The Term Nature or Nurture and Cognitive Development

Abstract

Both environmental and genetic factors influence a person’s intelligence level. A Sweden study, involving twins with a mean age of 82.3, show the increasing influence of genetic factors on intelligence with age. A study done by psychologist Plomin and others suggest that genetic influence play a large role in intelligence levels as well. However, these studies also show that environmental influences on intelligence decrease in significance as a child matures to adulthood while genetic factors increase in impacting intelligence levels.

When combined, the correlation coefficients of environmental impacts decrease with age, suggesting that an environment is insignificant in impacting intelligence after a certain age. It is important to understand how environmental factors can have an influence because an environment that deprives a child can have major consequences in adulthood. A child lacking certain aspects can have a decreased intelligence level which can lead to less opportunities in later life. The impact of environmental influences has created intelligence gaps between races and certain cultures.

Nature or Nurture? Francis Galton, a well-known English explorer and anthropologist, first coined this phrase in 1869 with his research into the heritability of intelligence (Plomin, 2004). Since then the conversation has gone back and forth multiple times between nature and nurture. In the 1930s, with the emergence of the Nazis, the conversation swung more towards nurture, as people saw it as the only acceptable explanation for the horrors they were witnessing. Then in 1963 the first influential article about twin, family, and adoption data for IQ scores led researchers to the conclusion that genetic influence played an important role in intelligence. In 1994 the publication of The Bell Curve by Charles A. Murray and Richard Herrnstein pushed the conversation back towards the side of nurture (Plomin, 1997). Decades later, the debate still continues on whether nature or nurture is responsible for a person’s level of intelligence (Plomin, 2004). Although, the debate seems to have slowed down in between the two sides of nature and nurture due to the results of more recent studies on intelligence. These studies show as an infant grows up both genetic and environmental influences impact intelligence levels, however; once a child reaches adolescent and adulthood the effects of environmental factors on intelligence seems insignificant as genetic factors increase significantly. Two studies, both conducted in 1997, provide evidence on the influence of both environmental and genetic factors on intelligence, as well as showing how genetic influences increase in importance. Although environmental factors do not play as big of a role on intelligence levels over the span of someone’s entire life, they can have a detrimental impact on a child’s intelligence as they play a role in a child’s development.

One of the major challenges in answering the question of whether intelligence is heritable or influenced by one’s environment, is defining intelligence itself. Since Galton first coined the term nature or nurture, there have been hundreds of studies each with a slightly different definition of intelligence. The most basic definition of intelligence is the ability to learn from everyday experiences, and adapt ones behavior to what has been learned. (Myers, 2016:341). Certain researchers and studies have defined intelligence more specifically, but this broad definition can be easily understood and applied to each individual study. Researchers use a series of intelligence tests to assess an individual’s mental abilities and compare them to others. Psychologists either use an aptitude test, which predicts an individual’s potential to learn, or an achievement test, which is intended to measure how much an individual has already learned (Myers, 2016:345).

The Colorado Adoption Project (CAP), was developed in 1975 by Robert Plomin, David Fulker, and John DeFries to measure cognitive development overtime. The goal of this study was to test the change and continuity of general and specific cognitive abilities. The CAP is the longest adoption study, lasting sixteen years, as well as the largest, with 490 participating families. From 1975 to 1982, researchers in Denver, Colorado sought out children and their biological and adopted parents with cooperation of adoptions agencies. On average, adopted children found their new homes at 29-days-old, and these families were contacted for this study when the children were seven months (Plomin, 1997:442). Mothers who were in their third trimester and planning to give their babies up for adoption were tested using the Wechsler Adult Intelligence Test, while the adoptive and parents with biological kids, who were used as the control group, were tested around the child’s first year. The parent’s test lasted 3 hours and consisted of 13 tests of specific cognitive ability. When the children reached 7 years old, all parents were tested again using the same IQ test. Children who were tested in their homes at ages one through four years old were retested in a laboratory at ages 7, 12, and 16 , and these findings correlated with Plomin, Fulker, and DeFries’ hypothesis that genetic influences on intelligence increase as a child grows into an adult and adolescent, while environmental influences decrease to insignificant (Plomin, 1997:443).

Plomin, Fulker, and DeFries found through their research that the average correlation between adoptive parents and their adopted children in intelligence levels was .09 at 3 years, .06 in early adolescence, and .03 in late adolescence (16 years old). These relationships are relatively insignificant, which suggests a children’s cognitive ability is minimally influenced their adopted parent’s general cognitive ability. The correlation between the intelligence between parents and the children they put up for adoption is also low, starting at .12, however they increase all the way up to the age of 16. In middle childhood the correlation is .18, in early adolescence the correlation is .20, and in late adolescence the correlation in .38. These increased correlations suggests that there is a genetic influence on intelligence that develops throughout early childhood. The results of the control group, which consisted of biological parents and their biological children are similar to those of parents who gave their biological children up for adoption. The parents with biological children had a correlation of .19 in early childhood, .24 in middle childhood, and .31 in late adolescence suggesting that cognitive ability is greatly impacted by genetic influences, even if the children were not raised in the same environment as the biological parents (Plomin, 1997:443). The numbers obtained by these three researchers show an increase in correlated intelligence by genetic influences, as the correlation coefficient increase significantly from ages 1 to 16 with biological parents and their biological children. These numbers also show a decrease in correlated intelligence by environmental influences, as the correlation coefficient decreases to numbers almost insignificant from ages 1 to 16 with adoptive parents and their adopted children.

Another study that shows an increasing genetic influence on intelligence consisted of 110 pairs of identical twins and 130 pairs of fraternal twins of the same sex. The Swedish study consisted of twins with a median age of 82.3 years, with 74% of the participants being between 80 and 84 years old. Subjects were tested in their place of residence by licensed nurses, with each twin having a different nurse. Participants were tested on their verbal, spatial, speed of processing, and memory abilities using the five most widely used intelligence tests in Sweden, as well as the Wechsler Adult Intelligence Scale. The results of these intelligence tests were then submitted into an analysis program for twin data in order to estimate the genetic and environmental impacts of intelligence. These estimates for the proportion of results that can be attributed to heritability (genetic factors) was 62% for general cognitive ability. The influence of each cognitive ability was established, with 55% of verbal ability, 32% of spatial ability, 62% of speed of processing, and 52% of memory ability being contributed to genetic influence . These results show that even at the average age of 80, genetic influences are an important part of levels of intelligence, in fact when the error of measurement was removed for the above results, the heritability estimates accounted for more than half of what intelligence can be attributed to. The shared environment parameter, or amount that cannot be explained by heritability was only 15% for cognitive ability. When the error of measurement was dropped for this parameter there was no significant change in the numbers like there was for heritability (McClearn, 1997). The results from this Swedish study support the hypothesis that genetic factors impact intelligence more than environmental factors.

Both of these studies support the conclusion that genetic factors have a greater influence on intelligence than environmental factors. When looking at the correlation coefficient, the numbers are much higher for genetic factors on intelligence. The study by Plomin, Fulker, and DeFries included children ages 1-16 where genetic factors were 3 times higher than environmental factors. The Swedish study on twins with an average age of 80 showed that even at a later stage in life genetic factors still have a greater influence on intelligence, and environmental factors don’t play as great of a role in intelligence differences. The results of these two studies can also be compared to show that genetic factors on intelligence increase as a person gets older. In Plomin’s study the correlation coefficients were highest at age 16 and were around .38 (Plomin, 1997). In the Swedish study the average age was 82.3 years and the correlation coefficient was around .62 (McClearn, 1997). From the age of 16 to 82.3 there was an increase of almost .3 in the correlation coefficient, this increase would be even larger if the error of measurement was taken away in the second study. Whereas, the correlation coefficient between the ages of 16 and 82.3 for environmental factors influencing intelligence stayed roughly the same with no significant increase. These results suggest that as a person changes from an infant, to adolescence, and eventually an adult, genetic influences increase significantly, and environmental influences seem insignificant. Similar to what Plomin says, environmental influences do have some effect on intelligence during early childhood, however, the importance of environmental influence decline to negligible levels after adolescence (Plomin, 2004).

Even though environmental influences decrease with importance as a person gets older, it is still important to examine the impacts they can have on a young child’s intelligence. J. McVicker Hunt observed children in an Iranian orphanage, and his findings support how severe deprivation can permanently impact the brain. One example of this deprivation was Hunt’s observation of two-year-old children that could not sit up without assistant, or four-year-old children that did not know how to walk yet. Hunt discovered these children were beginning to feel they had no control over their environment and in turn their native intelligence was being crushed causing them to lack natural skills at certain ages. These observations motivated Hunt to begin a training program for Iranian caregivers. Hunt taught caregivers to imitate babies’ babbling, follow-the-leader games, and how to teach infants sounds from the Persian language. These small actions had an enormous impact on the children. The children being given extra care were able to name more than fifty objects by the age of twenty-two months, and charmed visitors to such an extent that most were adopted. Hunt realized that extreme conditions like malnutrition, sensory deprivation, and social isolation can severely impact an infant’s learning ability (Myers, 2016). His small contribution made an impact on children in Iranian orphanages, but this concept can be applied to a larger community.

Perhaps one of the best examples of how environmental influences can have an impact on someone’s intelligence, and therefore opportunities, is by looking at the ethnic differences in children’s intelligence test scores. Research done by Jeanne Brooks-Gunn, Pamela K. Klebanov, and Greg J. Duncan shows that black children do worse on intelligence tests than white children. Psychologists have researched many reasons to explain why this ethnic gap exists, some of the theories include a difference in family life, family income, and the education opportunities available (Brooks-Gunn, 1996:396). The main difference researchers noticed contributing to this ethnic gap is household income. Black children are three times as more likely than white children to live in families below the poverty line. Throughout a six-year period, 66.4% of black families and only 25.8% of white families are living under the poverty line at least one of those six years. For five out of that six-year period 40% of black families were living below the poverty line compared to only 5% of white families. A family’s income has a large impact on which neighborhoods they can live in, and more often than not black families are found living in poor neighborhoods regardless of whether or not they have an income below the poverty line. Poverty has been correlated with the performance of adolescents in school and the outcome of teenagers after graduation. Children who come from a poor family or neighborhood, as well as children who have parents with little education, have a higher chance of dropping out of high school. Poverty has also been associated with lower achievement in school and lower performance on intelligence tests (Brooks-Gunn, 1996:397).

In order to provide evidence that poverty is associated with lower intelligence test scores and school performance among children a study was conducted on both white and black children. An analysis done on 1,302 infants, with one-third of the group receiving intervention and two-thirds of the group received only a follow up visit. The children were assessed at ages 3 and 5 years old by two intelligence tests: The Stanford-Binet Intelligence Scale, and the Wechsler Preschool and Primary Scale of Intelligence. Variables including neighborhood conditions, family level poverty, family environment, and maternal presence, were controlled and distributed among the infants involved in the study. The results of this study showed that black children were more likely to live in poverty as well as a deprived home environment, which in turn impacted their IQ scores and intelligence levels. The results showed that the mean IQ for white children was an entire standard deviation higher than that of black children. In addition to a higher IQ, only 14% of white children were living below the poverty line, compared to 57% of the black children (Brook-Gunn, 1996:400). Since the black children were more likely to live below the poverty line, they also had less access to resources like tutors. Results suggest that a child’s home environment is one of the main factors in influencing the ethnic gap in intelligence. The results of this study show the impacts that a home environment and level of income can have on a child’s intelligence level. The ethnic gap between household income leads to a gap in IQ because as shown in the previous studies environmental factors do have an influence on intelligence levels during early childhood and early adolescence (Brooks-Gunn, 1996:404).

The impact environmental factors can have on a child’s intelligence level can lead to greater consequences when the child reaches adulthood. Scholars have examined the potential association between IQ and antisocial behavior, as well as an association between delinquency and IQ (Silver, 2018). If a child grows up in a home that deprives them of certain aspects like maternal care, then they are at risk of these aspects influencing their IQ level. If their IQ is influenced in this way, then they lose out on opportunities in life including higher paying job opportunities or chances at higher education. Therefore, it is important to look at environmental influences on IQ as a child, even though they decrease in importance as a child matures into an adult. Plomin and Sweden’s study providence evidence in their research of twins and families that genetic influences on intelligence play a more significant role in IQ. In addition, these studies show, through correlation coefficients, that the significance of environmental influence decreases with age. The debate between nature or nurture will continue to exist as new studies surface with new evidence, however, the bulk of evidence supports nature and nurture, with nature prevailing throughout someone’s entire life. Understanding the importance of this research, people can now take steps to attempt to enrich the environment to better the opportunities that people can have in life.

References

1. Brooks?Gunn, J., Klebanov, P. K., & Duncan, G. J. (1996). Ethnic Differences in Children's Intelligence Test Scores: Role of Economic Deprivation, Home Environment, and Maternal Characteristics. Child development, 67(2), 396-408.
2. McClearn, G. E., Johansson, B., Berg, S., Pedersen, N. L., Ahern, F., Petrill, S. A., & Plomin, R. (1997). Substantial Genetic Influence on Cognitive Abilities in Twins 80 or More Years Old. Science, 276(5318), 1560-1563. http://dx.doi.org/10.1126/science.276.5318.1560
3. Myers, D. G., & Dewall, C. N. (2016). Exploring Psychology in Modules (10th ed.). New York, NY: Worth.
4. Plomin, R., Fulker, D. W., Corley, R., & DeFries, J. C. (1997, November). Nature, Nurture, and Cognitive Development from 1 to 16 Years: A Parent-Offspring Adoption Study. Retrieved April 2, 2019, from https://journals.sagepub.com/doi/pdf/10.1111/j.1467-9280.1997.tb00458.x
5. Plomin, R., & Spinath, F. M. (2004, January). Intelligence: Genetics, Genes, and Genomics. Retrieved April 3, 2019, from https://psycnet.apa.org/record/2003-11198-008

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