FT Alphaville discusses Morgan Stanley's Spyros Andreopoulos on the probability of a double-dip recesion:

He sifts through all the slowdowns — defined as two successive quarters of growth not exceeding 1 per cent — recorded since 1950. There are 13 in total – and, as the charts above show, not all of them presaged a double-dip recession. Partly because, of the 13, only four occurred in a “young” expansionary period. That is, within eight quarters of a recession ending.

And of those four, only two resulted in a double-dip recession. One was 1959, in which a recession commenced three quarters after the the slowdown, and 1981, when a recession immediately followed a slowdown.

Andreopoulos writes:

But what were the catalysts?

…both because of monetary tightening: It turns out that both these recessions were precipitated by monetary policy. The 1981 recession was – deliberately – induced by the Fed in order to squeeze inflation out of the system (the recession essentially marked the beginning of the ‘Volcker disinflation’). And even the 1960/61 recession is thought by economic historians to have been caused by “the drastic tightening of money that occurred in 1959/60".

Conclusion: double-dips have only occurred upon Fed tightening: Whenever in post-war US history expansions have died young, the catalyst has been monetary policy tightening. Put differently: double-dips have occurred only when induced by the Fed.

A commenter writes:

RTRS GREEK GOVERNMENT TO BAN THE EXPORT OF TARAMASALATA AND TZATZIKI
RTRS LAST DITCH ATTEMPT TO STAVE OFF DOUBLE DIP RECESSION

On fiscal policy, Andreopoulos writes:

The outcome here is binary, with adoption of the president’s proposals bringing about 0.8% of GDP of net new stimulus; a rejection by Congress would mean expiration of these measures and bring about an automatic fiscal tightening of 1.2% of GDP. And of course the eurozone debt crisis – a fiscal problem – could yet prove a catalyst for a potentially vulnerable US economy.