Why are identical twins useful in determining the impact of nature vs nurture on intelligence?

Research Paper (postgraduate), 2017

6 Pages, Grade: 1

Introduction

In an attempt to determine the factors that influence intelligence in humans, psychologists have investigated the issue from two principal perspectives: nature and nurture. As a result, research studies on these aspects have confirmed that both nature and nurture influences the development of intelligence. According to Heffner (2002), genetic factors have been identified as the influential forces of nature that shape intelligence, whereas environmental factors influence intelligence through exposure or rather nurture. In both schools of thought, it is apparent that tests for intelligence are aimed at determining the level of intelligence through the use of Intelligence Quotient (IQ) test. Similarities and differences in IQ among individuals explain the degree at which genetics and environment influences development of intelligence. Therefore, this research paper discusses nature-nurture basis of intelligence.

Roles of Genetics in Determining Intelligence

According to reviews, heredity has been found to be one of the most influential factors determining the development of intelligence in humans. Deary, Spinath and Bates (2006) presents a summative study on reviews on the role of genetics on intelligence. From this study, it is reported that approaches for testing human intelligences have not changed significantly over the past 100 years. Some of the most principal cognitive domains that have been identified by researchers in determining the influence of genetics on intelligence include processing speed, verbal comprehension, working memory, and perceptual organization (Deary, Spinath & Bates, 2006).

Despite the divergent perspectives presented by psychologists and biologists regarding the influence of genetics on intelligence, Plomin & Spinath (2004) reaffirms that heredity plays a key role in the development of intelligence. He states that genetics studies have defined clear boundaries between nature and nurture, and provided the magnitude of influence caused by genetic factors on the development of intelligence.

According to research studies, the role of genetics in determining intelligence is explained by hereditary similarities between fraternal (dizygotic twins) and identical (monozygotic) twins. Devitt & Ormrod (2007) report that identical twins exhibit similar IQ, even when they are raised in different environments. This implies that genetic factors act as the principal force for the development of intelligence in identical twins. On the other hand, fraternal twins share similarities in IQ. As such, it is apparent that genetic factors determine their intelligence (Devitt & Ormrod, 2007). Moreover, adoption studies indicate that adopted children express intelligence related to their biological parents.

Genomic studies show that identical twins arise from a single zygote which splits into two identical products. Therefore, identical twins share similar genetic components. This explains why identical twins exhibit similarities in IQ. In IQ test, identical twins have been found to produce similar IQ scores, even when they are raised in different environments (Devitt & Ormrod, 2007).

Moreover, intelligence has been found to run along family lines. Studies demonstrate that children born of parents with high IQ are likely to exhibit high IQ. This implies that genes concerned with intelligence are passed over from parents to their children. This has been justified through allelic studies which reaffirm that children possess hereditary genes from their parents. It is also reported that children show similarity with their parents in brain structure. According to Thompson et al (2001), who carried out magnetic resonance imaging in in vivo studies of brain structure, brain volume serves as one of the heritable element involved in the development of intelligence. In addition, genes concerned with cognitive abilities have been found to be heritable. For instance, Winterer & Goldman (2003) states that chromosome 22 of the human genome bears a heritable gene coding for catechol-O-methyltransferase, which is expressed more similar in people who are genetically linked such as relatives than those from different populations (Egan et al., 2001). These studies provide evidence that heredity plays significant roles in determining the development of intelligence (Deary, Spinath & Bates, 2006). Ordinarily, children bear genetic components of the parents because they are products of sexual reproduction which involves the fusion of both male and female gametes through a well understood biological mechanism. Therefore, genes linked to intelligence are inherited by children from their parents the same way as the other gene segments, and they are expressed in equal magnitudes as those concerned with other traits such as height, pigmentation and gender.

Roles of Environment in Determining Intelligence

Environmental factors have also been found to influence intelligence. Heffner (2002) reports that environmental factors account for the differences observed in children’s cognitive development. Devitt & Ormrod (2007) highlights some the environmental factors that influence intelligence as home environment, nutrition, toxic substances, and formal schooling. Home environment is considered influential in determining intelligence. For instance, socioeconomic status of the family in which a child is raised influences their IQ scores. Wahlsten (1995) identified those children who are brought up in homes with a high socioeconomic status record high IQ scores than those raised in low income homes. In general, the environment in which a child is raised influences his intelligence. Evidence for this phenomenon can be provided by twins and adoption studies. Devitt & Ormrod (2007) presents IQ correlations between monozygotic (identical) twins and adopted children in two different environments to explain how environmental factors influence intelligence. It is reported that IQ correlation for identical twins raised together is .86 compared to IQ correlation of .72 recorded in twins raised in separate homes. Similarly, adoption studies indicate that adopted children record a higher IQ correlation than non-adopted children (Devitt & Ormrod, 2007). Therefore, it is apparent that intelligence is not influenced entirely by genetic factors, but environmental factors also play significant roles.

On the other hand, nutrition has been identified to play significant roles in the development of intelligence. According to Heffner (2002), children who consume proper nutrition exhibit high IQ scores than malnourished children. It is also observed that toxic substances in the environment influence the development of intelligence. Evidence for this phenomenon is provided by the effect of fetal alcohol syndrome on neurological development of children. This syndrome has been found to cause delayed language development, reduced motor coordination and mental retardation. As such, it is apparent that toxic substances influence intelligence (Devitt & Ormrod, 2007).

Another significant environmental factor that influences intelligence is formal schooling. It is reported that school attendance promotes intellectual growth through the acquisition of cognitive processes such as metacognition, rehearsal and organization, and this aspect is supported by Vygotskian theory. Research studies indicate that IQ for school dropouts decline year-by-year for every year school time missed (Devitt & Ormrod, 2007).

Roles of both Genetics and Environment

Currently, there is consensus that nature (genetics) and nurture (environment) influences intelligence. However, IQ correlation studies have not yet enabled psychologists to ascertain the degree at which each parameter influences intelligence. It is reported that none of the two factors contributes to the development of intelligence in entirety; both heredity and environment are believed to interact to produce intelligence (Devitt & Ormrod, 2007).

During intellectual development, genetic factors are believed to determine the susceptibility of an individual to environmental influences. As such, heredity creates an array of abilities whose expression in the form of IQ are shaped by environmental factors. This is phenomenon can be explained the Skinner model of learning; the operant conditioning which relies on enforcers. Therefore, it is apparent that environment plays a significant role in determining hereditary traits related to the development of intelligence. Research shows that intelligence is a product of heredity-environment interactions; thus, inherited abilities interact with environmental factors. In nature, some children who possess particular inherited traits are suited to particular environmental conditions. This phenomenon has been described as ‘niche-picking’ (Rodgers, 2014).

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