This principle is supported by Bouchard’s correlational twin study. The study observed two types of twins, monozygotic twins reared-apart (MZA) and monozygotic twins reared together (MZT), in order to determine the influences of genes and environments on behavior. As the word monozygotic implies, they used identical twins; in other words, each twin shared same genome. Bouchard then preceded lots of psychodynamic tests and compared the differences between MZA and MZT. Going against the fact that the MZT should be more similar than MZA if the environment is the major factor of one’s behavior, the result demonstrated that 70% of behavior was based on genetics.
The study clearly indicates that intelligence is primarily determined by genetic factors. However, Bouchard noted that traits like IQ could be enhanced through the environmental factor. He also claimed that even though family environments exert less influence, genes are not necessarily destined. Furthermore, he suggested that it is the genetic tendencies that influence the environment. For example, if a person is musically inclined, he will choose musical environment.
The major criticism is called ‘equal environment assumption’, which goes against Bouchard’s assumption that MZT is developed in identical environments. It claims that such assumption is invalid and different treatment exists. However, most accept the assumption and supports Bouchard’s idea.