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.