Brother and sister dating another brother and sister
Segal (1997) was interested in siblings of no biological relations.He found that intellect and behaviour is associated with rearing situations.
They also gathered twins who were reared together: 160 pairs of identical twins, and 212 pairs of fraternal twins.
Theoretically, there is a chance that they might not share genes.
This is very rare and is due to there being a smaller possibility of inheriting the same chromosomes from the shared parent.
They knew that past studies suggested that genetics played a larger role in one's personality in the earlier years of their life.
However, they were curious about whether or not this was true later on in life.