"The International Security Advisory Board (ISAB) was asked to judge the likelihood of a 'cascade' of new nuclear weapons states, what factors might be supporting a possible upcoming cascade, and what the U.S. can do to prevent such a cascade. The ISAB's findings are as follows. There are many factors that have led to concerns about a rapid rise in the number of nuclear-weapons states. Among these factors are:  The nuclear weapons programs of Iran and North Korea;  The expected rapid rise in nuclear power reactors;  The aging of the NPT and demonstrated lack of international will to respond rapidly and effectively to evidence of new nuclear weapons programs;  Regional tensions, especially in the Middle East and East Asia;  Distributed networks and suppliers groups that make detection of rogue programs more difficult;  Expected continuing rise in international terrorism coupled with worries of 'loose nukes'. The ISAB's research and discussions with experts, both foreign and domestic, has concluded that a nuclear weapons cascade is not inevitable. Unlike past years, a nuclear cascade now is more likely to begin in advanced industrial states, not rogue states. A region-by-region look at incentives and disincentives to initiate a nuclear weapons program shows powerful political, economic, and national security reasons for not initiating a regional arms race. While the probability of a cascade."
United States Department of State, http://www.state.gov