Individuals differ as much in mental features as they do in physical aspects. In fact, no two individuals are alike physically or mentally in the world. This difference is due to heredity as well as environmental influences.
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Influence of Heredity
Let us take heredity first of all. As individuals differ in intelligence, therefore, their offspring are not expected to be similar. If both the parents are very superior in intelligence, their children are expected to be intelligent and similarly mentally weak parents beget dull children.
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If one of the parents is superior in intelligence and the other one is dull, the young ones will be mixed generation, some of them intelligent, others dull, still others in between and most of them having reverse influences in themselves in latent forms apt to influence the coming generations only.
Due to this fact we come across some intelligent children whose parents both mother and father are dull. The heredity influence may be coming down from distant forefathers either from father’s side or mothers. Ordinarily we do not find dull children with parents who are both superior in intelligence, and a genius born in the house of dullards.
Influence of Environment
Environment, too, affects intelligence tests although to a very limited extent. Injury to brain, constant bad health, long and continued emotional disturbances either impair intelligence or make it ineffective in life.