To mark the publication of “Go Figure”, a collection of The Economist’s explainers and daily charts, the editors of this blog solicited ideas on Facebook and Twitter. This week we publish five explainers suggested by our readers, who will each receive a copy of the book. 

“ACCORDING to such-and-such, a think-tank,” is a phrase familiar to readers of any newspaper, not least The Economist. Sharp quotes, intriguing facts and bold new policy proposals are attributed to the mysterious tanks (as is plenty of rubbish). What exactly are these outfits, which churn out reports on everything from Brexit to badgers?

The “think-tank” label became popular in the 1950s, by which time there were already plenty of such organisations in existence. Many of America’s most venerable tanks, including the Brookings Institution and the Carnegie Endowment for International Peace, were founded in the early 20th century. Britain’s Royal United Services Institute, a military analyst, was created in 1831 by the Duke of Wellington. But think-tanks really blossomed in the second half of the 20th century. Researchers at the University of Pennsylvania reckon there are now 6,846 of them worldwide.

Think-tanks aim to fill the gap between academia and policymaking. Academics grind out authoritative studies, but at a snail’s pace. Journalists’ first drafts of history are speedy but thin. A good think-tank helps the policymaking process by publishing reports that are as rigorous as academic research and as accessible as journalism. (Bad ones have a knack of doing just the opposite.) They flourished in the 20th century for two reasons. Governments were expanding everywhere, meaning there was lots of demand for policy expertise. And the arrival of 24-hour news created an insatiable appetite for informed interviewees. The same trends are now causing think-tanks to take off in developing countries.

Yet the world may have reached peak tank. The Pennsylvanian researchers found that in 2014 the number of new tanks declined for the first time in 30 years. One reason is that donors nowadays prefer to make project-specific grants, rather than funnelling money into mere thinking. Another is increased competition. Professional consultancies such as McKinsey publish a fair bit of brainwork, and members of opinionated “advocacy organisations” can make for more compelling interviewees than balanced think-tankers. So some tanks are rethinking themselves. The Pew Research Centre describes itself as a “fact tank”, focusing on information rather than policy recommendations. And the Sutton Trust calls itself a “do tank”, putting its own recommendations into practice.