Four US states are considering new legislation about teaching science in schools, allowing pupils to to be taught religious versions of how life on earth developed in what critics say would establish a backdoor way of questioning the theory of evolution.

Fresh legislation has been put forward in Colorado, Missouri and Montana. In Oklahoma, there are two bills before the state legislature that include potentially creationist language.

A watchdog group, the National Center for Science Education, said that the proposed laws were framed around the concept of "academic freedom". It argues that religious motives are disguised by the language of encouraging more open debate in school classrooms. However, the areas of the curriculum highlighted in the bills tend to focus on the teaching of evolution or other areas of science that clash with traditionally religious interpretations of the world.

"Taken at face value, they sound innocuous and lovely: critical thinking, debate and analysis. It seems so innocent, so pure. But they chose to question only areas that religious conservatives are uncomfortable with. There is a religious agenda here," said Josh Rosenau, an NCSE program and policy director.

In Oklahoma, one bill has been pre-filed with the state senate and another with the state house. The Senate bill would oblige the state to help teachers "find more effective ways to represent the science curriculum where it addresses scientific controversies". The House bill specifically mentions "biological evolution, the chemical origins of life, global warming and human cloning" as areas that "some teachers are unsure" about teaching.

In Montana, a bill put forward by local social conservative state congressman, Clayton Fiscus, also lists things like "random mutation, natural selection, DNA and fossil discoveries" as controversial topics that need more critical teaching. Meanwhile, in Missouri, a bill introduced in mid-January lists "biological and chemical evolution" as topics that teachers should debate over including looking at the "scientific weaknesses" of the long-established theories.

Finally, in Colorado, which rarely sees a push towards teaching creationism, a bill has been introduced in the state house of representatives that would require teachers to "respectfully explore scientific questions and learn about scientific evidence related to biological and chemical evolution". Observers say the move is the first piece of creationist-linked legislation to be put forward in the state since 1972.

The moves in such a wide range of states have angered advocates of secularism in American official life. "This is just another attempt to bring creationism in through the back door. The only academic freedom they really want to encourage is the freedom to be ignorant," said Rob Boston, senior policy analyst at Americans United for Separation of Church and State.

Over the past few years, only Tennessee and Louisiana have managed to pass so-called "academic freedom" laws of the kind currently being considered in the four states. Barbara Forrest, a philosophy professor at Southeastern Louisiana University and close observer of the creationism movement, said that the successes in those two states meant that the religious lobby was always looking for more opportunities.

She said that using arguments over academic freedom was a shift in tactic after attempts to specifically get "intelligent design" taught in schools was defeated in a landmark court case in 2005. Intelligent design, which a local school board in Dover, Pennsylvania, had sought to get accepted as legitimate science, asserts that modern life is too complex to have evolved by chance alone. "Creationists never give up. They never do. The language of these bills may be highly sanitized but it is creationist code," she said.

The laws can have a direct impact on a state. In Louisiana, 78 Nobel laureate scientists have endorsed the repeal of the creationist education law there. The Society of Integrative and Comparative Biology has even launched a boycott of Louisiana and cancelled a scheduled convention in New Orleans. Louisiana native and prominent anti-creationist campaigner in the state Zack Kopplin said that those pushing such bills in other states were risking similar economic damage to their local economies. "It will hurt economic development," Kopplin said.

There is also the impact on students, he added, when they are taught controversies in subjects where the overwhelming majority of scientists have long ago reached consensus agreement. "It really hurts students. It can be embarrassing to be from a state which has become a laughing stock in this area," Kopplin said.

Others experts agreed, arguing that it could even hurt future job prospects for students graduating from those states' public high schools. "The jobs of the future are high tech and science-orientated. These lawmakers are making it harder for some of these kids to get those jobs," said Boston.