Even in a world with a pristine environment, no cigarettes and the ability to fix faulty genes inherited from our parents, most of the cancers diagnosed today still would occur — a combination of biology and bad luck.

Every new case of cancer depends on a collection of specific mutations in our DNA, and a sweeping new study finds that 66 percent of the mutations that put us at risk for cancer are the result of unavoidable errors made by cells as they copy themselves millions of times throughout our lives.

In research published Thursday in the journal Science, geneticist Bert Vogelstein and biostatistician Cristian Tomasetti demonstrate that most cancer risks stem not from bad genes, environmental toxins or poor lifestyle choices, but from simple random mutations.

The study authors said their findings “complement rather than oppose” authoritative estimates suggesting that 42 percent of cancer cases worldwide could be prevented with improvements to lifestyle and environment. People should keep practicing behaviors that reduce their cancer risk, they said.