Nature versus Nurture on Intelligence and Personality
By
Enes McCaig
ENGL1502A
Robert G. Cooper
March 6, 2014
The subject of nature versus nurture has been commonly debated in the field of
psychology. Human traits such as intelligence, personality, language and many others can
be attributed to genetic and environmental factors. Some traits can be explained through
genetic factors alone, such as genetic disorders. Genetic disorders typically occur as a
result of inheriting an affected gene from a parent. There are traits such as language,
which are entirely dependent on environmental factors, and cannot be attributed to a
genetic inheritance. Many of the human traits studied in psychology, such as intelligence
and personality, cannot be attributed to genetic or environmental factors alone. Both
genetic and environmental influences play a role in human development, but it is unclear
whether one factor outweighs the other. Are we the way we are because of the traits we
have inherited from our family, or is the course of our development dependent on
environmental factors? The debate continues from decade to decade. Substantial
evidence exists for both arguments, some of which will be reviewed and discussed. It
seems likely that we have the capacity to influence our own development, despite varying
genetic inheritance.
One of the earliest major publications in this field was a book titled “Hereditary
Genius” by Francis Galton. This book represents many of the major early arguments for
the role of heredity in human development. Galton argued that human development can
be compared to animal development. In the same way that animals can be bred to exhibit
certain traits (for example, racehorse), human abilities can be passed on through family
members. According to his book, Galton believed that people displaying exceptional
talent in any given measure, inherited their abilities from their ancestors.
To support his theory, Galton examined the family history of individuals whom
he considered to be “geniuses” in their field. Galton did not measure the abilities of these
individuals, but selected them based on success, popularity and general consensus. Galton
first examined English Judges, stating that “a judgeship is a guarantee of its possessor
being gifted with exceptional ability” (Galton 1892). The claim is debatable, and it is
unlikely that all judges possess exceptional abilities, but Galton goes on to examine the
possible source of these abilities. After examining the family history of 282 English
judges, Galton found that approximately one out of nine judges had a father, brother or
son who was also a judge. He concluded that there is no doubt that the type of ability
required to become a judge is often transmitted by descent. Galton also went on to study
statesmen, military commanders, famous authors, scientists, poets, musicians, painters
and prominent religious figures. As with the judges, he found that many of these
successful, esteemed individuals came from families of other great individuals in the
same field. Galton believed that this type of trend represented the inheritance of superior
abilities, resulting in a unique or gifted individual.
Galton also provides arguments against the claim that everyone is born equal, and
that greatness can be achieved through the consistent application of effort. He argues that
extensive training and practice, while it can improve certain abilities, has its limitations.
A man who is naturally born small and weak will never be as strong as someone who is
naturally large and muscular, no matter how much he trains. Galton believed that the
same principle can be applied to students. Thousands of ambitious students go through
school each year, while only a small fraction of them ever achieve a position of great
importance in their field. In examining gross cultural differences, Galton compares the
Anglo-Saxon race with African Americans. Not surprisingly, he concluded that African
Americans seemed less likely to posses the qualities necessary for success by AngloSaxon standards. He also stated that the abilities of Australians fell slightly below those
of African Americans. Galton refers to numerous books in which African American
servants were described making simple “childish” mistakes as a source of information for
comparing the two races. (Galton, 1892, 339)
Many of Galton’s conclusions represent one of a number of different possible
interpretations of his findings. His research was purely co-relational, and some of his
sources obviously represent biased opinions. Galton’s assertion that a family history of
ability or talent in successful individuals provides evidence for the heritability of these
traits can also be seen as evidence for the contrary. In other words, the influence that
family imposes on an individual must be considered in environmental terms as well as
genetic. It does not make sense to put so much weight on the genetic influence of family
members while ignoring the environmental influence, which may be substantial,
especially during childhood. It seems equally negligent to use an author’s opinion of
African American slaves as a source of information on the African American race. The
fact that they were used as slaves in the first place implies an insulting opinion toward the
African American race. Therefore, is does not seem unlikely that the authors in question
would be more likely to point out the mistakes and shortcomings of slaves, rather than
their positive traits.
Regardless of the nature of his research, it seems that some of Galton’s
conclusions are supported by modern research. Developments in the field of genetics
have made it possible to examine the role of genetics and environment much more
closely. As a result, many studies have been conducted in recent years to examine the
relative influence of genetic and environmental factors on human development, with
many results supporting the idea of heritable traits. Traits such as intelligence, personality
and behaviour have shown to have a genetic component.
Recent studies have supported the idea of heritability of intelligence. In this type
of study, heritability (h2) refers to the proportion of variance in a trait that can be
attributed to variation in genes. The remaining variance ( that cannot be explained by
genetic factors) is attributed to differences in environmental factors (Neisser, et. al.
1996). In Neisser’s 1996 study, “The Heritability of Intelligence in Adolescents” was
calculated at 0.75. This implies that a significant proportion of the variance in IQ scores
can be attributed to genetic factors, providing strong evidence for the heritability of
intelligence. The study also examined the role of intelligence in other life factors, mainly
grades in school, years of schooling, socio-economic status and income. It was found that
the correlation between intelligence and grades, years of schooling and socio-economic
status was around 0.50 to 0.55 (Neisser, et. al. 1996). The correlation between
intelligence and income was weaker, at around 0.40 (Neisser, et. al. 1996). This implies
the variance in schooling and socio-economic status can be accounted for differences in
intelligence and other factors and intelligence only accounts for a small percentage of the
variance in income.
Twins are commonly studied to determine the genetic component of certain traits.
Monozygotic twins, formed from the fertilization of a single egg which later splits into
two identical eggs, share 100 percent of their genetic information. Dizygotic twins,
formed during the simultaneous fertilization of two separate eggs only share about 50
percent of their genetic information (Malykh, S.B. et al., 2002). Studies involving
monozygotic twins provide strong evidence for the role of genetics and environment in
various traits. In a Russian study involving monozygotic twins, the correlation between
the full-scale IQ of twins was very high. It was found that 89 percent of the variance in
intelligence could be explained by shared genetic factors (Malykh, S.B. et al., 2002).
Other twin studies examined the genetic component of personality and
behavioural traits. The correlation between monozygotic twins for depression has been
calculated at around 0.40, and about 0.20 for dizygotic twins (Kendler, K.S. et.al. 1992).
For happiness, genetic differences were found to explain as much as 40 to 50 percent of
the variance between individuals (Lykken, D. 2000). In another study involving female
twins, genetic influences were found to account for 37 to 44 percent of the variance in
extraversion, neuroticism and social non-conformity (Pergadia, M.L. et.al, 2006). The
genetic component of personality is generally lower than the genetic component of
intelligence. This implies that personality development is largely affected by
environmental factors. In turn, personality factors significantly influence human
development, in terms of achievement, health, relationships and other factors (Caspi, A.
et. al., 2005). From these results, one can see that environmental factors involved in
personality development play a significant role in overall human development.
Often it is found that a combination of environmental and genetic factors
determine the emergence of a trait. A long-term study compared children possessing a
specific piece of genetic information (an allele for a dopamine-related gene) with children
who did not possess the allele (Keltikangas-Jarvinan, L. et.al. 2004). They were
monitored over a period of 14 years, and compared in terms of their level of novelty
seeking behavior. Within both genetic conditions were children raised in a “more hostile”
or “less hostile” environment, classified in terms of emotional closeness with the mother
and disciplinary style of the parents (Keltikangas-Jarvinan, L. et.al. 2004). The study
measured novelty seeking behaviour using the child’s novelty seeking (NS) score on the
Temperament and Character Inventory. It was found that for children possessing an
infected gene, those raised in a hostile environment demonstrated significantly higher NS
scores than those raised in a less hostile environment (also significantly higher than the
average person). Children who did not possess the infected gene did not differ
significantly in terms of NS scores, regardless of their childhood environment
(Keltikangas-Jarvinan, L. et.al. 2004). This study demonstrates a case in which a genetic
predisposition promotes a certain trait, only under specific environmental circumstances.
One can be convinced that this type of genetic-environmental interaction may be the
means of development for a large number of human traits that cannot be explained
through one of the factors alone (Keltikangas-Jarvinan, L. et.al. 2004).
While researching these developmental theories, the most notable factor found is
they are based on the combination of nature and nurture. There are some that suggest a
more complicated type of mechanism. Instead of considering development in terms of
two separate and independent factors (genes and environment), it can be seen as a more
joint process. Scarr and McCartney of Yale University proposed a theory in which people
“make their own environment and influence their own development” (Scarr, S. &
McCartney, K. 1983). They argue that environmental factors should not be considered
only in terms of different external circumstances and events, but also in the way an
environment is experienced by an individual. In essence, it is human experience that
drives development.
To demonstrate this, they refer to speech development in children. A child
becomes attentive to language long before he or she begins to use it. If it was for the
awareness of language, a child would never develop the ability to use it. In this situation a
child’s awareness of language comes about as a result of an in born developmental
system, but it is a useful analogy for explaining the role of experience. A child’s external
environment has not changed when language is first developed, although it is experienced
in a completely different way by the child. A similar mechanism may drive the
development of other human traits; they may emerge because of a change in the way the
environment is perceived by the individual. Many would argue that it is simply a person’s
genotype that determines the way they will respond to their environment.
Scar and McCartney propose that optimal conditions for development occur when
a person’s genotype corresponds to his or her environment. In terms of speech, we all
possess the ability to learn language and were all raised in an environment filled with
language. In the same way, if our environment corresponds with another genotype,
developments will occur. In child development, both the genotype and environment are
influenced by the parents. Parents who enjoy reading may have a genotype for reading
that is passed on to their child. At the same time, the child may be exposed to books at a
young age, which compliments his or her genotype and promotes the development of
reading abilities. As children become more mature and independent, they may begin to
select environments for themselves which compliment their individual differences. By
selecting appropriate environments, a person can essentially influence his or her own
development.
Research that was done has demonstrated a strong genetic component for specific
human traits such as intelligence. Genetic predispositions can account for some
variability in other traits as well, but are not necessarily the driving force behind human
development. It has been argued that gene-environment interactions are the dominant
mechanism for developing certain traits. The driving force behind this mechanism
however, is the selection of appropriate environmental conditions for the development of
specific traits. Researching the studies, one can be convinced that personal development
can occur in a wide range of directions, depending on the intentions of the individual. We
all possess in born abilities that can be attributed to the genetic code, such as the capacity
for language, consciousness and abstract thought. Despite variation in individual
genotypes, these abilities are common to the human species, and correspond to the
environment in which our species evolved. It is because of these abilities that we are able
to function effectively in our environment.
Through conscious awareness and understanding of our environment, both
internal and external, we can become aware of our developmental capacities. Once we
are aware of language, we can learn to use it. Once we are aware of components of our
environment, such as subjects we learnt in school like mathematics, art, music or science,
we can learn to develop in those areas as well. This is true for all individuals regardless of
genotypic differences. We have the ability to modify our environment in accordance
with personal needs. The process of development depends on the choices we make in
adapting our own environment to meet our personal requirements. There are no in born
traits towards a specific ability or a certain career, and each has the ability to recognize
and differentiate the components of their environment. Similarly, we all have the capacity
to choose which components we focus our attention on, and develop our understanding
of. Essentially, individual development depends on the conscious actions of the
individual, selecting desirable environmental elements and effective adaptive strategies
for the surrounding environment. The ability to develop in this way is present regardless
of genotypic variations, much in the same way that we all have the capacity to learn
language. The specific language that we learn depends entirely on environmental factors.
Similarly, we all have the capacity to learn any human ability, but the abilities we
develop depend entirely on the environment we are exposed to. Despite genetic
contributions to intelligence and personality, we each have a flexible developmental
capacity, and the ability to choose and adapt to any given environment.
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