(1) Francis Galton himself studied twins reared apart and came to the conclusion that heredity plays a more dominant role in determining an individual’s personality.
(2) One study of 2500 high school twins conducted in America concludes that “about half the variation among people in a broad spectrum of psychological traits is due to differences among people in genetic characteristics”, while the other half is due to environment.
(3) The most extensive twin study ever made was by the Medicogenetical Institute of Moscow which separated 1000 sets of identical twins at infancy. They were placed in controlled environments for two years of observation.
The findings supported strongly a hereditary basis for many characteristics, including intelligence differences. (This conclusion displeased Stalin who abolished the institute and executed its director).
(4) Miss Burks made a study of children in foster homes and arrived at the conclusion that the role of heredity is about 80% and that of environment 20%.
(5) H.H. Newman, a biologist; FN. Freeman, a psychologist; and K.J. Holzinger, a statistician, made a study of 19 pairs of identical twins reared apart. They compared the data concerning 50 pairs of identical twins and 52 pairs of fraternal twins reared together.
The authors concluded that physical characteristics are least affected by the environment and psychological characteristics are more subjected to environmental influence.
(6) F.N. Freeman made a study of 671 children who were placed in Chicago foster homes. His study led him to conclude that I.Q. of the children placed in homes other than their own, would increase in proportion to the quality of the foster homes. It means the level of intelligence varies with the nature of environmental influence.
(7) H.M. Skeels made a study of 150 illegitimate children at the State University of IOWA which led him to conclude that intelligence is more responsive to environmental changes.
The above-mentioned and several other studies do not tell us whether heredity or environment is the ‘more’ important factor. But they tell us why each is important. Individual differences in biological inheritance are real regardless of whether this fact makes one happy or unhappy.
For some traits, biological inheritance is more important than others. While individual differences in I.Q. are more highly determined by heredity than by environment, other trait differences are almost entirely environmental.
One recent study has revealed that certain qualities such as sociability, compulsiveness, and societal ease are said to be more influenced by heredity, while certain traits such as leadership, impulse control, attitudes and interests are believed to be more sensitive to environmental influence.
We may conclude that biological inheritance is important for some personality traits and unimportant for others. In no case the respective influence of heredity and environment is precisely measured. But most scientists agreed that the degree to which one’s inherited potentials are fully developed is determined by one’s social experience.
It is evident that those who study the influence of environment see only one side of the coin and those who study the effect of heredity see only the other side. They have failed to realise that they are inseparable. “Neither can ever be eliminated and neither can ever be isolated”.
Environment is complex and changing, heredity is not completely known. Hence we must take into account the interaction of the two factors rather than the absolute action of any one factor. Heredity is what the new life starts with, and environment is what makes its maintenance and development possible. Both are equally essential.
Personality is the product of both environment and heredity. Heredity provides the potentialities and environment brings them out into a definite form. All the inherited qualities become actualities only within and under the conditions of environment. No amount of environment can turn a mediocre person into a genius. “Heredity determines what we can do, and environment what we do”.
Heredity is potentiality made actual within an environment. Hidden potentialities are revealed when the favourable opportunities are given. Man being the final product of evolution has greater capacity of adaptability and can adjust himself with any environment. Hence heredity and environment are equally important. Each human trait requires both heredity and environment for its development.
As Lumley said, “It is not heredity or environment, but heredity and environment”. Both have been operating in determining human behaviour. As Maciver has pointed out, “Every phenomenon of life is the product of both, each is as necessary to the result as the other” No society or no organism is the product of either heredity or environment.
We may conclude that “Nature and nurture are so obviously necessary and inseparable that the important question is not which is more important but rather how together they determine our qualities.”