Identical twins (monozygotic twins) develop from a single fertilized egg that splits into two embryos. Therefore the twins share 100% of their alleles, meaning that they are genetically identical. Consequently, their fluid intelligence (brain structure and neural efficiency) will be similar. Any difference in their IQs will be a result of different environmental effects that affect the crystallized component (knowledge) of their intelligence. Since IQ measures the combined fluid and crystallized components of intelligence, identical twins are likely to have IQs that are closer than the IQs of fraternal twins (dizygotic twins) and much closer than the IQs of two randomly chosen people.
Bouchard et al. in their studies of identical twins raised apart reported, “about 70% of the variance in IQ [in the general population] was found to be associated with genetic variation.” At least five subsequent studies involving samples of identical twins in the United States and Europe, who were reared apart, yielded heritability’s of IQ between 0.68 and 0.71, confirming the results of Bouchard and his team.
This means that identical twins will have IQs that are very close unless their environments, including education, nutrition, sleep and intellectual stimulation have been dramatically different.
