Sex & The Brain

Some Basic Ideas• Significant differences in brain structure and

function between men and women are known. – 1880: an English surgeon reported slight differences

in the brain anatomy of men and women.

• On average, women’s brains are smaller.– Absolute size is not the key.

• Number of neurons is more critical.– On average, women have 11% more neurons. – “Extra” neurons are found in 2 layers of cerebral

cortex involved in recognition of language, melodies, and tone of voice.

• Number of connections, organization is the key.

The Corpus Callosum• Neural bridge connecting right and left

hemispheres. • Best documented anatomical difference

– Larger in women than in men.

• Researchers from UCLA examined 146 cadavers.– Found the rear portion of the corpus callosum was

up to 23% larger in women than in men.

• Are women better able to integrate the left and right brain?– Unknown - No studies of actual brain activity

• Significant only if it contained more neurons– Not demonstrated to date

Sex Hormones and Development

• The most important factor in differentiation of males and females and in differentiating individuals within a sex is the level of exposure to various sex hormones early in life

• Sex hormones can stimulate or repress the development of neurons

• The brain of a developing fetus is bathed in sex hormones

• At birth, environmental & cultural factors are acting on the already differently wired brains of boys and girls.

The Role of Testosterone

• Testes produce male hormones, or androgens (especially testosterone)

• Hormones are responsible for transformation of the genitals into male organs

• Also for organization of corresponding male behaviors early in life

• Female anatomy (and probably most behavior associated with females) is the default mode in the absence of androgens.

The Effect of Hormones is Life-altering

• Effects of early exposure to sex hormones are lifelong

• Changes are 'organizational' • Alter brain function permanently during a

critical period in prenatal or early postnatal development.

• Administering the same sex hormones later in development or in the adult has no similar effect

Animal Studies• A rodent with functional male genitals is

deprived of androgens immediately after birth (by castration or by the administration of a drug that blocks androgens)

• Male sexual behavior, such as mounting, is reduced, and female sexual behavior is expressed.

• If androgens are administered to a female directly after birth, she will display more male sexual behavior and less female behavior in adulthood.

Human Studies• Controlled experiments in humans are not

ethically feasible. • Scientists have studied defects which affect

hormone levels.• Boys with a syndrome that makes them

insensitive to testosterone are better at language than their unaffected brothers, but they are less adept at spatial tasks

• Girls with a rare birth defect called CAH have also been studied, as well as girls exposed to the hormone DES.

CAH & DES• The birth defect, CAH causes production of high

levels of testosterone during fetal development in girls.

• CAH girls score better than normal girls on spatial and mathematical tests.

• Girls exposed to the hormone DES in utero (a drug given to prevent miscarriage) are also exposed to excess testosterone.

• Unlike CAH girls, DES girls are entirely feminine in all other respects.

• DES girls do better than their normal sisters in spatial and mathematical tasks.

Hormones & Intellectual Functioning

• Women with high testosterone perform better on a spatial task than women with low levels

• Men with low levels outperform men with high levels.

• On a mathematical reasoning test, low levels correlate with performance in men

• On a test in which women usually excel (bottom), no relation is found between testosterone and performance.

The Role of the Hypothalamus• Female and male reproductive behavior is

regulated by the hypothalamus. • The hypothalamus connects to the pituitary, the

master endocrine gland. • One region of the hypothalamus is visibly larger

in male rats than in females – this size difference is under hormonal control.

• Scientists have also found parallel sex differences in a clump of nerve cells in the human brain– parts of the interstitial nucleus of the anterior

hypothalamus are larger in men than in women

The Hypothalamus & Sexual Orientation

• Sexual orientation and gender identity have been related to anatomical variation in the hypothalamus.

• Another part of the hypothalamus has been found to be smaller in male-to-female transsexuals than in a male control group.

Differences in Intellectual Function

• Major sex differences in intellectual function occur in types of ability rather than in overall level of intelligence (measured as IQ)

• Men and women overlap enormously on many cognitive tests that show average differences

• Variation between men and women is generally less than deviations within each sex

Male Dominated Tasks

• On average, men perform better than women at certain spatial tasks. – Tests that require the subject to imagine

rotating or manipulating an object – Navigating through a route.

• More accuracy in tests of target-directed motor skills--that is, in guiding or intercepting projectiles.

• Better at mathematical reasoning tests

• They do well on tests that involve mentally rotating an object or manipulating it in some fashion, such as imagining turning this three-dimensional object

• Or determining where the holes punched in a folded piece of paper will fall when the paper is unfolded:

• Men also are more accurate than women in target-directed motor skills, such as guiding or intercepting projectiles:

• Men do better on disembedding tests, in which they have to find a simple shape, such as the one on the left, once it is hidden within a more complex figure:

• And men tend to do better than women on tests of mathematical reasoning:

Female Dominated Tasks

• Women, on average, excel in language tasks:– tests that measure recall of words – tests to find words that begin with a specific letter

or fulfill some other constraint.

• Women are better than men at rapidly identifying matching items

• Women also excel at certain precision manual tasks

• Women tend to perform better than men on tests of perceptual speed, in which subjects must rapidly identify matching items for example, pairing the house on the far left with its twin:

• Women remember whether an object, or a series of objects, has been displaced:

• Women outperform men on tests of ideational fluency, for example, subjects must list objects that are the same color.

• Women also outperform men on tests of verbal fluency, in which participants must list words that begin with the same letter.

• Women do better on precision manual tasks-that is, those involving fine-motor coordination - such as placing the pegs in holes on a board:

• Women do better than men on mathematical calculation tests:

Who Needs Directions?

• Studies show that men complete a computer simulation of a maze or labyrinth task more quickly and with fewer errors than women do.

• A different study used a path on a tabletop map to measure route learning.– Although men learned the route in fewer trials and with

fewer errors, women remembered more of the landmarks, such as pictures of different types of buildings, than men did.

• Women use landmarks as a strategy to orient themselves in everyday life more than men do.

Landmark Memory

• Research suggests that women have superior landmark memory.

• Researchers tested the ability of individuals to recall objects and their locations within a confined space, such as in a room or on a tabletop. – Women were better able to remember whether items

had changed places.

• Other investigators found that women are superior at a memory tasks such as remembering the location of pictures on cards turned over in pairs.

• At this kind of object location, in contrast to other spatial tasks, women have the advantage.

Right Brain/ Left Brain?

• Research has shown greater differences in activity between the hemispheres of men than of women during certain language tasks, such as judging if two words rhyme or creating past tenses of verbs.

• Other research has failed to find sex differences in functional asymmetry.

• Different results may reflect different language tasks used in the various studies– the sexes may differ in brain organization for some

language tasks but not for others.

The Evidence from PET & FMRI

• Positron emission tomography (PET) & functional magnetic resonance imaging (FMRI) can reveal how male & female brains actually function.

• By detecting areas of the brain using the most blood glucose, PET pinpoints active regions.– Glucose is the basic fuel of all cells in the body,

including brain cells. – Thus, active regions of the brain use more glucose

than quiescent areas. – Glucose is radio-labed– The brain areas in use then will emit more

radioactivity, which can be detected by the PET scan.

So Who Likes the SAT?• Researchers at U.C., Irvine used PET to study

22 male and 22 female students while they were solving SAT math problems.

• Half of the men and half of the women had SAT math scores above 700.

• PET scans of male students with high SAT scores showed intense activity in the temporal lobes compared with men with average scores. – This indicates that in men, ability is related to how

hard the brain works.

• Women with comparably high scores expended no more neural effort than average SAT women

Banana Fana Fo Fana . . . Banana ?

• Researchers at Yale University found that men and women use different parts of their brains to figure out rhymes.

• Pairs of nonsense words were flashed on a screen & brain activity was monitored by FMRI

• In all male subjects, one region in the left inferior frontal gyrus lit up. – This is an area known to be involved in language.

• In more than half of the female subjects, the same area, plus an additional area, lit up. – This additional area is involved in emotion.

And This Means?

• One interpretation of this finding is that women are more adept at language because they draw on feelings as well as reason when processing language

• Since sounding out words is a necessary first step to reading, researchers were looking at rhyming as an indicator of reading skill.

Your Mind is an Open Book• Researchers studied men and women who were

asked to clear their minds, to think of nothing. • Examined 37 men & 24 women using PET

scans. • In men’s idling brains, the action was in the

temporal-limbic system. – This is one of the oldest regions of the brain from an

evolutionary standpoint– Controls emotional responses linked to basic

survival, such as aggression and sexual behavior.

• In women, the most active area at rest was the posterior cingulate gyrus– A more developed area of the brain which is

associated with language.

Men are From Mars . . .

And Women Are From Venus