Genetic and Environmental Influences on Individual Intelligence

Identical twins are many times seen as genetic reproductions representing variations from twin to the other twin and are created because of environmental factors (Cervone & Pervin, 2010). Since identical twins share identical genetics, his or her personality seems to also share similarities. Moreover, the environment can impact fraternal twins, resulting in fraternal twins personalities and his or her characteristics similar to each other. For example, identical twins. Some research shows twins that are raised in the same residence have little differences from those who are raised in a different home.

Many psychologists agree that nurture play a greater part in the development of a human being. Studies such as those performed on monozygotic twins (identical), who share the same genetics and those performed

Table 3-1 examines the correlations between selected characteristics for the identical twins separated at birth and the twins raised together. When compared to each other, they both have very similar brain wave activity, blood pressure and heart rate in the physiological characteristics. The Raven intelligence test was very similar, but the WAIS IQ displayed a bit of difference when compared to the other comparisons made. Personality wise, they typically had similar personalities at the same rate as twins raised together. Psychological interests were off a little bit, but still very similar. The twins that were separated at birth had less similar psychological interests compared to the twins

Most scientists agree that genes have some influence over general intelligence and special aptitudes in such activities as athletics, mathematics, music, and science. But genes are not the only factor involved in producing these characteristics.

Twin studies are a mainstay of the nature verse nurture debate because they allow researchers to conduct “natural experiments” on human beings. When researchers try to determine the relative importance of genetics and environment with regard to a certain condition, they have a hard time because they cannot generally get people with the same genes. This is where twin studies come in. Twins studies are part of the method used in behaviour genetics, which includes all data that are genetically informative. E.g. siblings, adoptees, pedigree data etc. Researchers use this method to estimate the heritability of traits and to quantify the effect of a person 's shared environment (family) and unique environment (the individual events that shape a life) on a trait.

Identical twins may share the same DNA make up and grow up in the same household, yet they have very distinct personalities. On the other hand, twins that may have been separated at birth, find that years later when they meet up for the first time that they have very similar character traits and experiences. Researchers have been trying to figure out for years if DNA or environment has a bigger factor in determining a twin’s personality. When researchers try to determine how personalities are developed, they often look to twin studies to determine if DNA or life experiences are influential in personality development.

In reading stories about identical twins my point of view is easily understood. Various types of psychologist have analyzed the study of identical twins. It shows that individuals with the same genetic make-up can be totally different, when it comes to personality. Nurture promotes the twins to grow up as separate individuals. Its a lot better if a twin has his or her own personality rather than the same exact personality of the other half. I think this great debate weighs heavier on the nurture side. As a child my parents, and sibling overshadowed my genetic make-up. No matter how much I thought my physical traits made me behave. It did not affect me more than the things that I had seen and been exposed to. The more things that I had seen around and experienced made me the person that I am today. If my genetic make-up was totally different, to a certain extent, I am sure that I?d behave the same way that I do presently. There are exceptions to that. For instance if I were six foot five and one hundred and ninety pounds, I?d most likely be in the NBA and be really rich. I think if I was really rich and in the NBA I wouldn?t have the same worries that I have today. Even if before I were in the NBA I would have lots of attention and have a lot more confidence. So to an extent I would change a little if my genetic make-up were different. In a sense, genetic make-up is just a competitor to the environmental factors

Charles Spearman’s theory (1904) also takes the psychometric approach that there is a general intelligence. Spearman maintains that intelligence is hereditary and an individual is born with their maximum mental ability. This suggests that intelligence cannot be changed or strengthened. The concept of ‘general intelligence’ suggests that an individual has an underlying intelligence, in which their performance in one type of cognitive task is often similar to their performance in another (Boundless 2013). Spearman demonstrated this through the correlations between tests, where individuals who performed well in one test, often performed well in others. This led Spearman to the conclusion that intelligence is defined as a single factor. However, critics refute Spearman’s conclusions, as they argue that one cannot reduce all factors of intelligence to an indefinable “g” (Neisser et al. 1996).

Using the information provided by Belmont and Marolla, Zajonc and Markus proposed the theory that as family size increases, the average intellectual climate of the family decreases. Zajonc and Markus put the data from the Belmont and Marolla study into a table that provided information about the number of children and the average intellectual score for the children. They noticed that in larger families there is somewhat of a leveling off or even an increase in scores for the extreme later-born children. The reason for this increase is because the other siblings start to leave the family and contribute towards the average intellectual climate. Zajonc and Markus also found that twins and triplets performed lower on tests of intelligence than non-twin siblings. Zajonc and Markus didn't try to claim that the only

His findings have concluded that environmental factors explains 58 percent of IQ variations around the world. If IQ tests were able to compare different backgrounds of different races, it would be more apparent. An American clinical psychologist Terrie Moffitt, said that negative environments (low social status or ineffective parenting) could affect a child 's intelligence even more (Parker/Morton 1). There is a way to make a child smarter and that starts from an upbringing environment that is positive (Hurley). While bringing in the genetics of an individual based on their skin color, there are also different environmental factors that can contribute to the correlation between race and intelligence.

Researchers suggested that the “admixture hypothesis” is responsible for the connections between birth order and intelligence (Page & Grandon, 1979). Parts of the hypothesis argue that parental IQ and social status are responsible for large families and low IQ, which would make it seem that high birth order causes lower IQ. It would appear that parents with low IQs would have more children, but it this was proven to be true, the IQ scores would decrease given the population size.

Identical twin studies are most useful in determining genetic influence on intelligence. Since the identical twins share the same exact genes, the straightforward comparison of intelligence can display if this human trait is truly heritable. Similarity of intelligence scores between identical twins supports the genetic influence on intelligence. According to Myers (2007),

While this assumption is accepted by a majority of geneticists and behavioral scientists, there is great disagreement on the degree of influence each contributes. Arguments for environmental influences are compelling; at the same time there is growing evidence that genetic influence on intelligence is significant and substantial (Eyesenck, 1998; Mackintosh, 1998; Plomin, 1994; Steen, 1996). The purpose of this paper is to explore the question: "How is intelligence influenced by heredity and environment?"

Intelligence is used by everyone to help them in everyday life. Where people get their intelligence, however, is debatable. A person is born with an amount of inherited intelligence; this intelligence is considered nature, genetic, or heredity. Then there is intelligence that comes from the ability to learn from experience; this intelligence is impacted by nurture or the environment. It is known that both genetic and environmental factors determine intelligence, but the subject is controversial among professionals. Does nature or nurture have a bigger impact on intelligence? Is it true that some people are born with high intelligence, but are limited by their environment? Given the proper tools, can a person increase their intelligence? How accurate are IQ tests?

From these studies, researchers have found that twins who have grown up in the same household share similarities in the way they think, act, dress, speak, etcetera. However, with the first type of twin studies, researchers could not differentiate whether the similarities were due to them having identical/similar genetic makeups or growing up in the same environments. Therefore, in order for psychologists to know whether similarities in twins are genetic or socially influenced, they studied twins who grew up in completely different households and environments. The study showed that despite growing up with a different family and in a different environment, they still had noticeable similarities in fashion, interests, and thought processes (K.W. Brown, personal communication, August 29, 2017). Twin studies, as well as other studies, have greatly contributed to our understanding of overall role that genes play in the development of psychological traits and behavior.