A number of studies have been conducted regarding the influence of heredity. The studies are either case studies or genetic studies or experimental studies.
1. Everyday Experience:
We have experience of observing that like begets like, that the children of intelligent and cultured people tend to grow bright from the very beginning.
Great men had great mother or great father. There are individual variations in the achievement of children brought up in the same environment. Such a difference must be largely the result of heredity or native difference in endowment.
2. Biographies of great men:
Great men were exceptionally bright from the very beginning:
(i) Goethe, at an early age, had done extensive reading in 4 languages viz. German, French, Latin and Hebrew. He also played piano and flute. He knew the history of chief European countries in detail.
(ii) Sir Francis Galton could read when two and half year old. Darwin, his cousin was a similar genius.
(iii) Voltaire began to read at three. At twelve he wrote a tragedy.
(iv) Macaulay could read at four. At eight, he wrote a treatise to convert the natives of Malabar to Christianity.
(v) Sir Issac Newton, in his childhood could devise many types of machines, especially water- clocks and kites.
(vi) Sir Walter Scot and John Stuart Mill were child prodigies.
(vii) Shankaracharya, the great Indian philosopher had a complete mastery over the whole of Sanskrit literature at the age of 16. At that age he wrote his famous treatise on Vedanta.
(viii) Swami Rama Tirtha, Lala Hardayal, Ramanujam (the mathematician), Vivekananda, and many other Indian genii vouchsafe the fact that they had some inner potentiality.
3. Studies of Family Histories:
Some psychologists have studied different generations of some families to find the connection between the traits of the individual with the family.
(i) Galton Francis, in 1 869, prepared a list of 977 famous individuals, their nearest blood relations out of whom 536 were famous. Hence, there was high correlation. In another list 977 common individuals he found only 4 near relatives who were famous.
(ii) Dr. A.E. Winship, studied some Edward family. The children of Edward born of a talented lady, and their successive generations came out to be famous. But the children of Edward born of another ordinary lady came out to be ordinary.
(iii) Karl Pearson studied the family of Galton, Darwin and Bezwood (all cousins). For five successive generations, the members of this family had a place of the ‘Royal Society’ of England.
(iv) Dugdall and Istabruke studied the Juke family. Juke was a corrupt man and he married a corrupt woman. Out of the 5 generations of this family, about 1000 persons were born. Out of these 300 died in infancy, 310 were in orphanages, 440 were chronic patients, 130 were criminals, and only 20 could learn some vocation.
(v) H.H. Goddard studied Kalikakfamily and its five generations. During the American Revolution Martin Kalikak (the name is fictitious) had an illegitimate son by a girl known to be mentally defective. From his son have come 480 descendants. Of these 143 were feeble-minded, 46 normal, and the rest abnormal. Among the total lot, 36 were illegitimate, 33 were sexually immoral (mostly prostitutes), 24 were drunkards, 3 were epileptics, 82 died in infancy, 3 were criminals, and 8 were ill-famed.
After the American Revolution, Martin married a normal woman. Goddard traces 496 descendants of this union, all of whom were normal mentally and morally. Several were superior too. In this group, we find lawyers, professors, doctors etc. The two lives of generations thus differed sharply.
4. Study of Siblings:
(i) Leathly in one of the best controlled study of adopted children found a considerable high relationship between the intelligence of children and their own parents, than between adopted children and their foster parents.
(ii) Pearson found that the coefficient of correlation between 2000 pairs of siblings as regards their personality traits was 50.
5. Study of Twins:
Twins are of two types- identical and fraternal. The twins that are developed from one single fertilized ovum are identical. Thus their genetic elements are the same. On numerous studies of identical twins, it has been found that identical twins, even though reared part had the same I.Q., with only a slight difference (maximum 5). Their correlation of traits was 9.
As regards fraternal twins, the correlation was .63 to .7, and the I.Q. differences were in no case more than 9.