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 ntelligence 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 previous processes.
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 onsider 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 dentical 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 hild’s level of intellectual performance, these studies demonstrate that they only do so within limits set by heredity. (Mussen, Conger, and Kagon, 1963 p. 2)
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 ask 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 elated 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 amilies 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 onditions 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 ell 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 sychology. 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 eared 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.
