According to plomin and daniels, "two children in the same family are (apart from their shared genes) as different from as are pairs of children selected randomly from the population."

In their book "Interaction between temperament and environment" behavior geneticists Robert Plomin and Denise Daniels state, "Two children in the same family are [apart from their shared genes] as different from one another as are pairs of children selected randomly from the population." There may be many reasons children in the same family are so different from one another but their finding boils down to the environment. To a large extent the environment influences personality, psychopathology, and cognitive abilities. Environmental differences between children in the same family represent the major source of character and environmental variance.