Kevin MacDonald, Ph.D.CSULB, Department of Psychology
Psychology 361: Chapter 11 notes, part 2.Note: Sections in italics are not going to be on the test. They are there in order to explicate the text for interested students.
Genetics and IntelligenceThe extent to which genetic differences affect differences in intelligence remains controversial.
Responsible estimates are that between 40-80% of the differences among people are due to differences in their genes.
Another way to say this is that heritability estimates for intelligence vary between 0.40 and 0.80.
HERITABILITY IS A MEASURE OF THE PROPORTION OF VARIATION FOR A GIVEN TRAIT, SUCH AS INTELLIGENCE, THAT IS CAUSED BY GENETIC VARIATION.
A HERITABILITY OF 1.00 WOULD MEAN THAT ALL THE VARIATION IN INTELLIGENCE WAS THE RESULT OF GENETIC VARIATION. (NONE OF THE VARIATION COMES FROM REARING IN DIFFERENT ENVIRONMENTS.)
A HERITABILITY OF 0.00 WOULD MEAN THAT NONE OF THE VARIATION IN INTELLIGENCE WAS DUE TO GENETIC VARIATION. (ALL THE VARIATION COMES FROM REARING IN DIFFERENT ENVIRONMENTS.)
Textbook: Consensus among psychologists that 40-50% of differences are
from different genes.
Arthur Jensen: 80% (Jensen argued that black/white differences in IQ were mainly the result of genetics.)
Stephen Ceci: Less than 40%.
The fact that a trait is caused by genes does not mean that it can't
be influenced by the environment.
Example: Blindness, deafness, Pheylketonuria (PKU).
Estimates of heritability apply to the entire population studied;
they are averages, and they do not apply to particular people.
Can't say: He has a heritability of .6 for IQ.
Can say: For a random sample of the Long Beach Public Schools, the heritability of IQ is .6. This would be compatible with some children's IQ being strongly influenced by the environment and other children's IQ not being influenced by the environment. The heritability estimate is an average of all the influences for the entire sample.
As was shown in the lecture notes on behavior genetics (see Lecture
Notes II: Behavior Genetics:
http://www.csulb.edu/~kmacd/361Notes2.html), the degree of genetic influence increases as children get older. Jensen argues that the 80% estimate would apply to intelligence in later adulthood, while in childhood only about 40% of the differences among children are due to genetic differences.
All agree that environmental differences have an effect; environmental differences may be social (family, peers, school) or non-social (nutrition, pollutants, diseases).
Jensen's Types of Learning:
Associative Learning (Level I learning): short-term memory, rote learning, simple associative skills.
Example: Child recalls list of familiar objects.
Cognitive Learning (Level II learning): abstract thinking, symbolic
processes, using language in problem solving.
Example: What is the next number in the following series? 2, 3, 5, 8, 12, 17 ...
How are an apple and a banana alike?
Jensen: Only tasks involving cognitive learning (Level II learning)
predict school performance; these are the skills that tap intelligence
and differentiate high IQ people from low IQ people. Level I abilities
are pretty evenly distributed among humans and don't predict academic success.
Heritability (degree of genetic influence) depends on the sample being studied. (pp. 431-432)
1595: 400 YEARS AGO, ONLY RICH PEOPLE ATE WELL.
RESULT: MOST OF THE PHENOTYPIC VARIATION IN HEIGHT WAS
THE RESULT OF ENVIRONMENTAL VARIATION.
IN SOME COUNTRIES THIS CONTINUES TO BE THE CASE.
2003: IN THE US, ALMOST EVERYONE EATS ENOUGH TO ACHIEVE THEIR
RESULT: MOST OF THE PHENOTYPIC VARIATION IN HEIGHT IS THE RESULT OF GENETIC VARIATION. BASICALLY THE IDEA IS THAT IF YOU GET RID OF ALL THE ENVIRONMENTAL SOURCES OF VARIATION, THE REST OF THE VARIATION IS GOING TO BE THE RESULT OF GENETIC VARIATION.
The textbook argues that heritability estimates will be lower for populations
in really bad environments. This relates to the lecture notes on behavior
genetics that we studied previously (see Lecture Notes II: Behavior Genetics:
http://www.csulb.edu/~kmacd/361Notes2.html). In those notes I had an illustration of what happens to IQ in abusive and normal environments. The point was that the environemnt does not have much effect on IQ in the normal range of environmental variation, but it does have a large effct in abusive environments. This is the figure from those notes:
IDEA OF ABUSIVE VERSUS NORMAL ENVIRONMENTS
The textbook is arguing that environmental influences are highest on the left hand side of this graph. (You will have to come to class to see how the graph actually looks.) In other words, if you had a sample of children who came from normal environments, the degree of genetic influence would be relatively high, but if you had a sample from abusive environments or even perhaps the the low end of normal environments, there would be more evidence for environmental influence. The empirical data supporting this is a bit mixed, but it is certainly true that the degree of genetic influence is less if you are studying only abused children. But it's not clear that the degree of genetic influence is less at the low end of normal than at the high end of normal. (The low end of normal environments in the US would be working class people who send their kids to school but are not well educated and are not much interested or able to provide a lot of intellectual stimulation to their children. The high end of normal would be yuppie-type, well educated parents who desperately try to get their kids into the best schools, hire tutors, and take them on trips the the local art museum.)
TEXTBOOK: Black/White IQ gap and Black/White achievement gap has
declined over time, from 1/3 to 1/2 of its previous level. However, the
text notes that the gap has not declined since the late 1980s and data
from the early 1990s show that the gap is widening. TEXT notes that the
widening trend is not statistically significant.
However, a recent study is not encouraging:
From the article:
There are a number of reasons explaining the continuing and growing black-white SAT scoring gap. A major factor in the SAT racial scoring gap is family income. There is a direct correlation between family income and SAT scores. For both blacks and whites, as income goes up, so do test scores. Some 28 percent of all black SAT test takers came from families with annual incomes below $20,000. Only 5 percent of white test takers came from low-income families. At the other extreme, 5 percent of all black test takers came from families with incomes of more than $100,000. The comparable figure for white test takers is 24 percent. But income alone does not explain the racial scoring gap. Consider these facts:
' Whites from families with incomes of less than $10,000 had a mean
SAT score of 980. This is 123 points higher than the national mean for
' Whites from families with incomes below $10,000 had a mean SAT test score that was 46 points higher than blacks whose families had incomes of between $80,000 and $100,000.
' Blacks from families with incomes of more than $100,000 had a mean SAT score that was 142 points below the mean score for whites from families at the same income level.
ENVIRONMENTAL FACTORS INFLUENCING IQ
Pregnancy and birth problems: Prenatal malnutrition, maternal disease (e.g., AIDS)
Family: IQ correlated with family environments that are "emotionally and verbally responsive to their children, provide appropriate play and reading materials, encourage children in school, etc. (BE ABLE TO EXPLAIN WHY THESE FINDINGS COULD BE DUE TO PASSIVE GENOTYPE-ENVIRONMENT CORRELATIONS.)
Schools and Peer Groups:
Ceci: 1.) Lack of formal education or "dropping out" of school associated with DECLINES in IQ.
2) IQ DECLINES DURING SUMMER VACATION
3. CHILDREN WHOSE BIRTHDAYS BARELY QUALIFY THEM FOR SCHOOL ENTRY HAVE
Peer culture: Compared to European-American children, peer groups of Asian Americans supported each others' academic pursuits and participated in education-related activities such as studying together. They were less involved in dating and general socializing. Linked with higher achievement.
African-Americans: Peer groups express anti-academic attitudes, ridiculing and isolating students who try to succeed at school. Children may try to fit in by not taking school seriously.
Community: Isolated rural areas tend to be associated with lower IQ.
Social class: Lower SES children score 10-15 points below Middle SES children. Differences can be found at 18 months of age.
Race: black children score about 20 points below white children.
THESE DIFFERENCES ARE OBSERVED BY THE FIRST GRADE AND ARE CONSISTENT THROUGH THE SCHOOL YEARS. (Implies that schools don't cause the IQ differences.)
Cumulative Risk: Adding risk factors is correlated with lower and lower
IQ. Children with 7-8 risk factors scored 30 points below children with
1 risk factor. Risk factors: hunger, poor clothing, family stress, premature
CAUTION: NOT CONTROLLED FOR GENETIC FACTORS.