Many researchers in the field of psychology have conducted experiments and observations of twins in a twin study. Twin studies compare the correlations in a trait in two types of twins: identical (monozygotic) and fraternal (dizygotic). Higher identical than fraternal twin correlations strongly suggest genetic influence. In fact, almost in most cases, studies of twins reared together have offered evidence of considerably higher identical than fraternal twin correlations for IQ. I think that this evidence is interesting, and also true. I had two sets of twins in my grade in high school. The identical pair seemed to have their intelligence and many other traits positively correlate. The fraternal ones seemed like they were very opposite than the identical pair's case. This is just supporting the fact that the traits in identical twins are more positively correlated than what fraternal twin's might be. However, I know a pair of identical twins that are completely different in every way. Obviously there are outliers in most sets of data, but this is showing the replicability of the research findings in the textbook.