Nature vs. Nurture
The dubious history of the heredity environment controversy can be
easily traced as far back as the start of the present century with at least some
historical evidence placing the roots of this dispute in the time of John Locke.
This controversy has continued despite continual reiteration that the critical
question is not how much of a trait is due to heredity and how much is due to
environment, but rather how environment transact to influence development. (
Wachs , 1983, p. 386).
This paper will focus on the nature/nurture controversy and the extent
to which an individuals intellectual level is determined either by inborn
intelligence or by environmental factors.
The relative powers of nature and nurture have been actively pursed by
psychologists and biologists striving to determine how heredity and environment
influence the development of intelligence.
Before we can go on to discuss the relationships between intelligence
and the controversy that exists between the different schools of thought
regarding inherited or environmental issues we must have an understanding of
what intelligence really is.
Of all the words used in pressed day psychology, intelligence is one of
the most difficult to define and is also one of the most controversial. There
is however, a general agreement that intelligence refers to the overall
faculties of the mind which concern themselves with the sorting of information
in the brain after it has been received by the senses, the perceiving of
relationships between this new data and information which is already in memory,
and the capacity to make rapid and appropriate decisions as a result of the
The intellectual faculties of the brain are dynamic and interactive and
relate to the capacity of the central nervous system to respond speedily and
appropriately in a rapidly changing and potentially threatening environment.
Raymond J. Corsini provides us with a somewhat more simplistic
definition of the term intelligence. According to Corsini (1984) the term
intelligence can be employed to indicate the amount of knowledge available and
the rapidity with which new knowledge is acquired; the ability to adapt to new
situations and to handle concepts, relationships, and abstract symbols.
While the heredity/environment topic continues to be a controversial
issue, a great deal of evidence has been gathered to support both arguments.
In order to investigate the topic of nature/nurture it is important to
consider a variety of research elements. Among these elements are some of the
most relevant issues pertaining to this subject including: twin, adoption,
family, orphanage life, IQ, and race studies. It is to these studies we will
now turn our attention.
The importance of twin studies is evident if we look at the studies
objectively, if intelligence is basically hereditary, identical twins who have
the same genetic legacy, should be concordant for that trait than are fraternal
twins, which are no more alike genetically than other siblings.
Burt’s (1958) famous study show that the intelligence test scores of
identical twins, whether reared together or apart , display considerably higher
correlation than the scores of fraternal twins.
Burt’s work is currently viewed with caution due to the manner in which
he gathered and interpreted his data (Vernon, 1979). However, Burt’s research
provides an important foundation for this research.
Jone’s study (1946) shows that there is a modest difference in the
intelligence test scores of twins reared apart, and the more divergent the
environments, the greater the difference.
While environmental factors are important in raising or lowering a
child’s level of intellectual performance, these studies demonstrate that they
only do so within limits set by heredity. (Mussen, Conger, and Kagon, 1963
The Louisville Twin Study (Wilson, 1983) showed that environmental
considerations such as characteristics of home and the interaction of the mother
with the infant, have a prominent effect upon the infant’s mental development.
Vermon (1979) concludes that we may attribute 60 percent of the
determination of IQ status to heredity, 30 percent to environment and 10 percent
tot he combined effects of the two.
David Layzer’s (1976) study indicates that the more relevant a given
task is to an individuals specific environmental challenges, the more important
are the efface of this interaction. A child grown up in circumstances that
provide motivation, reward and opportunity for the acquisition of verbal skills
will achieve a higher level of verbal proficiency than his twin reared in an
environment hostile to this id of development.
According to Layer if two egg twins are reared together we cannot assume
the environmental factors are the same for both. If one twin has a greater
verbal aptitude he will devote more time and effort to this kind of learning
than his twin. So test results on verbal proficiency will not reflect genetic
difference, only, but differences between the ways in which the genetic
endowments of the twins have interacted with their common environment.
Longitudinal studies have also found the influence of heredity on
intelligence increases with age. Among 500 pairs of twins, identical twins
became more and more alike in IQ from infancy to adolescence, while fraternal
twins became less alike. The home environment had some impact, but genetic
factors had more (Plomin, Pedersen, McClean, Nesselroade, and Bergman, 1988).
IQ of identical twins are more highly correlated than less closely
related people and IQ’s of children growing up in similar circumstances tend to
be more highly correlated than those of children growing up in dissimilar
circumstances. This fact helps to illustrate that IQ is strongly influenced by
both genetic and environmental factors.
Adoption studies are important for us to look at because they provide
one of the few methods available for separating the effects of environment and
heredity in intellectual development.
When adopted children are more like their biological parents and
siblings, we see the influence of heredity; when they resemble their adoptive
families more, we see the influence of environment.
The Texas Adoption Project (Horn, 1983) conducted intelligence tests
from parents and children in 300 adoptive families and compared them with
similar measurers available for the biological mothers of the same adopted
children. The results of this study supported the hypotheses that genetic
variability is an important influence in the development of individual
differences for intelligence.
The study also concluded that adopted children resemble their biological
mothers more than they resemble the adoptive parents who reared them from birth.
In an extensive study dealing with the mental growth of foster children
after they had lived in their new homes found that on the whole, they improved
their mental status, the extent of improvement being contingent upon the quality
of the foster home, the length of residence there, and the age at which the
child entered the new environment. It found, also, that siblings living in
different foster homes resembled one another much less than brothers and sisters
ordinarily do. In general, the results demonstrate that improved environmental
conditions which endure can raise the level of intelligence, if optimal
conditions are provided early in life.
A child born into a poor home often shows an improvement in intelligence
if adopted by a more intelligent and stimulating family, and it has been found
that Negro children born into backward rural families improve steadily if they
move to the city. But the amount of improvement is always limited by the mental
capacity that was there a t birth. Freeman. (1928)
The Minnesota Adoption Studies of 1974 included the Transracial Adoption
Study to test he hypotheses that black and interracial children that are reared
by white parents would perform on IQ tests and school achievement measures as
well as other adopted children.
Results were that black and interracial scored as well as adoptees in
other studies. The high IQ scores of the black and interracial children showed
that genetic racial differences do not account for a moor portion of the IQ
performance difference between racial groups. The study also found that black
and interracial children reared in the culture of the tests and the school
perform as well as other adopted children in similar families.
Marie Dkoday and Harold Shell’s 1949) report of a longitudinal adoption
study of IQ is one of the most frequently cited articles in developmental
psychology. The IQ scores of adopted children tested four times between infancy
and adolescence were compared to characteristics of both their adoptive parents
and their biological parents. The results of the study were impressive, the
correlation between the IQ of 63 biological mothers and their adopted children
indicated increasing hereditary influence.
However, a study done on adopted children in France found that white
children abandoned at birth by lower-class parents and adopted at an average age
of 4 months by white professionals, when compared with their siblings who we
reared by their biological parents, the adoptees scored about 14 points higher
than the average IQ and were less likely to be held back in school (Schiff,
Duyme, Dumaret and Tomkiewicz, 1982).
Two of the largest adoptive studies were conducted by Horn, Scarr and
Weinberg. They concluded that individual differences in IQ are substantially
influenced by genetic differences among individuals and that family environment
also has a significant impact.
Plomit and Defries (1980) in their studies found that genetic factors
account for 50 percent of the variance in IQ scores and that environmental
factors 15 percent.
Bouchard and McGud (1981) conducted a review of 111 studies in the area
of familial resemblance’s in measured intelligence. The purpose of the review
is to provide a comprehensive summary of the literature on the IQ correlation’s
between relatives. In general the review reported that the higher the proportion
of genes two family members have in common the higher the average correlation
between their IQ’s.
The patterns of averaged correlation’s is remarkably consistent with
pyloric theory. This is not to discount the importance of environmental
factors; monozyotic twins reared apart are far from perfectly correlated,
dizygotic twins are more similar than other biological siblings. That the data
support the inference of partial genetic determination of IQ is indisputable;
that they are informative about the precise strength of this effect is dubious.
Buouchard and McGue. (1981).
Burk’s (1928) study indicates that the maximum contribution of the best home
environment to intelligence is about 20 points and that the least cultured kind
of American home environment may depress the IQ by as much as 20 points.
Bradly and Caldwell (1976) conducted research regarding infants early
home environment as related to the children’s mental test performance at age
Their studies indicated that home environment scores during the first
two years of life were strongly related to fifty-four month IQ scores. The
studies produced evidence linking the quality of stimulation provided in early
years of life to cognitive ability in young children.
Orphanage life is a form on environmental deprivation, since the
children lack personal attention that would occur in normal families. These
children generally show slower intellectual developments, as reported by Skeels
(1940) and J.M Hunt (198O).
Jense, (1969) found that orphanage children gained in IQ from an average
of 64 at an average age of nineteen months to 96 at six years of age as a result
of being given social stimulation and placement in good homes at between two and
three years of age.
Children who were let in an orphanage environment continued to show a
decrease in IQ points, and as adults their lifestyle reflected the past deprived
environment. Many of these adults who had spent all their early life in an
orphanage worked at jobs requiring minimum skills. Thus demonstrating the
environmental impact on intelligence.
IQ AND RACE
There is no evidence that differences in IQ scores between cultural,
ethnic, or racial groups are due to hereditary factors. But many studies point
of a strong genetic influence on differences between individuals within a group.
About 50 percent of the difference in intelligence between persons in a group is
believed to be genetically determined, with the remaining variation due to each
person’s experience (Weinbergt. 1989).
Jensen (1976) argues however that reported differences in average IQ
between black and white children is due in part to systematic genetic difference.
At this point one or two somewhat consistent differences can also be
mentioned about race. Comparisons of black and white samples in the United
States show typical IQ differences of nearly 15 points in favor of whites. The
black white difference depends somewhat on sex membership and upon what special
ability is measured. The differences IQ and in school achievement are smaller
for black females. black do relatively better in rote memory tests and
relatively poorer is visual test (Vernon, 1979).
For other special groups Jews tend to score higher than other white
ethnic groups. Japanese and Chinese tend also to score higher than the average
white groups. Corsini. (1984).
In conclusion this paper had included the findings of numerous studies
in order to examine the heredity/environment controversy. From the studies we
have gained an understanding of the importance of genetic constitution, the
significance of which in the formation of intelligence and individuality is
clearly demonstrated. The studies have also provided ample evidence to support
the importance of environment influences in individual development. Clearly,
the studies demonstrate that the contribution of nature and nurture may no
longer be set in opposition for each is conceivable as one factor, or as a set
of factors in a complex situation.
Indeed, one has no meaning without the other, both nature and nurture
are inextricable related in intellectual development. Thus, instead of writing
nature and nurture we should write nature/nurture for the two sets of facts
exist as integrals in a single process of development. The two are mutually
inclusive, since innate tendencies are capable of expression on in terms of
environments, and environmental influences can only act together with the
genetic bases of individuality.
Bochard, J.,M McGue, M. (1981). Familial Studies of Intelligence: A Review.
University of Minnesota.
Burks, BS. (1928). The relative Influence of Nautre and Nurture Upon Mental
Development. Yearbook of the National Society for the Study of Education.
Bradley Hand Caldwell, B. (1976). The Relation of Infants Home Environments to
Mental Test Performance at Fifty-For Months. University of Arkansas.
Cancro, Rober. (1971). Intelligence, Genetic and Envronmental Influences. New
Corsini, Raymond J. (1984). Encyclopedia of Psychology Volume 2. New York.
Freeman, Fin. (1928). Influence of Environment of the Intelligence, School
Achievement, and Conduct of Foster Children. New York.
Horn, J. (1964). The Texas Adoption Project: Adopted Children and their
Intellectual Ressemblance to Biological and Adoptive Parents. University of
Jesen, (1976). The IQ Contraversy. New York.
Plomin, Robert, De Fries J.C. (1983) The Colorado Adoption Project. The
Univeristy of Colorado.
Scarr, Sandra (1983). The Minnesota Adoption Studies; Genetic Differences and
Malleability. Yale University.