At what point do progressive ideas become liberal orthodoxy? Historians and futurologists alike could do worse than identify the take-off point as being when an idea first receives endorsement in the Liberal Democrats’ election manifesto.

Take the cause of transgender rights. It wasn’t until Freedom To Be Yourself, a LGBT supplement to the Lib Dem manifesto for the 2001 general election, that the party made championing transgender rights a platform pledge. But what was meant by a transgender person?

For the Lib Dems, clarity was reached by the time of the passage of the 2010 Equality Act. When Labour politicians were still thinking in terms of protecting those who had undergone gender reassignment, the Lib Dems had moved on and were arguing for legal recognition to those who self-identified as a different gender to their birth sex, regardless of what biology or medical opinion might suppose.

The Lib Dems fought the subsequent 2010 election explaining that the definition of transgender should be “based on how someone considers themselves rather than an expectation that they are seeking medical help. We will therefore change equality law to encompass this broader definition.”

So it should have been no surprise when the party-fluid ex-Labour, ex-Change MP and now Lib Dem candidate, Luciana Berger, appeared on BBC Radio 4’s Today programme and gave Nick Robinson the disarmingly direct answer “yes” to his question, “is it the Liberal Democrats’ view that if I now identify as a woman you want the law to treat me as a woman?”

To make it even clearer, the Lib Dems’ 2019 equalities and human rights plan, unveiled yesterday, repeats the Party’s 2017 election pledge for “complete reform of the Gender Recognition Act to remove the requirement for medical reports, scrap the fee and recognise non-binary gender identities.”

This would sweep away the 2004 Gender Recognition Act’s requirement that individuals seeking the legal reclassification of their birth-sex must first have been medically diagnosed with gender dysphoria. The Act also requires that they have spent two years living in their preferred gender.

In the Lib Dem grand narrative, trans rights are the logical extension of women’s rights and gay liberation. The problem is that this narrative arc contains many diverting sub-plots.

From women’s changing rooms, dormitories, hospital and prison wards to the sports’ field, there are plausible safety and fairness reasons for why it may not be appropriate for individuals who remain anatomically male to have unrestricted access to women’s spaces merely because they unilaterally declare themselves women. As many leading voices in feminist thought point out – is the experience of being a woman actually just as readily tradable as a fashion label that can be put on and discarded at will? Luciana Berger’s answer that “I don’t think they are in competition” because “there shouldn’t be a hierarchy of equalities,” suggests that this is a debate the Lib Dems are keen not to exhaust during an election campaign.

There is a secondary issue about self-identification that risks undermining other aspects of the Party’s equalities platform. The Lib Dems remain committed to corporate reform in which FTSE 350 company boards should be at least 40% female. Additionally, the Party is promising to compel companies employing more than 250 staff to publish data on gender, BAME and LGBT+ employment levels and pay.

But what is the value of classifying people by gender, race, and sexuality whilst at the same time insisting that gender and sexuality (but not race) are fluid concepts? If these identities can be so easily traded, why are they deemed to trump all other defining characteristics?

There are practical difficulties too. One problem with seeking to identify the pay-packet size of LGBT+ employees is the non-cooperation of those who regard their sexuality to be no business of their employer or government body.

Nor is this the only limitation with attempting to create a more equal society having first established a narrow and contestable set of means by which to measure it. If a Lib Dem government will legislate that there is to be no objective determination of gender, then what is to stop insolent company boards or insubordinate employees from simply submitting answers framed to make a mockery of the required thresholds? In a “I am what I say I am” legal order, they would be breaking no law.

A final thought. To what extent do such initiatives demarcate the new identity politics between left and right, progressive and reactionary?

It was not a liberal activist but Theresa May who in her efforts to tackle the “burning injustices” of British life set-up the Race Disparity Audit in 2016. The following year, it was the Conservatives’ women and equalities minister, Justine Greening, who announced “we’re going to be launching a consultation in the Autumn on reform of the Gender Recognition Act looking at how we can make sure it’s more streamlined and de-medicalised.”

Greening raised hopes that when the consultation was concluded, trans people could have the sex on their birth certificates and passports changed without any medical diagnosis or period living in the assumed gender. Her successor, Penny Mordaunt, appeared to be similarly minded.

Thus perhaps Lib Dem manifestos are the wrong place to look. Progressive ideas go from promotion to orthodoxy when they become Conservative Party doctrine. The irony is that if Liz Truss is retained after the general election as the minister responsible, then that doctrine may be abandoned. An irony, because Ms Truss started off her political life as a Lib Dem.