In chapter 9, the author introduced the concept of intelligence quotient, which is defined as systematic means of quantifying differences among people in their intelligence (Page 327, Lilienfeld textbook, intelligence qoutient). The author also mentions different means of calculating IQ and the influence factors of IQ. Among these ideas, I found the genetic influence on IQ most interesting.
First of all, in the textbook the author introduces three studies to research genetic and environmental influences on IQ: family studies, adoption studies and twin studies. The family studies allow us to examine the extend to which a trait "runs" or goes in intact families, those in which all family members live together in the same home (Page 337, Lilienfeld textbook, family studies). The trait we are studying here is the intelligence. The research from the family studies shows that intellectually brilliant individuals had many first-degree relatives (parents, siblings and children) who were also brilliant, but fewer second-degree relatives (such as cousins) (Page 337, Lilienfeld textbook, family studies).
Secondly, the adoption studies is another way to research influences on IQ. Compared to the family studies, the adoption studies emphasize on the environment influence in the research. Adoption studies allow us to separate environmental from genetic effects on IQ, because adoptees are raised by parents with whom they share an environment, but not genes. It has shown a clear contribution of the environment in IQ. However, research also shows that adoptees's IQ tend to be similar to their first-degree relatives (Page 338, Lilienfeld textbook, adoption studies)
The last comes the twin studies. Compared to the family studies ad the adoption studies, the twin studies mostly focuses on the biological perspective of IQ research, but it also shows that environmental factors influence IQ. The twin studies is based on the idea that identical twins share same genes, and therefore we can research their IQ based on the assumption of same genetic structure. A research conducted by Paul Thompson at the University of California in Los Angeles shows that, in identical twins, certain brain areas related to intelligence showed a 95 to 100 per cent correlation between one twin and the other - they were essentially the same. Other researches show that the identical twins' IQ correlation have been .7 to 1.0 where fraternal twins' IQ correlation is between .3 to .4. The higher the identical than fraternal twin correlations shows IQ is influenced by genetic factors. On the other hand, the twin finds shows that identical twins' correlation for IQ is not 1.0 but a little less than that. Therefore we can conclude that environmental influences also matters on IQ. In my opinion, the twin studies focuses both biological and environmental perspectives, and it shows most results and evidences in researching of genetic influences on IQ.
In addition, while research shows that both genes and environments have influences on IQ, the correlation and causation scientific thinking principle tells us there is a possibility that neither of the two factors is the main influence on IQ. There might be a third party contribute to IQ, genes and environment. For example, the education one received can contribute to higher IQ, which will change the environment he/she is in and may also change the genes for his/her offspring.In this case, genes and environment are correlated.
IQ is inherited, suggests twin study
Twins: Nature or Nurture