But merger waves also tend to coincide with periods when share prices are high and, arguably, over-valued.

ECONOMIST: Hostile bids are back again. Who should rejoice?

Some would say that bubbles tend to coincide with periods of great economic change, such as the development of the railways or the internet.

ECONOMIST: Buttonwood

On the other hand, bear markets also tend to coincide with rising interest rates whereas now, for all his tough talk, Alan Greenspan, chairman of the Federal Reserve, shows little inclination to put rates up.

ECONOMIST: A wobble down Wall Street

The study, published in 2009 in one of the journals of the Public Library of Science, PLoS Computational Biology, also showed that patients who developed diseases that tend to coincide with many others were more likely to die sooner than people whose diseases were more tangentially connected.

WSJ: Genes May Link Disparate Diseases