later that
risk factors for certain neuropsychiatric disorders cross over from male
preponderance to female preponderance from childhood to the post menopausal
years. These phenomena (the ones I am aware
of anyway) are all hormonally mediated,
and our challenge will be to try to disentangle the various factors,
biological and cultural, which cause these interesting cross-overs.
Now it must not be forgotten that cross-over of sex
differences during development remains the exception rather than the rule. For example, metabolism is higher in human
males throughout life. Boys and men
expend more energy per hour per square meter of body surface than girls and women
do, having consumed an equal amount of calories. Men have more absolute muscle
mass than women, and they have a higher proportion of muscle mass to fat. The higher male metabolism is associated with
this greater muscle mass and even precedes it in development. There even exist parameters routinely
observed in blood samples of normal people which are significantly different
for the two sexes throughout life. Such
blood parameters are related to muscle mass and metabolic rate. One of the most commonly used machines for biochemical analysis of blood for medical
diagnostics is built by the Coulter company. The company provides crude norms
which clearly show a substantial sex difference in blood biochemistry. Normal men have higher concentrations of
hemoglobins, hematocrites and erythrocytes,
and greater average globular size
-all indicative of higher blood
oxygen metabolism in the male sex.
This sex difference seems to be mediated directly by
sex hormones. Indeed, the blood of 18
untreated transsexuals and 20 castrated or non-castrated transsexuals was
investigated in one study. The
erythrocyte (red blood cell) counts were significantly higher in untreated
males and treated female-to-male transsexuals than in untreated females and
treated male-to-female transsexuals, a
finding perfectly compatible with that of St Marseille described above. This apparently innocuous sex difference
seems to have implications for health issues.
Indeed, it has been found that
these blood paramneters are related to blood pressure, and hypertension in particular, and it is well known that men are more at
risk for hypertension than women -with the ensuing greater risk of men for more
dangerous and deadly vascular diseases.
The study of blood parameters could be surprisingly relevant to the
neuropsychology of cognitive function.
One recent study analyzed six distinct protein markers in blood serum of
normal men and women who completed an episodic memory task. Episodic memory is the recall of an episode
(event) not destined to be memorized (such as what one ate for breakfast two
days ago). The serum markers and
episodic memory performance strongly interacted with gender.
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