Inheritance and environmental factors are both responsible for the level of intelligence in a human being.
To start with, everybody has a potential intelligence range (although there has been shown to be a number of different types of intelligence, it is easier to understand this in terms of IQ) which is inherited from their parents. Say for example, little Johnny has reasonably smart parents, so his possible IQ range is from 115 to 140.
This model, known as the reaction-range model (the reaction range being the possible range of IQ of an individual) proposes that a person’s genetic makeup determines the limits on his or her IQ, and that the environmental factors determine WHERE within this range the person’s IQ will actually fall.
If little Johnny grows up in a poor area where conditions are far from good, and he receives little or no education or encouragement, then his intelligence level will be lower, somewhere down around the 115 mark. Therefore his low intelligence is due to the deprived environment in which he lives.
However, if Johnny were to grow up in a rich family who could afford him decent conditions and an impeccable education, with family support and encouragement (an enriched environment), his IQ score would be at the higher end of his intelligence range, around the 140 mark.
As a final point; an individual’s IQ CAN increase or decrease if his/her environment changes. For example, if Johnny has spent most of his life in a deprived environment and has an IQ of 115, and then he is moved to a more enriched environment where he is encouraged to learn, his intelligence WILL increase further up his intelligence range. This can also be reversed if he is moved back.
This goes to show that inheritance and environmental factors BOTH play vital roles in the development of human intelligence, although there is still some debate about the actual extent to which both factors have an effect. Despite there already being some studies conducted on twins, even more research is needed in this area, in order to find some stronger evidence.