The complex causes of gun crime "can't be an excuse for inaction", he said.

BBC: Newtown shootings: Obama says tragedies must end

He told the BBC News website the relationship between hCG and production of the muscle relaxant was still not completely clear, and the causes of prematurity were complex.

BBC: NEWS | Health | Drug may help avoid prematurity

America's rising income inequality (which has far more complex causes than the tax system) means that the richest Americans have been getting relatively richer and hence should be paying more in taxes, quite independently of tax cuts.

ECONOMIST: Soaking the rich | The

We will be told that the causes of such violence are complex, and that is true.

WHITEHOUSE: The White House

Although the causes of obesity are undoubtedly complex, only a limited number of cases are primarily due to genetics.

FORBES: Obesity: The New 'Just Say No' For 2013

There's complex causes for violence throughout the country.

NPR: Afghanistan Five Years After Sept. 11

Existing vaccines work against viruses and bacteria, but the parasite that causes malaria is a more complex type of organism.

ECONOMIST: Malaria research gets a shot in the arm

But the rise of single-parent families in itself has complex social causes, and it will not be reversed by politicians.

ECONOMIST: The poor are still there, and need helping

Its causes were manifold and complex, but we know that somehow the wrong decisions were made, and the wrong risks were taken, and they weren't stopped.

FORBES