Twin studies play a very important role into determining whether or not personality is genetically influenced. Identical twins are siblings who share 100 percent of genes with each other. They are more or less the best participants to prove whether or not personality is genetically influenced. Fraternal twins share 50 percent of genes with each other. They are excellent participants to compare with identical twins. Twin studies generally rely on findings of identical and fraternal twins; if identical twins have more similar behaviour than non-identical twins then this provides evidence for genetics influence on personality.
James Olson et al. (2001) looked at whether attitudes are genetically influenced by evaluating earlier studies and also carried out research on identical and fraternal twins. While traditional psychological theory claimed that attitudes are learned and mostly influenced by environment, Olson et al. and colleagues argued that biological and genetic factors also influence attitudes. They assumed that genes perhaps influence natural inclinations, then environmental factors will develop different traits and attitudes.
Another interesting study by Amy Abrahamson, Laura Baker, and Avshalom Caspi (2002) looked at the genetic influences on attitudes of teenagers. The idea of their study was to investigate causes of family influence on teenagers’ social attitudes in order to understand whether and how much families influence the attitudes of them. Abrahamson, Baker and Caspi (2002) wanted to discover the age when genetic influences come to view and to look at how much parents and siblings affect the teenagers’ views on contractive issues. Genetic influences in social attitudes were being investigated in 654 adopted and nonadopted children. From the age of twelve to fifteen, children were annually measured for their conservatism and religious attitudes. During the twelve year old visit, parents were also being measured. The study showed that shared-family environmental factors strongly influenced both conservatism and religious attitudes. Families’ similarities for conservative attitudes occur from both genetic and environmental factors, and religious attitudes are mostly influenced by family environmental factors. Compare to the findings from the twin studies before, this study by Abrahamson, Baker and Caspi (2002) suggests that until adulthood, genetic influence on social attitudes do not occur.
However, the Colorado Adoption Project study identifies that genetic influence in conservatism happens since the age of twelve, but there was no evidence on genetic influence on teenagers’ religious attitudes. Abrahamson, Baker and Caspi (2002) concluded that genetic factors influence social attitudes much earlier than they previously claimed.
Adoption studies were also done to investigate genetic influences on personality. They are important because they include biological parents and environmental parents that affect differences in behaviour and personality.
In the first adoption study on schizophrenia done by Plomin et al. (1997), the findings suggested that environment factors have very little influences on a child’s risk of getting disorders such as schizophrenia. During the study, interviews were given to both adopted children: the adopted children whose biological mothers suffered from schizophrenia and the ones whose biological parents did...