THE NELSON MANDELA RULES 27 2. There should be in connection with every prison social workers charged with the duty of maintaining and improving all desirable relations of a prisoner with his or her family and with valuable social agencies. Steps should be taken to safeguard, to the maximum extent compatible with the law and the sentence, the rights relating to civil interests, social security rights and other social benefits of prisoners. Rule 89 1. The fulfilment of these principles requires individualization of treatment and for this purpose a flexible system of classifying prisoners in groups. It is therefore desirable that such groups should be distributed in separate prisons suitable for the treatment of each group. 2. These prisons do not need to provide the same degree of security for every group. It is desirable to provide varying degrees of security according to the needs of different groups. Open prisons, by the very fact that they provide no physical security against escape but rely on the self-discipline of the inmates, provide the conditions most favourable to the rehabilitation of carefully selected prisoners. 3. It is desirable that the number of prisoners in closed prisons should not be so large that the individualization of treatment is hindered. In some countries it is considered that the population of such prisons should not exceed 500. In open prisons the population should be as small as possible. 4. On the other hand, it is undesirable to maintain prisons which are so small that proper facilities cannot be provided. Rule 90 The duty of society does not end with a prisoner’s release. There should, therefore, be governmental or private agencies capable of lending the released prisoner efficient aftercare directed towards the lessening of prejudice against him or her and towards his or her social rehabilitation. Treatment Rule 91 The treatment of persons sentenced to imprisonment or a similar measure shall have as its purpose, so far as the length of the sentence permits, to establish in them the will to lead law-abiding and self-supporting lives after their release and to fit them to do so. The treatment shall be such as will encourage their self-respect and develop their sense of responsibility.