It becomes difficult to differentiate difficult to differentiate between heredity and environment when it comes to intelligence because there have been evidences like -
Hereditical evidences-
a) The generations of an high IQ person posses high IQ.
b) Siblings have high level of similarity in IQ of same family
c) An adopted child seems to be more intelligent than the parents
However for the same inferences there can be the following environmental evidence-
a) The generations of the family must have been residing at a rich environment which causes the IQ similarity
a) Sibilings might be possessing similar genetic maps and thus seems to be one having high IQ
c) An adopted child must have come from a good environment that the environment of his current parent.
All these gave a clear indication that IQ level can be very well explained by both heredity factors as well as the environmental factors an thus this similar explanation causes the confusion (also there is no strong scientific backing )