In August 2006, at the general assembly of the International Astronomical Union in Prague, astronomers voted on a new definition of a planet, and Pluto was demoted.

Pluto didn’t change, only its definition.

Now New Zealand’s parliament is preparing to vote on the legal definition of “man” and “woman”. If scientists couldn’t end the controversy over what counts as a planet, why should we expect politicians to define who counts as a woman or man?

There is something strange about deciding what something, or who somebody, is by counting votes. The result was awkward enough when scientists were voting in a domain where they knew what they were talking about.

The planet vote didn’t settle the debate at the time, nor nearly two decades later.

And now we want to do the same with humans?

The Legislation (Definitions of Woman and Man) Amendment Bill was introduced by New Zealand First MP Jenny Marcroft and passed its first reading on May 20. It asks parliament to define woman in law as “an adult human biological female” and man as “an adult human biological male”.

The bill assumes there is a settled biological test for whether a person is female or male – one the law can........

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