The theory that there is one, over-all general kind of intelligence is not scientifically demonstrable; the profusion of intelligence testing and the assignment to an individual for most of his or her student life of an arbitrary intelligence quotient rating may be an infringement of civil liberties. Such ratings may be used to unfairly screen out students for educational programmes, or applicants for job or promotional opportunities. The tests themselves are often biased towards privileged backgrounds and the dominant ethnic culture. Thus gifted but disadvantaged individuals' scores will not reflect their capabilities.
Aptitude, intelligence and general ability testing are inseparable phenomena illustrating the belief that human nature and human potential are fully understood by scientists; that they can be classified in 7, 13, 22, or 66 categories, and individuals pigeon-holed according to their predominant top scores. These premises underlie a large amount of educational thinking that seeks an assembly-line efficiency by putting students on the supposed right track in accordance with their abilities at the earliest age. Such educational perspectives accord very well with state politics, centrally-controlled social behaviour, and 'scientific' human breeding programmes. Intelligence testing has been turned into a tool for role modification if not for behaviour modification, and is one of the growing techniques for the factory-farming of human beings.
Identical twins separated at or soon after birth have IQs which correlate higher than either the IQs of ordinary siblings reared apart or the IQs of fraternal twins reared together. This suggests that, according to some studies, genetic factors account for nearly 60% of the variability in IQs. Most experts in the field, while agreeing that there is no adequate understanding of the nature of intelligence, believe IQ tests measure something that can be reasonably called general intelligence. IQ tests also predict reasonably well the academic and job performance of both blacks and whites suggesting that they are not culturally biased.