Is Intelligence Inherited?

Is Intelligence Inherited?

There are many different arguments relating to whether intelligence is inherited or whether it is something that is developed through the environment, which is otherwise known as the ‘Nature versus Nurture’ debate. A little of this will be discussed later on, along with trying to define intelligence, heritability and how inheritance takes place. In additional to this; various studies will be discussed with regards to research into whether intelligence is in fact inherited, and what effect the environment can have on a person’s intellect.

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To be able to define intelligence, there are so many different approaches that have to be looked at such as mathematical or logic; linguistic or verbal; kinaesthetic or movement; rhythmic or musical; spatial or visual; inter and intrapersonal. Which is why Sternberg and Grigorenko (1997) comment that there is no definitive description to what intelligence is. Whereas Piaget (1950) stated that “intelligence constitutes the state of equilibrium towards which tend all the successive adaptations of a sensori-motor and cognitive nature, as well as all assimilatory and accommodatory interactions between the organism and the environment” (pg 11).

Intelligence is indeed affected by the environment as stated by Piaget, but heritability also has an effect on intelligence and the environment also has an effect on heritability. Sternberg, Grigorenko and Kidd (2005) have suggested that heritability does not illustrate what percentage of the characteristic is related to heritability, whereas is does demonstrate that the characteristic is attributable to genetic variation within the population. Furthermore they go on to say that because trait variation is also known as phenotypic variation and genetic variation is also known as genotypic variation and because of this they conclude by defining heritability as “a ratio of variation in the phenotype being considered due to relevant genetic variation to phenotype variation” (p53).

Many people will think of inheritance and automatically think that it is something that is bestowed to someone in a will, which it is but in this case it is the transmission of genes from a mother and father to their son or daughter. This means that the child is given half of the mother’s genes and half of the father’s genes.

The genes that are inherited from a person’s parents are known as the phenotype, these usually include characteristics which are passed from one generation to another, which include physical characteristics such as ear shape, eye colour, and nose size and physically attractiveness, other characteristics can be intelligence and behaviour but these characteristics can be influenced by environmental factors, whereas the previous ones cannot be.

There have been quite a number of research studies in to intelligence and whether it is inherited or not. One of the first to do this was Sir Francis Galton, who was the first psychologist to see a relationship between nature and nurture, where he believed that intelligence was something that was actually passed from generation to generation but he also thought that there was another influencing factor; namely the environment. (Maltby, Day and Macaskill, 2007)

Due to this discovery the study of identical twins began to help develop research in to heritability. These studies were conducted around identical twins that were brought up together in shared environment and identical twins that were brought up apart in a non shared environment.

A shared environment is where the twins are living together with their biological parents, attend the same school, and have the same friends, where in contrast a non shared environment is where the twins have been separated, with one twin live away with adoptive parents, possibly in a different area of the country, attend different school and they would defiantly have different friends. By conducting the research in this way it is easy to see if there is any correlation between the groups of twins.

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One of the most famous studies that were carried out on twins and triplet that were reared apart was the work done by Dr Thomas Bouchard Jr in 1979. This was a longitudinal study that went on for a period of approximately 10 years. This study was looked in the identical and non identical twins and triplets and compared their IQ scores, to see if genetics or the environment is the reasons behind the differences. The study was carried out twins and triplets because there share their DNA (Deoxyribonucleic Acid).

The results that were obtained from this research was that 70% of the difference in the IQ scores was a result of inheritance, unlike previous studies that have show that only 50% of the difference is due to inheritance (Bouchard Jr, 1990). Previous studies such as these where carried out by Plomin (1998) where here studies several hundred pair of Swedish twins who were reared apart found that half of the variance in the test scored was down to heredity, which was consistent with other studies.

The study carried out Bouchard was only carried out with a small sample size of around 100 twins and triplets. If the research was to be carried out with a large sample size and included other relatives of the twins and triplets then maybe the result of the variance may decrease, and therefore showing that the effect of genetics and the environment is a 50/50 split.

There needs to be lots more research into the area of intelligence and influencing factors, such as the effects of child poverty and social exclusion, maternal factors such as alcohol, smoking and drugs use, biological factors such as race, age and gender, and geological factors. Sternberg (1997) has stated that hereditability estimates vary depending on the type of test used when the study is being carried out, as well as the population, country and ethnic group.

To conclude intelligence is in fact inherited, but the other factors that have to also be included are the environmental factors. For example if a child is born intelligent, if the influencing and nurturing family environment is not available the child will not know how to channel the intelligence that they have. For example Bradley, et al (1989) stated that “Measures of specific aspects of the child’s home environment, such as parental responsibility and availability of stimulating play materials, were more strongly related to child development”.