Sex Differences in Intelligence

1
Sex Differences in Intelligence
2
Key Themes

• Sex differences in intelligence
• General intelligence
• Spatial intelligence

3
Learning Outcomes

• Outline the extent of sex differences between men
  and women on general intelligence and specific
  aspects of intelligence
• Describe possible biological and environmental
  variables and considerations that may explain sex
  differences in intelligence

4
Sex Differences in Intelligence

• Sex differences on measures of general
  intelligence
• Sex differences in specific intelligences.

5
Sex differences on measures of general
intelligence

• In the first instance we are going to look
  whether men and women score differently on
  general measures of intelligence, such as IQ
  scores, or scores on intelligence tests such as
  the Weschlers tests and the Ravens Progressive
  Matrices.

6
Sex differences on measures of general
intelligence

• Louis Terman (1916) was the academic who decided
  to use the Binet-Simon test among Californian
  school children, and, consequently, developed the
  Stanford-Binet test. Terman reported that when
  comparing nearly 1,000 4- to 16 year olds, girls
  had a slightly higher score than boys for overall
  IQ scores on the Stanford-Binet test.
• Charles Spearman (1921), the English
  psychologist, who introduced the way of
  conceptualising intelligence as a single factor,
  g, argued that there were no sex differences in
  intelligence.
• Raymond B. Cattell, who theorised about the
  differences between fluid intelligence and
  crystallised intelligence, suggested that there
  were a number of studies that show no significant
  differences between men and women on these two
  dimensions of intelligence (of all ages including
  children).

7
Sex differences on measures of general
intelligence

• A first systematic review of evidence on sex
  differences in intelligence was presented by J.H.
  Court (Court, 1983).
• In his review, Court reviewed nearly 120 studies
  that had provided information on sex differences
  on the Ravens Progressive Matrices. He found
  that some studies suggested that women scored
  higher than men in other studies men scored
  higher than women, and in the majority of studies
  there was no difference.
• Mackintosh (1998) in an analysis of the research
  literature on sex differences on the Ravens
  Progressive Matrices. Mackintosh argued that the
  sex differences are, generally, small, amounting
  to no more than 12 IQ points in favour of males
  or females.
• Anderson (2004) reviewed the literature and
  concluded that on the Weschler and the Ravens
  tests, no significant differences occurred
  between males and females.

8
However..

• Richard Lynn and Paul Irwing, over the course of
  two studies (Lynn Irwing, 2004 2005), argued
  that there had been no actual statistical
  meta-analysis of sex differences in general
  intelligence.
• They pointed out that reports, such as that made
  by Court, are known as a narrative analysis.
• Narrative analysis is where the researcher weighs
  up the evidence presented across a number of
  studies, but the size of samples of these studies
  vary, can be taken from a number of different
  samples, and results are only analysed in terms
  of whether the study supports, or doesnt support
  the main hypothesis
• Sample sizes used by Court were relatively small
  (i.e. much less that n 500).

9
Lynn and Irwing

• To remedy this situation, Lynn and Irwing set
  about collecting data on sex differences for
  scores on the Ravens Progressive Matrices. In
  all, Lynn and Irwing collected the data for large
  population samples (i.e. greater than n 500)
  from 1939 to 2002.
• In all, Lynn and Irwing collected data from 57
  studies from 30 countries covering 195 samples,
  numbering a total excess of 80,000 people.

10
Sex Differences in General Intelligence

• Reporting of Effect Size is evident in this
  area..Comparing differences you get d..
• d .2 is a small effect size,
• d .5 is a medium effect size and
• d .8 is a large effect size.

11
Effect Size of Lynn and Irwings Findings
Regarding Sex Differences in Intelligence
Table 14.1 Effect size of Lynn and Irwings
findings regarding sex differences in intelligence
12
Sex differences in specific intelligences

• In what is considered as a seminal study of the
  literature on sex differences, Maccoby and
  Jacklin (1974) concluded that men, on average,
  perform better on tests of spatial ability than
  women.
• Spatial ability is the ability to visualise
  spatial relationships and to mentally manipulate
  objects. It is a mental process, which is
  associated with the brains attempts to interpret
  accurately incoming information.
• On the other hand, they argued that women, on
  average, do better on verbal abilities, such as
  comprehension (The act of understanding the
  meaning, nature, or importance, of things) and
  language vocabulary (a knowledge of words in a
  particularly language)

13
Stop and think. What is spatial ability?

• a. Spatial perception requires participants to
  identify the horizontal or the vertical object
  (usually a line) in a display while ignoring
  distracting information. One example of this task
  is the Piaget water level task, which requires
  participants to draw in the water level in a
  picture of a tilted glass that is half filled
  with water (see below).

14
Stop and think. What is spatial ability?
(Continued)

• b. Spatial visualisation refers to analysing
  spatial information. Examples of tests that tap
  spatial visualisation are the embedded figures
  test (see Figure below), in which a target figure
  is hidden in the contours of a larger figure
  and the participant is expected to find it.
  Another example is a paper-folding task, in which
  participants have to imagine what the result is
  of folding a piece of paper in several ways.

15
Stop and think. What is spatial ability?
(Continued)

• c. Mental rotation involves being able to
  visualise objects from different angles and
  different positions. Examples of this sort of
  tests include tasks where participants are
  presented pairs of drawings and asked people
  whether the objects were the same, only rotated,
  or mirror images of each other (see Figure below).

16
Stop and think. What is spatial ability?
(Continued)

• d. Spatiotemporal ability involves the
  participants making judgments about moving visual
  stimuli. Often responses are asked to guess when
  certain objects will arrive at a certain point
  (see Figure below).

17
Stop and think. What is spatial ability?
(Continued)

• Mechanical reasoning measures are the ability to
  understand basic mechanical principles of
  machinery, tools, and motion. These tasks
  comprise principles that involve reasoning rather
  than specialised knowledge or training. The task
  illustrated below is an example of mechanical
  reasoning. Participants would be asked to
  indicate which direction vehicle A would go if it
  collided with vehicle B at point X.

18
Summary of the Findings From FiveMeta-Analyses
Examining Sex Differences on Specific
Intelligences
Table 14.2 Summary of the findings from five
meta-analyses examining sex differences on
specific intelligences
19
Summary of the Findings From FiveMeta-Analyses
Examining Sex Differences on Specific
Intelligences (Continued)
Table 14.2 Summary of the findings from five
meta-analyses examining sex differences on
specific intelligences (Continued)
20
Sex differences in specific intelligence

• In terms of men scoring higher on spatial
  abilities, we can see that across all ages, men
  score higher on measures of spatial perception,
  though it is a small to medium effect size.
  There is a larger effect size (medium to large)
  for mental rotation. Among adolescents, the
  greatest effect for men scoring higher than women
  is for mechanical reasoning (See Figure 14.3 if
  you need to be reminded of what these
  intelligence tasks involve).
• In terms of women scoring higher than men in
  verbal ability, the only effect size, of above
  .2, is for verbal production (i.e. 0.33).
  However, among adolescents, there is evidence for
  girls doing better than boys (effect size gt .2)
  on verbal abilities for spelling, language, and
  perceptual speed.

21
Sex differences in specific intelligence Summary

• There is a small effect for men over women on
  measures of general intelligence (IQ scores)
  (based on Lynn and Irwings findings)
• There is a medium effect for men over women for
  spatial intelligence
• There is no evidence for a general effect of
  women over men for verbal intelligence abilities

22
Looking for explanations for sex differences in
measures of intelligence

• Biological explanations for sex differences in
  measures of intelligence (IQ scores).
• Environmental explanations for sex differences in
  measures of intelligence.
• Going a different route to Chapter 13 which talks
  about problems with biological explanations and
  differences..

23
Biological Explanations for Sex Differences in
Intelligence

• The effect of brain size and maturity rates on
  general intelligence
• Biological variables for sex differences in
  spatial intelligence
• Evolutionary perspectives and sex differences in
  spatial ability
• Brain functioning and sex differences in spatial
  intelligence
• Testosterone and sex differences in spatial
  intelligence.

24
Biological variables for sex differences in
measures of general intelligence

• Lynns theory of the effects of brain size and
  maturity rates on general intelligence seeks to
  explain why there are differences in measures of
  general intelligence (IQ scores) between men and
  women in adulthood, but not in adolescence.
• Willerman et al. (1991) examined brain size and
  IQ scores among 40 U.S.A. university students and
  reported a correlation of r .35. While Raz et
  al. (1993) reported a correlation of r .43
  between brain size and IQ scores among 29 adults
  aged between 18 and 78 years of age. In a
  meta-analysis of 37 samples across 1530 people
  (McDaniel, 2005), U.S.A. psychologist Michael
  McDaniel estimated the correlation between brain
  size and IQ was 0.33. The correlation was higher
  for females than males and higher for adults than
  children. However, across both sexes and all
  ages, it was clear that brain volume is
  positively correlated with intelligence.
• Lynn (1994) points out that, on average, men have
  10 larger brains than women by about 10 percent,
  and larger brains suggest larger brain power,
  this explains the difference between men and
  women on measures of general intelligence (IQ
  scores).

25
Biological variables for sex differences in
measures of general intelligence

• However, why no differences among adolescents?
  Well, Lynn (1994) has proposed a developmental
  theory of sex differences in intelligence, as
  boys and girls mature physically and mentally at
  different rates
• Lynn suggests boys and girls mature at the same
  rate up to the age of seven years. However,
  during crucial times during development girls
  mature faster than boys. This growth spurt starts
  at 8 years and slows down at fourteen and fifteen
  years. Therefore, girls brain size may be
  similar to boys around the start of adolescence
  at twelve to thirteen years and this leads to
  girls evening up with boys in intelligence scores
  in adolescents. Then, at the age of sixteen
  onwards into adulthood, while growth rates
  continue, boys start to develop larger average
  brain sizes and the differences in general
  intelligence, that last into adulthood, start to
  develop.

26
Summary of Effect Sizes of Sex Differences on
Intelligence Across the Age Range
Table 14.3 Summary of effect sizes of sex
differences on intelligence from Lynn and Irwing
across the age range Source Lynn and Irwin (2004)
27
Evolutionary perspectives and sex differences in
spatial ability

• Jones, Braithwaite Healy (2003) summarised
  three evolutionary hypotheses that might explain
  why men might be better at spatial abilities
  Foraging, Range Size and Warfare.

28
Evolutionary perspectives and sex differences in
spatial ability

• Foraging (see also Silverman Eals, 1992).
  Foraging is the act of looking or searching for
  food or provisions. Men took the skills such as
  finding their way around the countryside,
  demonstrating awareness of the physical
  environment to be able to hunt and intercept
  animals, and throwing weapons to kill animals,
  all require spatial abilities.
• Range (see also Gray Buffery, 1971 Gaulin,
  1995). This hypothesis focuses on the view of
  mating roles in evolutionary theory, and the
  theory that while females seek out monogamous
  relationships (single relationships), males seek
  out multiple relationships (polygynous). To
  pursue and maintain multiple relationships,
  polygynous males need to cover a larger area of
  land and therefore has increased range size.
• Warfare (see also Geary, 1995 Sherry Hampson,
  1997). To some extent this theory is a
  combination of the first two points. Sex
  differences in human spatial ability as a result
  of direct male-to-male small-scale warfare. Men
  travel long distances in order to ambush other
  men, compete for resources and also compete for
  females. In fighting men develop their spatial
  abilities.

29
Brain Functioning and sex differences in spatial
intelligence

• Richard J. Haier, Professor of Psychology in the
  Department of Pediatrics at the University of
  California, presented findings suggesting men and
  women achieve intelligence through different
  areas of the brain (Haier, Jung, Yeo, et al.
  2005).
• They used MRI (Magnetic Resonance Imaging) to
  find out where intelligence was in the brain. An
  MRI scan is able to provide clear pictures of
  parts of the body that are surrounded by bone
  tissue, so the technique is useful when examining
  the brain and spinal cord. They used a particular
  technique called voxel-based morphometry that
  looks for grey and white matter around the brain.
  Grey matter can be understood as the parts of
  the brain responsible for information processing,
  whereas, white matter is responsible for
  information transmission.
• Participants were given the Wechsler Adult
  Intelligence Scale battery and underwent the MRI
  scan. The analysis (voxel-based morphometry) was
  used to examine whether IQ scores were related to
  brain areas where grey matter and white matter
  occurred. Haier et al. found two interesting
  findings.

30
Summary of Haier et al. (2005) FindingUsing
Brodmann Definitions of Areas of the Brain
Figure 14.1 Summary of the Haier et al. (2005)
finding using Brodmanns definitions of areas of
the brain Source Haier et al. (2005)
31
Brain Functioning and sex differences in spatial
intelligence

• The second finding was that women have more white
  matter and fewer grey matter areas related to IQ
  scores compared to men. This finding suggests
  that intelligence is related to white matter in
  women, and grey matter in men. Haiers et al.s
  findings suggest that, among men, IQ score is
  related to those aspects of the brain responsible
  for information processing (grey matter)
  whereas, for women, intelligence is generally
  related to those aspects responsible for
  information transmission (white matter). This
  finding suggests that intelligence in men is
  related to information processing. Consequently,
  this might explain mens superior abilities in
  spatial intelligence.

32
Testosterone and sex differences in spatial
intelligence

• Hormones are the substances that travel around
  the human body to effect physiological activity,
  such as growth or metabolism.
• Gonadal hormones are those substances that create
  physiological growth in organs in animals that
  produce sex cells, e.g. ovaries in females (part
  of the female reproductive system) and testes in
  the male (part of the male reproductive system).
• Testosterone (a gonadal hormone) is the male sex
  hormone. Testosterone is necessary for the
  individual in the fetus, for the development of
  male genitalia. Additionally, increased levels of
  testosterone at puberty are responsible for
  further growth of male genitalia and for the
  development and maintenance of what are known as
  male secondary sex characteristics, such as voice
  changes and facial hair.
• Estradiol (also a gonadal hormone) is the most
  potent naturally occurring oestrogen. Oestrogen
  is produced chiefly by the ovaries and
  responsible for promoting oestrus and the
  development and maintenance of female secondary
  sex characteristics, such as pubic hair and
  breasts.
• Males and females have both hormones, but greater
  concentrates of testosterone are found in men and
  greater concentrates of estradiol are found in
  women.

33
Testosterone and sex differences in spatial
intelligence

• Choi and Silverman (2002) starts from a general
  finding in the literature, that male and female
  humans and non-humans use different strategies to
  learn routes.
• Routes through the environment require
  individuals to use spatial ability. However,
  there is a sex difference in the way men and
  women use routes (Choi Silverman, 1996).
• Women tend to use relative directions (left,
  right, front, and back) and landmarks (buildings,
  bridges, traffic lights etc) to learn routes (for
  example, you turn left at the bridge and then
  right at the next traffic lights).
• Men use distance (mileage, metres) and cardinal
  (the four principal compass points north, south,
  east, and west) directions (for example, You
  head down there for about 100 metres and then
  head west until you get to the next turning 50
  metres down that road, and then head north).

34
Testosterone and sex differences in spatial
intelligence

• The authors gave 46 male and 60 female
  undergraduate students a route-learning task,
  which comprised of a map that depicted a
  fictional town, with various landmarks and
  streets, and a compass indicator and distance
  scale.
• Participants were asked to learn the shortest
  route from one point to another on the map in 2
  minutes, and then were asked to recall the route
  back to the experimenter.
• The experimenter then coded the number of times
  the participants made a reference in their route
  to landmarks, relative direction, cardinal
  directions and distance.
• Respondents testosterone and estradiol levels
  were measured by taking some saliva from the
  participants.

35
Testosterone and sex differences in spatial
intelligence

• The authors gave 46 male and 60 female
  undergraduate students a route-learning task,
  which comprised of a map that depicted a
  fictional town, with various landmarks and
  streets, and a compass indicator and distance
  scale.
• Participants were asked to learn the shortest
  route from one point to another on the map in 2
  minutes, and then were asked to recall the route
  back to the experimenter.
• The experimenter then coded the number of times
  the participants made a reference in their route
  to landmarks, relative direction, cardinal
  directions and distance.
• Respondents testosterone and estradiol levels
  were measured by taking some saliva from the
  participants.

36
Testosterone and sex differences in spatial
intelligence

• Choi and Silverman found that though estradiol
  was not related to strategies for route learning.
  However, they found among men, but not women,
  that the testosterone level was positively
  related to the use of male-biased route-learning
  strategies (the use of distance and cardinal
  rules).

37
Environmental Explanations for Sex Differences in
Intelligence

• Schemas
• Influence of stereotypes on sex differences in
  intelligence outside of education
• Influence of stereotypes on sex differences in
  intelligence within education
• Interactions between technology and
  socio-economic status and their influence on
  intelligence in the classroom.

38
An Illustration of the Three Aspects to Consider
When Considering Environmental Influences on Sex
Differences in (spatial) Intelligence
Figure 14.2 An illustration of the three aspects
to consider when considering environmental
influences on sex differences in (spatial)
intelligence
39
Stereotypes on sex differences in intelligence
outside of education

• Susan Levine (Levine, Vasilyeva, Lourenco,
  Newcombe Huttenlocher, 2005) suggests that,
  although not a lot is known about what type of
  toys can be used to increase spatial skills,
  playing with building blocks, puzzles and video
  games are related with spatial skills, and boys
  spend more time with these toys than girls.
• Evidence among infants suggests that there are
  differences in the toy choices of boys and girls,
  which may affect their intelligence, particularly
  spatial intelligence. Anne Campbell and her
  colleagues (Campbell, Shirley, Heywood Crook,
  2000) found that sex differences in preferences
  for toys started to appear as young as 9 months.
  Girls were found to want to play with dolls,
  while boys preferred trucks.

40
Stereotypes on sex differences in intelligence
within education

• Subject Choice
• Halpern and LaMay (2000) suggest that while men
  and women nowadays reach similar levels of
  intelligence and achievement at school, their
  choice of subjects at school tend to follow a
  pattern based on their sex. At school, there is
  a large distinction in sex choices in science
  subjects (and to some extent aspects of
  mathematics that involve spatial reasoning). This
  can comprise positive subject choice, as in
  selecting particular subjects, or negative
  subject choice, as in avoiding particular
  subjects.

41
Stereotypes on sex differences in intelligence
within education

• Subject Choice Plucker (1996) notes that one key
  idea on how sex differences may occur in
  intelligence is that self-fulfilling prophecies
  occur in the classroom. This is where positive or
  negative feedback influences a pupils ideas
  about their own abilities, and despite their real
  potential regarding a particular ability, their
  own ideas determine their attainment in that
  ability.
• Stereotype threat (Steele, 1997). Stereotype
  threat has been used to explain why certain
  groups may perform poorly in test situations,
  particularly in tests of intelligence .

42
Interactions between technology and
socio-economic status and their influence on
intelligence in the classroom

• Sex differences in spatial abilities may become
  more apparent as teaching relies more on learning
  techniques that use visual and technical
  environments (e.g. computers and the internet).
• Kirby Boulter(1999) compared two groups of
  students on a new spatial ability task. One group
  was given the task via pen and paper, and the
  other was given the task via visual stimuli.
  Kirby and Boulter found that one of the factors
  of predicting success at the visual stimuli task
  was spatial ability before the task. Kirby and
  Boulter suggest that people who are weak at
  spatial ability before the task may be
  disadvantaged by the use of complex visual
  stimuli used as a teaching method, and this will
  further exacerbate their problems with spatial
  ability.

43
Interactions between technology and
socio-economic status and their influence on
intelligence in the classroom

• Levine, Vasilyeva, Lourenco, Newcombe
  Huttenlocher, 2005). In a longitudinal study,
  children were administered two spatial tasks
  requiring mental transformations and a task of
  verbal comprehension. As with previous studies
  mentioned above, no sex difference was found
  between boys and girls in their verbal ability.
• However, on the two tasks, boys from the middle
  and high-socio-economic class did better than
  girls on both spatial tasks, whereas boys and
  girls from the low-socio-economic group did not
  differ in their performance on the spatial task.
  It is important to note that the individuals from
  the low socio-economic classes didnt find the
  test any harder.
• Susan Levine suggests that the main reason for
  this finding is that children from lower
  social-economic backgrounds spend less amounts of
  time engaged in activities that promote spatial
  intelligence outside the classroom and this gives
  them disadvantages in the modern-day classroom
  that emphasises the use of spatial abilities

44
A final consideration of sex differences in
measures of intelligence.

• Stereotype emphasis
• Placing the extent of the sex differences in
  intelligence within its proper context.

45
Stereotype emphasis

• Over-emphasised stereotypes (i.e. the estimated
  size for the sex difference for the variable was
  greater than the real difference between men and
  women on that variable). This might provide
  evidence that stereotypes might be overly
  attributed to difference between the sexes. For
  example, Swim found that we over-emphasised how
  aggressive men are.
• Under-emphasised stereotypes (i.e. the estimated
  size for the sex difference for the variable was
  smaller than the real difference between men and
  women on that variable). This may provide
  evidence that real sex differences are being
  underestimated in our stereotypes. For example,
  Swim found we might under-emphasise mens helping
  behaviour in an emergency.
• Accurate stereotypes (i.e. the estimated size for
  the sex difference for the variable is the same
  as the real difference between men and women on
  that variable). For example, we may be accurate
  in terms of viewing men as more restless.

46
Stereotype emphasis

• In terms of intelligence, Swims (1995) study
  suggested that we are accurate in our perceptions
  of men being better in their mathematical
  abilities, but subjects tended to overestimate
  women's verbal abilities (a finding that is
  consistent with our earlier discussion regarding
  research that suggests that there are no sex
  differences in verbal abilities).

47
Placing the extent of the sex differences in
intelligence within its proper context

• Yoder Kahn, 2003) have suggested that there are
  two ways in which sex differences are treated in
  the literature, the first is an alpha bias, in
  which there is assumed to be large differences,
  and beta bias, in which it is assumed that there
  a minimal differences (also known as the gender
  similarity hypothesis).
• Though research on sex differences in
  intelligence does tend to speculate on reasons
  for differences, we must consider to what extent
  sex differences in intelligence are important.
  Two points are important to such a consideration

48
Placing the extent of the sex differences in
intelligence within its proper context

• There is a small effect size of around .3 for
  general intelligence scores, and a medium to high
  effect size of between .5 and .7 for two aspects
  of spatial ability (mental rotation and
  mechanical reasoning among adolescents).
• However, U.S.A. psychologist Janet Shibley Hyde
  (Hyde, 2005) notes that when the ranges of these
  effect sizes for sex differences in intelligence
  scores are compared against the effect sizes for
  sex differences for other variables, their
  importance diminishes.
• For example, the effect sizes of the majority of
  aggression variables range from .33 to .84, the
  majority of sexuality variables range from .30 to
  .96, personality variables (particularly
  extraversion and agreeableness) range from .35 to
  .91. Within this context, the sex differences
  between men and women seem a lot less important.

49
Two Comparison Lines of Difference. Comparingthe
Range of Variability in IQ scores within
Population Groups of Men and Women with Average
Difference Between Men and Women (drawn to scale)
Figure 14.3 Two comparison lines of difference.
Comparing the range of variability in IQ scores
within population groups of men and women with
average difference between men and women (drawn
to scale)
50
Summary

• Outline the extent of sex differences between men
  and women on general intelligence and specific
  aspects of intelligence
• Describe possible biological and environmental
  variables and considerations that may explain sex
  differences in intelligence