WASHINGTON — Few Americans question that smoking causes cancer. But they express bigger doubts as concepts that scientists consider to be truths, an Associated Press-GfK poll found.

Americans have more skepticism than confidence in global warming, the age of the Earth and evolution and have the most trouble believing a Big Bang created the universe.

The survey asked people to rate their confidence in several statements about science and medicine.

On some, there’s broad acceptance. Just 4 percent doubt that smoking causes cancer, 6 percent question whether mental illness is a medical condition that affects the brain and 8 percent are skeptical there’s a genetic code inside our cells. More — 15 percent — have doubts about the safety and efficacy of childhood vaccines.

About 4 in 10 say they are not too confident or outright disbelieve that the earth is warming, mostly a result of man-made heat-trapping gases, that the Earth is 4.5 billion years old or that life on Earth evolved through a process of natural selection, though most were at least somewhat confident in each of those concepts. But a narrow majority — 51 percent — questions the Big Bang theory.

“Science ignorance is pervasive in our society, and these attitudes are reinforced when some of our leaders are openly antagonistic to established facts,” said 2013 Nobel Prize in medicine winner Randy Schekman of the University of California, Berkeley.

The poll highlights “the iron triangle of science, religion and politics,” said Anthony Leiserowitz, director of the Yale Project on Climate Change Communication.

To the public “most often values and beliefs trump science” when they conflict, said Alan Leshner, chief executive of the world’s largest scientific society, the American Association for the Advancement of Science.

Political values were closely tied to views on science in the poll, with Democrats more apt than Republicans to express confidence in evolution, the Big Bang, the age of the Earth and climate change.

Religious values are similarly important.