There have been many debates about whether intelligence is genetic or a learned behavior. It is very difficult to locate the origin of intelligence and how it can be measured. To test whether intelligence is genetic or not, Bouchard and McGue (1981) conducted a study. They reviewed 111 studies of IQ correlations between siblings from research studies in intelligence from around the world. The researchers used identical twins that had been separated at birth for the study which means that participants had a 100% genetic relationship, but had grown up in different environments. They assumed that any similarity between the IQ’s must have been due to genetics rather than environment. They found that the closer the kinship, the higher the correlation for IQ.
Another twin study that supports the statement that intelligence is genetic is the Minnesota Twin Study (Bouchard et al. 1990). In this study, MZA’s( identical twins raised apart) and MZT’s (identical twins raised together) were compared. Each twin completed around 50 hours of testing and interviews. Bouchard found that 70% of intelligence can be attributed to genetic inheritance. This study has many strengths and weaknesses. The strengths are that there were participants from all over the world and that the mean age of participants was 41 years old. The limitations were that Bouchard relied on media coverage to recruit participants, there were some ethical concerns about the way he reunited the twins, and there was no way to establish the frequency of previous contact between the twins.
In contrast to these two studies, there are studies that show that environment has a greater influence on genetics. Scarr and Weinberg (1977) and Horn et al. (1979) studied parents who had raised both adoptive and natural children. They assumed that all the children had the same upbringing, in the same environment, with the same parents. If there were any differences in parent-child IQ correlations in would be due to genetics. In all the families that adoptive parents were white, wealthy, and middle class with high IQ’s and the adopted children were from lower-class, poor backgrounds with lower IQ parents. The researchers found no significant difference in IQ correlations. This shows that environment and the way a person is brought up has a greater influence on intelligence than genetics.
Another study that is pro nurture in the nature-nurture debate is Jahoda’s research on the self fulfilling prophecy. This prophecy states that when society gives people a label, they live up to that expectation. Jahoda studied the Ashanti people in Ghana. These people have a tradition of naming their sons according to the day of the week when they are born. They believe that the day of the week predicts a child’s future temperament. For example, boys born on Monday are supposed to be calm and peaceful, but boys born on Wednesday are supposed to be problematic and aggressive. According to local police records, there was a very low number of arrests for boys born on Monday and a high number of arrests for boys born on Wednesday. This shows that the naming of children has resulted in a self fulfilling prophecy.
Hutching and Mednick’s (1975) study of criminal behavior proved otherwise. They studied criminal activity in biological and adoptive fathers and their sons. They found that if both the adoptive and biological fathers had criminal records, 36.2% of sons also did; if only the biological father was criminally active, 21.4% of sons also had criminal records; and if only the adoptive father had a criminal record, it dropped to 11.5% of sons. When neither father was a criminal, 10.5% of sons were. This study shows the importance of genetics in combination with environmental factors in the development of criminal behavior.
According to the studies mentioned above, both genetics and environmental factors influence behavior. Heston showed that schizophrenia is mostly genetic. Bouchard and McGue and the Minnesota Twin Study found that intelligence is genetic, but Scarr and Weinberg and Horn disproved them showing that intelligence is mostly learned. Jahoda found that criminal behavior is more influence by environment, but Hutching and Mednick found that criminality was more genetic with environmental factors. Today most psychologists agree that humans may be predisposed to a certain behavior, but without the right environmental stimulators, the behavior may not develop.
