Why do psychologists perfrom twin studies in orer to determine the influence of genetics on intelligence?

Psychologists perform twin studies in order to determine the extent to which genetics influence intelligence levels. Since identical twins share 100% of their genetics, it follows that their intelligence levels should be quite similar given that intelligence is genetically determined. However, when identical twins are found to have varying intelligence levels, it indicates that external factors (or nurture) play a role in intelligence. Therefore, p sychologists perform twin studies in order to determine the extent to which genetics and non-genetic factors determine intelligence levels.