However, the twin and family studies have been criticized for their reliance on a number of assumptions that are difficult or impossible to verify, such as the equal environments assumption ( that the environments of monozygotic and dizygotic twins are equally similar ), that there is no misclassification of zygosity ( mistaking identical for fraternal & vice versa ), that twins are unrepresentative of the general population, and that there is no assortative mating.
42.
From the late 1970s, it was generally accepted that " he had fabricated some of the data, though some of his earlier work remained unaffected by this revelation . " The possibility of fabrication was first brought to the attention of the scientific community when Kamin noticed that Burt's correlation coefficients of dizygotic twins'IQ scores were the same to three decimal places, across articles even when new data were twice added to the sample of twins.
43.
One way to consider it is using trait X in twin 1 to predict trait Y in twin 2 for monozygotic and dizygotic twins ( ie using twin 1's IQ to predict twin 2's brain volume ); if this cross-correlation is larger for the more genetically-similar monozygotic twins than for the dizygotic twins, the similarity indicates that the traits are not genetically independent and there is some common genetics influencing both IQ and brain volume . ( Statistical power can be boosted by using siblings as well .)
44.
One way to consider it is using trait X in twin 1 to predict trait Y in twin 2 for monozygotic and dizygotic twins ( ie using twin 1's IQ to predict twin 2's brain volume ); if this cross-correlation is larger for the more genetically-similar monozygotic twins than for the dizygotic twins, the similarity indicates that the traits are not genetically independent and there is some common genetics influencing both IQ and brain volume . ( Statistical power can be boosted by using siblings as well .)
