SEX DIFFERENCES AND ADAPTATION

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SEX DIFFERENCES AND ADAPTATION

• Empirical demonstration of sex differences has
  been a major strategy for showing adaptation in
  human evolutionary psychology
• When sexes have faced different adaptive
  problems/selection pressures in ancestral
  environments, expect different brain mechanisms
  to have evolved
• Given that men and women inherit genes from both
  parents, how can sex differences evolve?
• Y chromosome contains Sry gene that acts as
  testis-determining factor
• Fetal testes produce androgens like testosterone
• Testosterone turns on some genes rather than
  others and this differential gene activation
  contributes to sex differences in development

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Selection Pressures Related to Sex Differences

• 2 basic types of selection pressures that may
  cause the evolution of sex differences
• Division of labor History of males and females
  doing different things to survive
• Sexual selection Males and females having
  different strategies for reproduction/competition
  for mates

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SEXUAL SELECTION
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SEXUAL SELECTION

• Darwin argued that sexual selection, depends on
  the advantage which certain individuals have over
  other individuals of the same sex and species in
  exclusive relation to reproduction
• Trait is sexually selected if it confers an
  advantage over same-sex rivals but is not
  necessary for survival or for reproduction in the
  absence of rivals
• Sexual selection is a subset of natural selection
  (artificial category) and operates according to
  same mechanism (differential gene replication)

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PARENTAL INVESTMENT THEORY

• Trivers (1972) predicted that the sex with the
  greater typical parental investment in offspring
  becomes a limiting resource for the reproductive
  success of the opposite sex
• PI defined as any cost associated with raising
  offspring that reduces parents ability to
  produce or invest in other offspring
• Females of most species make larger typical
  parental investment than do males
• Batemans data with fruit flies

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PARENTAL INVESTMENT THEORY

• Trivers (1972) predicted that the sex with the
  greater typical parental investment in offspring
  becomes a limiting resource for the reproductive
  success of the opposite sex
• PI defined as any cost associated with raising
  offspring that reduces parents ability to
  produce or invest in other offspring
• Females of most species make larger typical
  parental investment than do males
• Batemans data with fruit flies
• The lower investing sex should typically exhibit
  greater eagerness to mate, greater competition
  for mating opportunities (often larger size), and
  more elaborate courtship displays/traits

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EAGERNESS FOR SEX/SEXUAL VARIETY

• Coolidge Effect
• Clark and Hatfield study

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Percent of subjects who agreed when approached by
an opposite-sex stranger
Go on a date tonight
Go to bed with me tonight
Women
Men
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EAGERNESS FOR SEX/SEXUAL VARIETY

• Coolidge Effect
• Clark and Hatfield study
• Sex differences in sexual fantasy/pornography
• Cross-cultural research on desire for number of
  partners, etc.

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Do Men Desire Sexual Variety More Than Women
Do? __________________________________________
Ideally, how many different sexual partners
would you like to have in the next month?
Source Schmitt et al. (2003, Journal of
Personality and Social Psychology)
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PHYSICAL ATTRACTIVENESS

• Do humans possess specialized mechanisms that are
  designed to find particular physical cues
  attractive?
• An adaptationist approach predicts that any such
  cues should have signaled health, fertility, or
  other beneficial traits (e.g., resource
  acquisition abilities, parenting abilities) in
  ancestral environments
• Can compare this position against null hypothesis
  that attractive traits are arbitrary (beauty in
  eye of beholder)

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Female Attractiveness

• Clearly strong selection pressure on ancestral
  men to choose fertile vs. infertile females as
  mating partners
• Age (post-pubertal and pre-menopausal) and cues
  to age
• Cues of poor health, insufficient energy should
  be unattractive
• Even among the class of fertile women, though,
  are there cues that mark some as more chronically
  fertile/healthy than others?
• Shorter birth spacing
• Higher rates of offspring survival?

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Fertile window
Estrogen
Progesterone
Age Energy balance Lactation Stress
ovulation
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Estrogen as Fertility Index

• Are there physical cues that predict individual
  differences between women in estrogen
  concentrations?
• Facial cues (Law-Smith et al. 2006)
• 18-24 year old women
• Face photographs, urine samples for estrogen
  assay
• Late follicular estrogen correlated with ratings
  of face attractiveness, femininity, and health
• Effects only for women not wearing make-up
• Body shape
• Waist to hip ratio
• Breast size

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WHR as possible cue to Age Sex Pregnant
status Health Fertility Fatty acids (brain
growth) Estrogen?
Singh (1993)
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Estrogen and Body Shape
Polish women, 24-37 years old. Daily saliva
samples for one cycle. Breast size and WHR split
into top and bottom quartiles. Jasienska et al.
(2004)
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Male Attractiveness
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Male Attractiveness

• Testosterone appears to act as a signal that
  allocates energy to mating effort (including
  growth of secondary sex traits) vs. survival
  functions (e.g., immune responses)
• T injections can increase 2nday sex traits but
  decrease immune function
• T declines during non-breeding season when 2ndary
  sex traits atrophy
• T declines during food shortages, illness
• Physical cues associated with higher testosterone
  may therefore signal that males in good enough
  condition to afford higher testosterone (could
  indicate better immune system for local
  environment genes passed on to females
  offspring)

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Testosterone Fitness Indicator in Humans?

• In humans, some evidence that higher testosterone
  associated with more masculine facial features
  (will test this in lab activity)
• However, most masculine faces not typically
  perceived as most attractive

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Mixed Strategy Model

• Masculinized faces rated lower on qualities like
  good father or quality as parent (Johnston et
  al. 2001 Perrett et al. 1998)
• When choosing most attractive face, women
  generally select faces that have slightly
  feminized relative to the average male face
  (Perrett et al. 1998)
• However, when women are tested near ovulation,
  they shift their preference toward the masculine
  end of the continuum (Johnston et al. 2001
  Penton-Voak et al. 1999)
• Mixed strategy choose feminine, high investing
  men as long-term partners cheat with masculine,
  high testosterone men when fertile to get good
  genes

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Selection for Extra-Pair Copulations

• This feature of womens preferences cycle
  shifts in preferences for the scent of
  symmetrical men can be explained as a special
  design feature evolved to enable ancestral women
  to obtain genetic benefits from extra-pair mates
  while avoiding costs of seeking such benefits
  during periods of infertility. It is difficult to
  understand the nature of these preferences
  otherwise. Gangestad (2000 pp. 57-58)

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Cycle Phase Shifts

• Stronger preferences for more masculine faces
  near estimated fertile window
• Penton-Voak et al. (1999) Follicular vs. luteal
• Penton-Voak Perrett (2000) Days 6-14 vs. all
  others
• Johnston et al. (2001) 23-14 days before next
  menses vs. others
• Stronger preferences for scent of symmetrical men
  near fertile window
• Gangestad Thornhill (1998) Days 6-14 vs. all
  others continuous estimates
• Thornhill Gangestad (1999) Same as above
• Stronger preferences for behavioral displays of
  symmetrical men
• Gangestad et al. (2004) Continuous fertility
  estimates interact w/ preferences

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Issues in Perception of Mens Faces

• Do men with faces that are perceived as more
  masculine actually have higher testosterone
  concentrations?
• Can observers tell how much men like children
  simply from photos of their faces?
• Does high masculinity and/or testosterone predict
  low interest in children?

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SEX DIFFERENCES AND SPATIAL COGNITION

• Gaulin Polygynous species (males mate with
  multiple females) males need to range widely to
  search for females or control territories with
  multiple females not so in monogamous species
• Should be selection on polygynous males (but not
  females) to have better spatial ability in order
  to travel w/o disorientation

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SEX DIFFERENCES AND SPATIAL COGNITION

• Meadow voles are polygynous and Pine voles are
  monogamous
• In meadow voles males home range gtgt females no
  difference in pine voles
• Meadow voles males maze performance gtgt females
  no difference in pine voles
• Meadow voles males have larger hippocampus than
  females no difference in pine voles
• Natural selection for larger brain structure
  related to spatial navigation in male vs. female
  meadow voles

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• Possible alternative explanations for (1) male
  better maze performance, (2) male larger
  hippocampus?
• Meadow vole males show (1) greater range size,
  and (2) greater maze performance, only during
  breeding season

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• Possible alternative explanations for (1) male
  better maze performance, (2) male larger
  hippocampus?
• Meadow vole males show (1) greater range size,
  and (2) greater maze performance, only during
  breeding season
• Gaulin compared lab-reared pine voles with
  wild-caught pine voles on maze performance
• Lab-reared lived in small cages with very little
  spatial navigation experience
• No differences in performance between wild-caught
  and lab-reared
• Argument for experience not driving sex
  differences
• Analogous argument for humans? if sexes differed
  in spatial tasks used in finding food, etc.
  during human evolution, would selection build
  different navigational mechanisms in males vs.
  females?

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SPATIAL COGNITION IN HUMANS

• Large and reliable sex difference on Mental
  Rotation Test (MRT), favoring men
• Men tend to use cardinal directions (north,
  south) and distances when giving directions,
  while women tend to use landmarks and left/right
• Often argued that this may be related to sex
  differences in ancestral environments
• In hunter-gatherer societies, men do nearly all
  of the hunting, inter-group violence/warfare that
  would require long trips
• Women tend to perform most of the foraging, plant
  food gathering
• Argued that ability to reverse routes or find way
  back from a different route may require ability
  to rotate all or parts of the route stored in
  memory
• Route learning and MRT
• When learning routes on a tabletop map, men on
  average make fewer errors and require fewer
  trials to reach zero errors
• Some research suggests significant correlations
  between MRT and route-learning performance (e.g.,
  r .60 between MRT and navigation through a
  computer-simulated maze)

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• What are possible alternative explanations for
  mens advantage (on average) in mental rotation
  and route learning?
• Alternatives to the idea that natural selection
  for navigational ability has designed sex
  differences in brain mechanisms related to
  spatial cognition?
• Men and women have same spatial navigation
  mechanisms men and women experience different
  spatial environments during development (e.g.,
  boys play more sports, etc.), experience accounts
  for sex differences in mental rotation, route
  learning, etc.
• There are practice effects on tests such as MRT
  however, men and women seem to benefit equally
  from practice

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HORMONAL EFFECTS ON SPATIAL COGNITION

• Organizational effects in rodents
• Female rodents navigate a radial arm maze by
  using landmark cues in room Performance is
  impaired if objects are moved
• Male rodents use geometric cues Moving objects
  has no effect, but a circular curtain that blocks
  angles of walls, etc. impairs performance
• If males are castrated at birth, they switch to
  use of landmark cues if females are injected
  with androgens after birth, they switch to use of
  geometric cues
• Spatial experience is controlled

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• Organizational Effects of Hormones in Humans
• Idiopathic hypogonadotropic hypogonadism Men
  lifelong deficiency in androgen production (often
  diagnosed at puberty) Tend to have worse
  spatial ability than normal men
• Congenital Adrenal Hyperplasia (CAH)
  Overproduction of testosterone-like androgen,
  androstenedione by the adrenal gland prenatally
  (usually diagnosed at birth)
• CAH girls show superior spatial ability to normal
  girls (including MRT)
• CAH boys usually no better spatial ability than
  normal boys

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SPATIAL REASONING FAVORING WOMEN

• Silverman and Eals reasoned that if male spatial
  advantages were due to an evolutionary history of
  men hunting, etc. perhaps there should be female
  spatial advantages related to a history of
  foraging
• Plant foods are embedded in complex arrays of
  vegetation and other objects, so ability to
  remember objects and their location within an
  array may have been selected more strongly in
  women
• Tests of both object memory and location memory
  show large advantages for women

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Basic Research Techniques

• Observational Research (naturalistic, systematic)
• Categorizing, quantifying behavior
• Correlational Research
• Measure two or more variables and test their
  association with each other
• E.g., self-report measures of happiness and
  archival measures of length of life
• Do happier people live longer?

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r .62, p lt .05
Happy
Age _at_death
Direction of causality Lurking variables or 3rd
variables
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0 3 6 9 12 15
Number of Drownings
r .76
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Partial Correlation
Differences between data points and regression
line are residuals
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Partial Correlation
Partial correlation between ice cream and
drowning, controlling for temperature, is r
-.01.
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