In practice, the default assumption seems to be that a couple will file jointly for the final year of the marriage.

FORBES: Nebraska Supreme Court Rules - State Court Cannot Order Joint Federal Return

Given the hazard of joint and several liability and the fact that filing jointly is an irrevocable election, I really think the default assumption should be separate filing.

FORBES: Nebraska Supreme Court Rules - State Court Cannot Order Joint Federal Return

The assumption is that countries will default before taxing their companies and individuals to oblivion.

WSJ: HEARD ON THE STREET: Bond Myths Exposed by Greece

Typically, they fall back on the traditional assumption that anyone who buys Credit Default Swaps, stocks or bonds has an exclusively economic motive.

CENTERFORSECURITYPOLICY: Warfare's new financial face

Their assumption seems to be that a real default (and along with it a triggering of the credit default clauses in all those CDS contracts) would be a bad thing for the banks and thus a credible threat to get them to line up for the voluntary version.

FORBES: Greek Default: Don't Fear a Credit Event

You can work out default expectations by looking at the spreads on speculative bonds and making an assumption about the amount investors will recover from companies that fail to pay up in full.

ECONOMIST: Buttonwood