Huw Richards reports from the American Political Studies Association's conference in Chicago.

"Who's next?" asked satirical songwriter and Harvard mathematician Tom Lehrer, as the small group of nuclear nations grew in the early 1960s.

Jim Walsh of the Massachusetts Institute of Technology, just down the Boston subway from Harvard, says this question has preoccupied analysts ever since - and that they have consistently over-estimated the growth of the nuclear club. President Kennedy reckoned there might be 21 nuclear states by 1973.

Professor Walsh, who emphasised that his Chicago paper represented work in progress rather than polished conclusion, notes that researchers have tended to concentrate on the small group of states that did develop nuclear capacity. But "the biggest puzzle of the nuclear age may be that there are so few nuclear states".

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Drawing on his own examination of Egyptian nuclear decision-making, he challenges two of the commonest explanations for proliferation: motivation, that is, feeling threatened, anticipating a future threat or wishing to threaten others; and capability - the technological, scientific and economic ability to develop weapons.

Other theories suggest that aspirant states are thwarted by the actions of other states through measures such as export controls, by fears that the development of weapons would place them at risk or through internal factors such as public opinion or the desire for economic development.

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He argues that these are at best a partial explanation of Egypt's failure to develop its own nuclear weapons, and in particular its lack of progress between 1960 and 1967 when, he argues "an unusual conjunction of enabling conditions" existed.

Israel's nuclear programme coincided with economic growth in Egypt and a favourable international attitude to technology transfer.

But progress was limited. He suggests that reasons were partly institutional, with President Nasser pursuing several goals and overstretching national diplomatic resources. There were also divisions within Egypt's development programme.