a pre-existing egg becomes mature

(Recall not until Weismann, 1885, was it known that egg and sperm cells are independent of influences from other cells. This was the death of Lamarckian inheritance of acquired traits though it was not fully accepted until about 1920 or so. Many psychologists and biologists educated around 1900--Freud, Watson, Piaget-- seemed to hang onto the Lamarckian idea for the progress to the human race it promised but could not deliver. Behaviorism in the USA took up the need for "progress" -- discounting heredity now that it could not be readily changed.)

Each female has about 500 eggs.

sperm competition

Males have millions of sperm constantly being produced. Only one cell can fertilize an egg --hence there's a race to be first. Among publically promiscuous primates such as chimps, sperm competition takes on even more significance when a fertile female mates with perhaps dozens of males in a short time period.


one sperm is allowed to enter egg
cell division
cell differentiation
form fetal gonad

Remember male and female anatomy must come from the same "plan" hence we should expect there are corresponding parts in both sexes anatomy, e.g. penis-clitoris, nipples on both male and female, etc. including corresponding brain-brain relationships.

Hormones, primarily androgens, shapes these basic common parts into mammalian males and females.

hormone input determined by M/F genes
differentiate female reproductive anatomy
(differentiate MALE anatomy ONLY if male hormones are present)

"The results reviewed here prompt the consideration of feminization as a process which functions parallel to masculinization. The two processes are qualitatively different and operate during different developmental periods. In order for the brain to become sexually differentiated, males need exposure to testicular androgens during the perinatal period (roughly from embryonic day 17 through postnatal day 10 in rats), and females need exposure to ovarian secretions including, but not necessarily limited to, estrogen, during a later period that may extend to or even beyond puberty." (psycoloquy.95.6.05.sex-brain.1.fitch; A ROLE FOR OVARIAN HORMONES IN SEXUAL DIFFERENTIATION OF THE BRAIN by Fitch & Denneberg.)

brain develops toward end of fetal period

brain differentiation
overproduction of neurons and synapses

birth (to about age two)

continued brain growth
begin mylination of neural fibers

These speed transmission of information; reduce errors.

begin pruning (loss) of surplus neurons and synapses

New synapses continue forming based on internal guidance (basically an unknown process) and on the basis of external feedback from receptors

form self-identity including gender identity

based on anatomy and neural "programming"
self observations and comparisons
behavior of others toward self (socialization)

adolescence (triggered by hormones)

gender identity established
erotic imagery established
secondary sex characteristics emerging
sexual activities increase

length of estrus in nonhumans varies greatly

(See overhead based on deWaal (1995.)

One day in gorillas to several weeks in bonobos.

This typically coincides with maximum fertility with the exception of bonobos and humans.

In humans, ovulation is concealed and the female is more or less continuously interested in sex.

sex differences maximized during periods of high fertility

(these differences may have unforseen consequences in the life of contemporary humans)

(of course this is the same process that enables change and speciation!)

greater chance of ruptured ACL (knee ligament) in females playing soccer/basketball

greater chance of permanent damage to language centers from strokes in males.

greater incidence of reading disorders in males

perhaps increased risk of breast cancer in modern females due to greater "exposure" to their own hormones due to early sexual maturity and relatively few --often late -- pregnancies.

Sex links

  1. A role for ovarian hormones in sexual differentiation of the brain
  2. Testost erone and dominance in men