To put it simply, they decide to stay informal because the total costs of entry, operation and exit associated with joining the formal sector are greater than the potential benefits.

FORBES: Informality: A Marker Of Opportunity

Many loyal Labour voters are uncomfortable with Mr Blair's move to the right, and may become crosser if co-operation with the Lib Dems becomes more formal.

ECONOMIST: The road forks

Formal adoptions require paperwork and the co-operation of sluggish bureaucrats, so they are rare: in 1996 there were only 18 in the whole of Zambia.

ECONOMIST: Orphans of the virus

One popular idea for minimising such instability is a more formal system of currency co-operation between the world's major economies.

ECONOMIST: Why non-Europeans should care about EMU

The Metropolitan Police launched a "formal criminal investigation" into Savile's alleged offences on 19 October 2012, called Operation Yewtree.

BBC: Jimmy Savile