In theory, groups of countries could pool reserves, so that a smaller amount would suffice than if countries each maintain their own buffers.

ECONOMIST: Reserve currencies

Advocates of this theory argue that both groups were the original inhabitants of Europe before migratory waves of Indo-Europeans came from the east.

BBC: The hard work begins

That's the theory of the poet who takes groups of men into the woods to bang drums and helps us get in touch with our proud, warlike selves.

NPR: Excerpt: 'The Practice of Deceit'

This guide and the related website are intended as a tool kit to help teachers and trainers, NGOs and youth groups to help raise awareness of Sustainable Consumption and empower youth to put theory into practice.

UNESCO: UK National Commission for UNESCO :: Education materials from UNESCO and partner organisations

Mr Davies said it was part of what is known as "nudge theory", which is based on a belief that groups and individuals can be influenced by positive reinforcement and suggestion.

BBC: Footprint art on pavements to tackle litter problem