Studies of persons in settings of psychological and social isolation, such as concentration camps, psychiatric hospitals, and Antarctic stations, have reported different forms of impairment induced by monotony and other conditions of deprivation. However, only 10 studies have been conducted in prison environments. The best-known of these studies were conducted on English prisoners and suggested that while there was no decline in general intellectual capacity with increasing length of imprisonment, there was a reduction in perceptual-motor speed. The present study's subjects were all male, but varied in age, type of crime, and length of incarceration. Seven subjects were eliminated from the study because they refused to cooperate. Regression analysis showed that intellectual functioning as measured by the tests was not influenced by the length of incarceration. Thus, the statement that incarceration leads to cognitive impairment was not supported. It was concluded that incarceration is associated only with a decline in perceptual-motor speed and that incarceration results in increased verbal functioning because the prison forms a training environment for verbal skills. However, these conclusions must be viewed with caution, since it is unclear whether the recorded effects are temporary and whether the assumption that deterioration is a linear process is appropriate. Furthermore, the study ignored the influence of arousal intensity and the complexity of the tasks used in intelligence quotient tests and overlooked possible individual differences underlying the average scores. Tables and a list of 11 references are provided.