Genetics of Intelligence

Twin Studies: The logic is to examine subjects whose has known genetic relatedness and see if it can predicts the correlation size. If yes, the IQ difference is likely genetic. The degree of relatedness of monozygotic (identical) twins and dizygotic (fraternal) twins are 1 and .5 respectively (Western, 2002). Twin studies compare IQ of twins reared together and apart. The result is that, identical twins have greater resemblance, which continues to increase (Boomsma & Van Baal, 1998; McGue, Bouchard, Iacono, & Kykken, 1993).
Adoption Studies: Middle class families in Belgium adopted Korean children when they were around 19-month old. At 10 year-old, they were given IQ test. They had higher visio-spatial abilities yet lower verbal abilities than Belgian children. The explanation for that is, it is genetically programmed in Oriental population (Frydman & Lynn, 1989).