We recently wrote to the UK & Scottish Governments, highlighting our concerns about data reliability risks at the sub-population level, as a result of the proposed conflation of sex/gender identity in the sex question.
In particular, we drew attention to a Swedish population-based study that found 6% of 22 to 29 year olds wanted to be treated as the opposite sex, as detailed in this blog. murrayblackburnmackenzie.org/2020/06/04/a-n…
Both governments have now replied. The UK Government has suggested that we liaise with ONS, who are undertaking further testing on the sex question due to concerns raised by senior academics who work with population data. 
Responding on behalf of the Scottish Government, NRS simply state that they have recommended framing the sex question in terms of self-declared gender identity.
Should ONS/NRS take a different approach, it would suggest a departure from the harmonisation agreement between census authorities: "Common definitions and classifications, typically based on international standards, should be agreed, used & published". ons.gov.uk/file?uri=/cens…
Along with today’s announcement that NRS will now undertake Scotland’s census in 2022, while ONS still intend to undertake the E&W census in 2021, using different definitions for a core demographic variable adds to the potential for UK-wide inconsistency in census outputs.
