If parents matter, why is it that two adopted children, reared in the same home, are no more similar in personality than two adopted children reared in separate homes?

• 如果孩子的性格与父母相关的话,为什么同是收养的两个孩子,在一个家庭中长大的,在性格方面的共同点并不比在两个不同家庭收养的孩子要多呢?