A/HRC/4/40 page 24 58. This does not mean that any and all commitments by the receiving State to take measures to prevent a person suspected of constituting a threat to the sending State even after removal have to be rejected. It might be acceptable for a receiving State to undertake to keep a person returned to its territory under surveillance, as long as such surveillance does not amount to a deprivation of liberty without charges, is not as intrusive as to violate other fundamental rights (e.g. the right to respect for privacy and family life), and is subject to periodic review. IV. OVERVIEW OF THE PENITENTIARY SYSTEMS AND THE CONDITIONS OF DETAINEES 59. Since its establishment, the Working Group on Arbitrary Detention has tried to cooperate in avoiding detention in violation of the rights enshrined in the Universal Declaration of Human Rights and the International Covenant on Civil and Political Rights, or at least diminishing the number of persons so detained. After 15 years of efforts and noting the changes currently introduced by the Human Rights Council, the Working Group believes this is a good moment to undertake a general assessment of what it had encountered on the penitentiary systems and the condition of detainees. 60. The Working Group has so far visited several types of detention centres in 21 countries and has received a significant number of communications from different sources claiming the arbitrariness of the detention of hundreds of individuals. It has thereby been able to acquire a better knowledge of the different penitentiary systems around the world, the conditions of the detainees in prisons, particularly of the detainees in pretrial detention. Moreover, it has been made aware of the increase in the number of people in detention in the whole world, but particularly in the developed countries. Troubled by this information,14 it felt that the issue had to be tackled and evaluated in this annual report. The Working Group, aware that some aspects of the penitentiary system fall outside its mandate, is nevertheless convinced that a decrease in the prison population contributes to a better functioning of prisons and, at least indirectly, to a more effective social rehabilitation. 61. The Working Group observes that the majority of persons in detention come from a poor milieu and that a large number of them are in pretrial detention. Furthermore, their situation is often extremely precarious and they do not enjoy the guarantees established by the relevant international norms, mainly the Universal Declaration of Human Rights, the International Covenant on Civil and Political Rights and the Body of Principles for the Protection of All Persons under Any Form of Detention or Imprisonment. 62. The Working Group also notes that, in spite of the fact that many States have ratified the main international instruments relating to detention, their implementation in many countries leaves much to be desired. The mechanisms of judicial control set forth are, in many cases, just formal and do not constitute a real safeguard against arbitrary detentions. 14 According to statistics prepared by the School of Law, King’s College, London, at the end of February 2005 there were 9 million people detained around the world, the majority of whom were pretrial detainees.