A vote by France's National Assembly last week to write the ‘precautionary principle’ into the country's constitution is misguided. It is part of an environmental charter, which Jacques Chirac, the French president and new-found champion of the environment, hopes to give the same constitutional standing as the 1789 Declaration of the Rights of Man and the Citizen.

The precautionary principle has featured in environmental law since the 1970s, and appears in several international treaties, national laws and the proposed European Constitution. Invariably vaguely worded, it states that if technology or actions might cause harm, and there is scientific uncertainty as to what and how much, then other actions should be taken to anticipate and prevent that harm.

The devil in this sensible proposition lies in its interpretation. Wisely, most texts leave this to the checks and balances of legislators, whose deliberations are usually balanced and flexible. The French charter, in contrast, makes the precautionary principle “directly applicable”, meaning that anyone can contest the desirability of a proposal even if it falls outside the scope of existing legislation.

The French Academy of Sciences has rightly warned that this may have “disastrous consequences”. Pressure groups are sure to seize on it to block research they dislike. Parliamentarians should listen to these concerns as the bill moves through the approval process.

When European leaders meet in Brussels next week to try to agree on the European Constitution, they will find a proposed amendment stipulating that the European Union (EU), in all its policies, “shall pay full regard to the welfare requirements of animals, as sentient beings”. It is probably the least of the negotiators' worries, and may slip through without due consideration of the licence it would give to obstruct research that most people would support. The same wording on animal welfare was previously added to the EU's founding treaties in 1997. Given that a coalition of European animal-rights and welfare organizations have lobbied so strongly for its inclusion in the constitution, scientists should anticipate that giving the EU direct powers on animal welfare will, at the very least, result in yet stricter controls and greater bureaucracy.

Issues such as the environment, animal welfare and bioethics should not, in most cases, be addressed in constitutions. A battery of legislation on such issues already exists, ranging from laws passed by national parliaments to codes of conduct produced by professional bodies and recommendations by ethical committees, which although devoid of the power of law, often have equivalent impact or directly influence legislation. Unfamiliar, complex and fast-moving areas involving science can often be dealt with at this lower legislative level. Indeed, it is better placed to juggle regulations to meet each nation's own mix of religious, moral and cultural traditions, as well as the sometimes conflicting streams of contemporary thought.