Genetics are known to play a role in how intelligent people can be but there are still other factors that should not be forgotten. Good study habits can be developed by people and this can make them more intelligent than their peers. There are also some people who are known to be exposed to different things in the environment that will affect the way that they think and perceive things.
The more skills that people have, the more that they will be perceived as intelligent. For example, there are some people who are observant. This will allow them to gain enough information about the things around them. This can lead to being more “intelligent” than others.
The two main factors affecting intelligence are the genes you are born with, and the opportunity for developing this potential. Without sufficient nutrition the brain, like other organs, will suffer. Similarly, without access to mental stimulation and challenge - basically the opportunity to learn - the potential for intelligent behaviour is limited to a degree. For instance, the recent case of the Californian youngsters starved, tied up and psychologically abused will have suffered detriment to their intelligence as well as to their health.
Added to this, the parents' failure to educate them has severely limited their opportunity to learn. It will be interesting, in future years, to compare the outcome of the oldest two children, the second of whom is the only one who ever went to school, whereas the oldest is the one who suffered the longest period of deprivation, physical and mental. Such cases are rare: most people in the western world have books, sufficient food and access to various modes of learning (Films, videos, internet) for their potential intelligence to be realised.