Proposed changes to a question about biological sex, asked in Scotland’s next census, risk undermining the reliability of the survey and set a difficult precedent for equalities protection, experts have said.

The Scottish government is proposing to add two questions, posed on a voluntary basis, regarding gender identity and sexual orientation to the 2021 census, via the Census (Amendment) (Scotland) bill.

Discussion has extended to the format of the sex question, which will be dealt with in secondary legislation. At present the question is a binary one, requiring a male or female answer.

The Equality Network, which campaigns for LGBTI rights in Scotland, wants to see a third option to the male/female sex question, ideally offering a write-in box for individuals to define themselves using the terminology they prefer.

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The government is testing this formula, in part to offer clarity, following online guidance for the 2011 census which said that trans people could select how they wished to be identified, irrespective of the details on their birth certificate. The formula was recently rejected by the Office for National Statistics for the next census in England and Wales in favour of maintaining the binary male/female sex question and adding a voluntary question on gender identity for those aged over 16.

But, as a Holyrood committee considers its draft report on the census changes this Thursday, policy analysts and data experts have warned that moving to a non-binary sex question puts the reliability of census data (key to policy development, research and strategic planning of public services) at risk.

Lucy Hunter Blackburn, an Edinburgh University researcher and member of the policy analyst collective Murray Blackburn Mackenzie, said: “Our concern is that this will make census data less reliable and less easy to understand. You are taking a risk with the usefulness of the data when you bring in an unknown, because you don’t know how many people are telling you their legal sex and how many something else. It is making the data harder to interpret because it is conflating two different ideas into one data set.”

And in oral evidence to the culture, tourism, Europe and external affairs committee last month, data experts from across the UK argued that the census should be collecting data on biological or legal sex (which they say refers to either an individual’s sex characteristics at birth, or as presented on their birth certificate, which can be legally changed with a gender recognition certificate) rather than self-identified sex (which can refer to a range of not necessarily static or legally recognised self definitions).

They also warned that the 2021 census could end up using a different interpretation of “sex” than was present in current law, most significantly in the Equality Act 2010.

Jackie Cassell, a professor and expert in public health research at Brighton and Sussex Medical School, told MSPs: “There is a real issue about precedent and the credibility of the census and, for the purposes that those many data sets are drawn on to provide sex data, it is key that we have a good representation of the definition as it currently stands in law.”

Susan McVie, also a professor and chair of quantitative criminology at Edinburgh University’s law school, said: “If we are to properly understand the relationship between sex and gender identity and how that impacts on factors such as health, the likelihood of getting a job and attainment in education, we need to disentangle those things so that we can have a much clearer picture.”

Blackburn said she believed that “the strongest argument is for collecting information both on legal sex and on trans people’s lived identity, without conflating or confusing the two”.

She said: “Legal sex has a specific value here, as it has a clear external definition. This gives the data greater reliability, particularly when linking data from different sources. Legal sex is also the data set needed to track how public bodies are fulfilling their duties towards the protected characteristic of ‘sex’ under the Equality Act 2010.”

Vic Valentine, policy officer at the Scottish Trans Alliance, a project of the Equality Network, rejected the suggestion that adding a third option would compromise census data. “We would argue that forcing people to answer male or female according to their birth certificate creates inaccurate data. It is significantly more accurate to allow people a third option, and all the evidence points to a very small number of people who are affected.”

The Network also supports Scottish ministers’ plans to include a new voluntary question on whether an individual considers themselves to be trans or to have a trans history. “Taken with the third option in the sex question, this gives us a way of counting those women who are trans, for example, without forcing them to answer ‘male’ when this does not reflect how they live or identify.”

A spokesperson for the National Records of Scotland, which is responsible for the census, confirmed that a change to the response options for the sex question was being considered for 2021, and that testing is continuing on a non-binary question which would allow people to record their sex as female, male or other. She added: “Although we are considering a non-binary sex question, we will still provide results on a binary basis, allowing continuity with previous censuses”.