Elements of sexuality and behavior

1
Gender and the Brain
2
Sexual dimorphism

• refers to differences between males and females
• sexual dimorphism exists both physically and
  behaviorally
  
• behavioral sex differences in childhood
• rough and tumble play
• maternal rehearsal

3
Sexual dimorphism, contd.

• behavioral sex differences in adulthood
• problem-solving tasks
• verbal fluency
• specialization of motor skills
• nature vs. nurture debate
• nature biological basis for sexual dimorphism
• nurture dimorphism has a social / cultural basis

4
Biological effects of sex hormones

• organizational effects permanent structural
  changes elicited by sex hormones during critical
  period of embryonic / fetal life
  
• activational effects reversible effects of sex
  hormones (e.g. estrus)

5
Sexually dimorphic behaviors in rats

• androgens appear to divert development from
  neutral (female) pathway
  
• critical period 1st 5-10 days of life
• treat females with testosterone masculine
  sexual behavior
  
• castrate males before critical period display
  female sexual behavior
  

6
Rats Effects of Testosterone on Brain

• hypothalamus (medial preoptic area)
• group of cells named sexually dimorphic area
• sex difference more prominent in males
• treat females with testosterone induce
  masculine hypothalamus
  
• sex differences in surge center (masculinized by
  androgens)
  

7
Rat Studies

• conclusions testosterone during critical period
  brain masculinized otherwise feminized
  
• findings differ somewhat in primates (including
  humans)

8
Non-human primates

• sexually dimorphic behaviors in monkeys
  mounting behavior
  
• normal females never exhibit mature mounting
  behavior
  
• if exposed to androgens during fetal life
  females show mature mounting behavior

9
Non-human primates

• results not as clear-cut as rodent studies
• females dont become fully masculinized
• still exhibit female sexual behavior
• exhibit cycles and can become pregnant
• possible experimental flaws
• critical period?
• dose of androgens?
• effect of masculinized genitalia?

10
Humans

• both children and adults display sexual
  dimorphism with respect to behavior
  
• biological basis for sex differences?

11
Female Pseudohermaphrodites

• congenital adrenal hyperplasia
• display more rough and tumble play, less maternal
  rehearsal than female peers
  
• show greater visual-spatial ability, less verbal
  ability
  
• conclusions about gender identity less clear
• study 1 assumed gender role that was reinforced
  during childhood
  
• study 2 even if raised as females, higher
  incidence of lesbianism
  

12
Testicular Feminization Syndrome

• merms
• adult gender identity and role completely
  consistent with that reinforced during childhood

13
Guevodoces

• ferms before puberty
• raised as females
• after puberty, assumed male gender identity,
  gender role
  
• due to societal acceptance?
• due to exposure to small amounts of androgens?

14
Gender differences in brain structure function

• less lateralization of cerebral hemispheres in
  women (in men, left more concerned with verbal
  skills, right with visual-spatial function)
  
• larger fiber tracts in female corpus callosum
• sexually dimorphic nucleus present in
  hypothalamus (also INAH 3)

15
Organizational effects of steroids?

• surge center still capable of being activated
  in males (appears to be an activational system)
  
• evidence from medical treatment of transsexuals
  argues against organizational effect
  
• Discrepancy evidence from CAH argues for organ.
  effect
  

16
Brain structure / function and homosexuality

• homosexual men have feminized gender role with
  respect to sexual orientation
  
• INAH 3 is small (feminized) in some gay men
• suprachiasmatic nucleus of some gay men is 2X
  larger than that of heterosexual males or
  females
  
• corpus callosum is feminized in some gay men

17
Homosexuality, contd.

• possible genetic contribution to homosexuality of
  Xq28 genes
  
• portion consists of 200 genes on X chromosome
• presence of this gene cluster results in
  increased incidence of male homosexuality
  
• passed on by maternal X chromosome
• not associated with lesbianism