FREE ESSAY ON INTELLIGENCE: GENETIC AND ENVIRONMENTAL FACTORS

INTELLIGENCE: GENETIC AND ENVIRONMENTAL FACTORS

Intelligence: Genetic and Environmental Factors
One of the most interesting and controversial areas in behavioral genetics, human
intelligence is currently assumed to be subject to both genetic and environmental
influences. 
While this assumption is accepted by a majority of geneticists and behavioral scientists,
there is great disagreement on the degree of influence each contributes. Arguments for
environmental influences are compelling; at the same time there is growing evidence that
genetic influence on intelligence is significant and substantial (Eyesenck, 1998;
Mackintosh, 1998; Plomin, 1994; Steen, 1996). The purpose of this paper is to explore the
question: How is intelligence influenced by heredity and environment?
What is Intelligence?
It is often difficult to remember that intelligence is purely a social construct, and as
such is limited to operational definitions. Binet & Simon (1905, as cited in Mackintosh)
defined it purely in terms of mental ability: the ability to judge well, to comprehend
well, to reason well. Wechsler (1944, as cited in Mackintosh) added behavioral factors:
the aggregate or global capacity of the individual to act purposefully, to think
rationally, and to deal effectively with the environment. Sternberg (1985) synthesizes
the previous definitions, defining intelligence as the mental capacity of emitting
contextually appropriate behavior at those regions in the experiential continuum that
involve response to novelty or automatization of information processing as a function of
metacomponents, performance components, and knowledge acquisition components. Gardner
(1993) took the definition to a societal level, as the ability or skill to solve problems
or to fashion products which are valued within a cultural setting. 
Measurement of Intelligence: IQ Tests
Alfred Binet developed the first IQ tests to identify children who would not benefit from
public ?school instruction. His concept involved the idea that certain mental tasks are
appropriate to certain ages, such as the ability to recite the names of the months: while
expected of a ten year old, such ability would be rare in a three year old. Binet
quantified intelligence as the Intelligence Quotient (IQ): the ratio of mental age to
chronological age, multiplied by 100. Reasoning that low intelligence stemmed from
improper development, Binet envisioned the test as a first step in treatment: a
diagnostic instrument used to detect children with inadequate intelligence in order to
treat them using mental orthopedics. 
Binet argued forcefully against the idea that intelligence is fixed or innate: We must
protest and react against this brutal pessimism (Lewontin, Rose, & Kamen, 1984). However,
those who translated his test into English tended to disagree, arguing that the test
measured an innate and immutable, genetically inherited characteristic. After Binet's
death in 1911, the Galtonian eugenicists assumed control, shifting the focus firmly
toward genetic explanations by insisting that differences in intelligence between social
classes and races were due to inherent genetic differences.
Over time, the tests were standardized to correspond to a priori conceptions of
intelligence by including items that correlated well with school performance. Test items
that differentiated between gender were removed; items that differentiated between social
classes were left in because it is these differences that the tests are meant to measure
(Lewontin, Rose & Kamin, 1984). 
There are many criticisms of the use of IQ test as a measure of intelligence. IQ tests
limit our definition of intelligence: they are powerful predictors only in the fields in
which literacy and mathematical ability are of central importance. Mental aptitudes not
requiring excellence in these two abilities are left out. The result is that we tend to
view creative abilities such as art, music, dance, cooking, and raising children as
having little connection with IQ. Other criticisms are more serious: there is a long and
ugly history of using IQ tests for eugenic purposes. One of the more benign eugenic
programs involves sorting people into categories for educational purposes.
In these programs (tracking in the US, streaming in England) children are sorted into
fast and slow learners and placed in classes accordingly, which may seriously impact
career and life choices.
Another use for IQ tests is to predict outcomes. Eyesenck (1998) cites a study in which
all five year olds on the Isle of Wight were given IQ tests and final school grades were
predicted. At age sixteen, the children were tested again; IQ scores changed very little
and grades had been very accurately predicted. Eyesenck takes exception to the idea that
IQ tests do not measure anything more than the ability to take IQ tests: he emphatically
states that IQ predicts achievement. While it is not difficult to see a relationship
between achievement and intelligence, defining intelligence as achievement precludes the
possibility that some children of lesser intelligence have greater motivation to succeed.
This author can cite several personal examples of brilliant students who lack motivation,
and less than stellar students who, through determination, achieve great successes.
Lewontin, Rose, and Kamen (1984) dismiss outright the idea that IQ tests alone are good
predictors of future social success, preferring to attribute it to family environment.
Numerous IQ tests exist; by 1978 there were at least 100 tests described as measuring
intelligence or ability (Mackintosh, 1998). All of them are validated by how well they
agree with older standards such as the Stanford-Binet (Lewontin, Rose, & Kamen, 1984).
The Wechsler Adult Intelligence Scale (WAIS-R) and the Wechsler Intelligence Scale for
Children (WISC-III) are the most often administered IQ tests today, and are composed of
eleven subtests and two subscales: verbal (information, vocabulary, comprehension,
arithmetic, similarities, and digit span) and performance (picture completion and
arrangement, block design, object assembly, and digit symbol). The next most popular
test, the Stanford-Binet, employs fifteen subtests with four subscales: verbal reasoning
(e.g. How are a porpoise, dolphin and whale different from a shark?, abstract/visual
reasoning (completion of matrix problems), quantitative reasoning (e.g. sequential
questions such as, What comes next: 5 10 9 18 17 34 33 __ __), and short-term memory
(repeating digits forwards and backwards). Raven's Matrices relies entirely on nonverbal
methods to measure IQ, by requiring the test subject to discern the relationships between
different objects. While both the Wechsler and Stanford-Binet are administered to
individuals, Raven's Matrices may be given to a large number of people at the same time.
Interestingly, despite the vast differences between the two tests, correlations between
Raven's and Wechsler scores in the general population fall between 0.40 and 0.75 (Burke,
1958, 1985; Court & Raven, 1995; as cited in Mackintosh, 1998).
Theories of Intelligence
Factor analysis has been employed to find patterns in the correlation matrices of IQ
tests, with varying results (Mackintosh, 1998). In 1927, Charles Spearman developed a
two-factor theory of intelligence, which involves the idea that there is a general
intelligence (g) that may be measured with varying degrees of accuracy by tests. He
proposed that the reason why all tests of ability correlate to a certain extent is
because they all measure g. Other theories began to arise: by 1941, Thurstone & Thurstone
suggested that IQ tests must measure a number of independent factors, which they called
primary mental abilities, and that researchers ought to focus on creating factorially
pure tests. They found six distinct primary mental abilities: numerical, verbal
comprehension, word fluency, space, reasoning, and memory. Guilford may hold the record
on distinguishing numbers of factors, which he defined in 1967 as the completion of a
specific type of operation on a specific type of product with a specific type of content.
He found five operations, six products, and four kinds of content, for a whopping 120 (5
x 6 x 4) factor total. Unfortunately, Horn & Knapp (1973; as cited in Mackintosh, 1998)
applied his factorial procedures to his test data and found they supported randomly
generated factorial theories just as well. 
Contemporary factor analysts generally cite nine different kinds of general mental
ability: fluid reasoning (critical thinking ability), acculturation knowledge (breadth
and depth of knowledge of the dominant culture), quantitative knowledge (mathematical
ability), short-term memory (involving events in the last minute or so), long-term
memory, spatial ability (measured in tasks such as comparing rotated objects for
similarity), auditory processing (perception of sound patterns under distraction or
distortion), processing speed (speed of response given an intellectually simple task),
and correct decision speed (speed of response given an intellectually challenging task).

Factor analysis is an incomplete solution to the problem of intelligence: while it
describes relationships between different IQ tests, it cannot tell us much about the
structure of human abilities. The existence of a general factor describing the
correlation of IQ tests does not imply that it measures a cognitive process; it is quite
possible that the tests measure any number of processes that happen to overlap, thus the
various theories mentioned above.
Genetic factors
Heritability of intelligence (or any other characteristic, for that matter) is the
proportion of the total variation in the characteristic in a population that can be
attributed to genetic differences between members of the population. Estimates of
heritability are made by attempting to separate genetic from environmental sources of
variance. Evidence exists that intelligence runs in families: the correlation of IQ
scores of genetically related people increases by the closeness of relationship, and this
correlation pattern remains even when controlling for social class, education, race,
gender, and the like (Herrnstein & Murray 1994). Lykken, Bouchard, & McGue (1993) found
an average correlation of about .45 for IQ scores of biological parents and offspring and
for siblings living together. 
Most of our understanding of the genetics of intelligence is grounded in twin and
adoption studies, which have documented significant and substantial genetic influence
(Plomin, 1994, Plomin & Petrill, 1997, Steen, 1996): for example, correlation between
scores of monozygotic (MZ) twins reared together is higher (approximately .85) than
correlations of dizygotic (DZ) twins and less closely related siblings (Plomin & Petrill,
1997). There is also a high correlation in IQ scores between MZ twins who were raised
apart (Joseph, 1998). Dudley (1991) found a higher correlation between IQ scores of
adopted children and their biological parents than with their adoptive parents. A major
concern with both twin and adoption studies, however, is the amount of correlation in the
environments of adopting and biological families (Lewontin, Rose, & Kamen, 1984). There
have also been a number of methodological problems (Steen, 1996), as well as several
instances of fraud (Cyril Burt, for example). Consequently, it is difficult to use these
studies to bolster arguments of heritability of intelligence.
Exactly how much intelligence is attributable to genetics is unknown, and estimates vary
widely. Arthur Jensen (1969, as cited in Eyesenck, 1998) placed heritability of
intelligence at 80 percent, Eyesenck (1998) at 70 percent, Herrnstein and Murray (1994)
between 60 and 80 percent, and Plomin and Petrill (1997) at 50 to 60 percent. However,
attempts to quantify heritability have serious problems in explaining some of the data.
Interestingly, heritability estimates vary with age: A study of 60 year old (average age)
Swedish twins (some reared together and others apart) indicates that heritability
increases from about 40 percent in childhood to about 60 percent in early adulthood and
about 80 percent in later life (Plomin, 1994, Plomin & Petrill, 1997). Eyesenck argues
that this is because we structure our environment based on genetic drives (p. 42). He
reasons that environment exerts a greater influence on children, who have little choice;
as they age, diversity and availability of choices expands, and if these choices are at
least partially determined by genetic factors, the influence of environment is thereby
diminished. Benno (1990) suggests the difficulty in determining relative contributions
lies in the interdependency of genes and environment.
Lewontin, Rose, and Kamen (1984) suggest that heritability of IQ is irrelevant and
unimportant, as heritability is not synonymous with unchangeability. They attribute this
confusion to a general misunderstanding about genes and development. They assert that the
genotype is inherited and unchanging; the phenotype is in a constant state of flux,
involving morphological, physiological, and behavioral properties. In simpler terms, the
loss of a limb is irreversible, but not heritable; Wilson's disease is heritable but not
irreversible. They may be mistaken about the heritability of genotype: Stoltenberg and
Hirsch (in press) explain that parental genotypes may not be passed down because they are
broken up at meiosis and a new genotype is formed at conception.
One of the consequences of the Human Genome Project, tasked with sequencing the entire
human complement of DNA, is a public perception that scientists are developing a
molecular understanding of the human condition. Seldom a month goes by without a media
article trumpeting a new genetic link to a behavior or disease. Everything from
schizophrenia to television watching is postulated to be linked to genetics, yet
scientists are a long way from being able to explain the ramifications of the human
genome sequence. Kaye (1992) suggests that phrasing used by the media such as gene for
alcoholism is misleading: Noble and Blum had only suggested a possible genetic component
contributing indirectly to the alcoholism of some individuals. As yet scientists have
been unable to fully trace a chain of events leading from genes to behavior, however,
scientists have recently begun to identify specific genes that influence cognitive
abilities and disabilities, most of which involve rare single-gene disorders such as
phenylketonuria (PKU), Fragile X mental retardation and the early-onset familial form of
Alzheimer's 5 disease (Plomin & Petrill, 1997; Steen, 1996).
Just how important are genes? The Central Dogma of molecular biology, attributed to
Francis Crick, is that DNA makes RNA, RNA makes protein, and 'proteins (to oversimplify
just a bit) are us' (Kaye, 1992). If this true, then DNA is simply a blueprint that
determines our humanity, but things are definitely more complicated. For example, we have
evidence that mutations do not arise solely from genes, but may arise from cell
structures as well: consider Sonneborn's melon-striped paramecia experiment (Goodwin,
1994). He was able to remove patches of cilia from the surface of a normal paramecium and
put it back in reverse orientation, creating a melon stripe. All future generations of
this paramecium were melon striped with the same reversed row of cilia. Sonneborn
demonstrated the same effect, cytoplasmic inheritance, on an asexually reproducing worm
(Stenostomum). 
In sexual reproduction, one cell from the female and one from the male unite to produce
the cell for the new organism. The male contribution consists solely of chromosomes,
which contain genes that influence development and form of the new organism. Chromosomes
are composed of deoyxribonucleic acid (DNA) and are located in the nucleus of the cell.
In the sex cells (testes and ovaries in humans), meiosis creates gametes with haploid
chromosome sets: each gamete contains half of the chromosomal information necessary to
create a new organism. The moment of conception occurs when the male and female gametes
unite, forming a zygote which has a complete, diploid chromosome set. While the male
contributes only his gamete, the female in addition to her gamete contributes cytoplasm
which nourishes the developing zygote as well as specific proteins that direct cell
differentiation. Along with mutation, meiosis assists in maintaining diversity of a
population: the homologues of each chromosome pair are split so that each gamete receives
one from each pair, assuring independent assortment; the homologues also exchange genetic
material during recombination. 
The genotype of an organism, uniquely formed at conception, contains its complete genetic
endowment; its phenotype is dependent on its interaction with the environment and
consists of its appearance, structure, physiology and behavior. While the phenotype is
dependent on the genotype, it may not be assumed that the genotype determines the
phenotype - a genotype may result in different phenotypes depending on the environment.
Quantitative genetics was developed to study traits such as behaviors that are
continuously distributed in a population (Stoltenberg & Hirsch, in press). To assess the
resemblance between relatives in terms of specific traits, the overall phenotypic
variance is partitioned into genetic and environmental components. Genotypic variance is
then partitioned into additive, dominance, and interactive variance components. Additive
genetic variance, breeding value, is not strictly additive in the mathematical sense, as
it is entirely dependent on the population from which the mate is selected. The dominance
deviation value is the difference of the genotypic values and additive values, and is
caused by the effects of one allele over another in the classic Mendelian
dominant/recessive sense. Interaction deviations occur when there are nonadditive
relationships between loci. Broad sense heritability is an estimate of the extent to
which the genotype determines the phenotype. Narrow sense heritability is used to predict
the outcome when selecting for a specific trait in a population, and is estimated as the
ratio of additive to phenotypic variance. Stoltenberg and Hirsch caution that
heritability is not the same as inherited: while heritability is an estimate of the
degree of relationship between genotype and phenotype, it does not give us the proportion
of a trait that is genetically determined. 
Environmental factors
No one argues for genetics alone in shaping behavior. Lewontin, Rose, and Kamin (1984)
cannot think of any significant human social behavior that is built into our genes in
such a way that it cannot be shaped by social conditions. Environment includes a broad
array of effects on intelligence; some influence whole populations while others
contribute to individual differences. These influences include biological as well as
social and cultural factors. 
Biological factors such as malnutrition, exposure to toxic substances, prenatal and
perinatal stressors may result in lowered intelligence. Prolonged malnutrition during
childhood has negative long-term intellectual effects, but interestingly, in a study on
prenatal malnutrition, Stenin, Susser, Saenger, and Marolla studied the test scores of
19-year-old Dutch males born during a wartime famine (Neisser et al., 1996), and found
that exposure to famine had no effect on adult intelligence. Exposure to lead can have a
negative effect on intelligence: Neisser et al. (1996) administered IQ tests to children
with high blood lead levels throughout childhood, and found they scored substantially
lower. Fetal alcohol syndrome, due to prenatal exposure to excessive amounts of alcohol,
may lead to mental retardation. Even smaller amounts of alcohol may harm: mothers who
reported drinking 1.5 oz of alcohol daily during pregnancy had children who scored an
average of 5 points lower on IQ tests by age four (Neisser et al., 1996). Perinatal
factors such as complications during delivery may also influence intelligence. The
correlation between very low birth weight (less than 2,500 gm.) and later intelligence is
fairly large (Breslau et al., 1994, Baumeister & Bacharach, 1996, as cited in Mackintosh,
1998).
Social and cultural aspects of environment may influence intelligence: Werner and Smith
(1992) found that temperament affected developmental outcomes for children, beginning at
birth. Infants who smiled often and were affectionate attracted more care and emotional
support from their parents and others. Schools promote the development of intellectual
skills such as systematic problem-solving, abstract thinking, and categorization;
children who attend regularly may be expected to benefit more than those who attend
sporadically. Plomin (1991) suggests other factors such as parental affection, birth
order, gender differences, experiences outside the family, accidents, and illnesses may
account for differences in IQ between siblings. In their study of adoptive and
biologically related families with children between 16 and 22 years of age, Scarr and
Weinberg (1983) found environment more powerful in influencing IQ level in the young
child than the young adult. They argue that by providing better schooling, nutrition,
health care, psychological services and the like, we can raise the level of intellectual
development. 
In the short run, a number of interventions have been shown to raise test scores. The
Venezuelan Intelligence Project involved exposing hundreds of seventh-grade children from
underprivileged backgrounds to a specialized curriculum focused on thinking skills
(Chipuer et al., 1990), and produced considerable improvements on a wide range of tests.
Another program, Head Start, is specifically geared to underprivileged children around
four years old and is able to document an improvement in test scores over the length of
the program. These effects dissipate over time, however, and by the end of grade school,
there are no significant differences for Head Start participants over other children from
similar environments (Darlington, et al., 1980). However, follow-up studies such as the
Perry Preschool study (Schweinhart & Weikart, 1988) indicate that children who
participated in such programs as preschoolers are less likely to be placed in special
education classes, less likely to be held back in grade and more likely to finish high
school. 
A change in a single gene may change the structure of an organism, but Goodwin (1994)
argues that we cannot logically step from this to the idea that genes contain all the
information needed to create the structure. Viewing organisms as the sum of their genes
is reductionistic: organisms must be studied as dynamic systems with distinctive
properties that characterize the living state (Goodwin). Development of an organism is an
epigenetic process: at every step of development, the next step depends on the organism's
current biological state, which is a function of both genetics and environment. Organisms
construct their environments, change them, interpret sensory experiences, and change the
pattern of variation (Lewontin, Rose, & Kamin, 1984). In humans, mental states affect
environments through conscious action; the relationship between human and environment is
a dual development of each. Kaye (1992) suggests that the question of human nature is not
simply a biological one, no matter how many genetic correlates of character are
discovered. Genes, environment, and the interaction between them all contribute to
influence the development of individual differences in the complex phenomenon known as
intelligence.
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