Teaching creationism in public schools has consistently been ruled unconstitutional in federal courts, but according to a national survey of more than 900 public high school biology teachers, it continues to flourish in the nation’s classrooms.

Researchers found that only 28 percent of biology teachers consistently follow the recommendations of the National Research Council to describe straightforwardly the evidence for evolution and explain the ways in which it is a unifying theme in all of biology. At the other extreme, 13 percent explicitly advocate creationism, and spend at least an hour of class time presenting it in a positive light.

That leaves what the authors call “the cautious 60 percent,” who avoid controversy by endorsing neither evolution nor its unscientific alternatives. In various ways, they compromise.

The survey, published in the Jan. 28 issue of Science, found that some avoid intellectual commitment by explaining that they teach evolution only because state examinations require it, and that students do not need to “believe” in it. Others treat evolution as if it applied only on a molecular level, avoiding any discussion of the evolution of species. And a large number claim that students are free to choose evolution or creationism based on their own beliefs.