A few years ago, I walked into a university to find several posters of my face. That was disconcerting enough. But it was advertising a graduate scheme for a science teacher – and I am a citizenship teacher. I made inquiries, and was told a focus group had suggested a female teacher was needed for the campaign to "break stereotypes" about scientists. The organiser quipped: "I think your being blonde was a bonus."

This event came to mind last week when reading yet another story about girls' collective failure to study physics at the same rate as boys. The schools minister, Liz Truss, has lamented this fact in many of her recent speeches. Her facts are stark: only a quarter of A-level physics students are female. And while half of all boys who receive an A* in GCSE physics continue to study the subject to 18, for girls that figure is less than one in five.

Like the university tutors who thought sticking a blonde woman on a science poster would somehow resolve gender disparities, Truss's main solution to the science gap appears to be gutsy role-modelling. In a recent speech to Google, Truss said: "It's up to all of us to encourage girls and say, you are just as good as the boys, you can do it."

But why are girls the ones constantly pathologised? This narrative makes out that girls are afraid to study the big scary sciences: but another way of thinking about it is that boys are the odd ones – crowding as they do into just a few subjects. They take up 60% or more of places in just five subjects, and two of these – maths and physics – are particularly dominated by boys, with the subjects accounting for one in five of all A-level examinations taken by boys.

Look across the full suite of A-level subjects, though, and girls dominate in more than 10 subjects, including English, psychology, and modern foreign languages: in some cases, by a lot. Girls make up more than 90% of performing arts students – yet when was the last time you heard a politician imploring teachers to tell boys they can dance "just as well as the girls"?

Looked at in this way, the question we need to answer is less "Why do so few girls choose physics?" and more "Why do so many boys only choose physics and maths-related subjects?" After all, if they spread themselves out across all other subjects, girls would no longer be regularly outnumbered in physics.

Sadly, few politicians question gender disparities in other subjects. The schools minister has referred on several occasions to the fact that 40% of students who take maths A-level are female. She is at pains to point out how disastrous this is. Yet male participation rates in psychology, sociology, and religious studies are regularly well below 40% and no one bats an eyelid. One might argue that these are not "core" subjects. But boys only make up 29% of English A-level students – why isn't that deemed a "problem"? Apparently only girls deserve that label.

Vocational education presents an even greater problem. Few things are more perplexing on sixth-form enrolment days than sitting opposite a male student desperately wanting to study childcare and having his eyes plead with you to lie as his father leans in and abruptly asks: "Tell me, how many boys normally take this subject?" In truth, just 2.5% of childcare BTec students are male. Construction, hairdressing and motor vehicle engineering have similarly large imbalances.

On the upside, some subjects' enrolments are surprisingly even. History, chemistry, business studies and politics are all fairly balanced between the sexes. Commentators point to the parity in these fields as a consequence of path-breaking women. But recent research warns against such simplistic notions.

US researchers found that girls who played with Barbie dolls for just a few minutes, even Barbies dressed as a medical doctor, were less likely to feel capable of doing male-dominated jobs. In contrast, children who played with a more androgynous "Mrs Potato Head" figure showed greater faith in their abilities to take on male-dominated roles. This suggests that while glitzy female role models sound like a good idea, they can actually be less effective for improving women's confidence than a simple "any person can do this" approach.

If we really want more girls to do science, then, we need to be concerned about gender disparities in all subjects, not just physics. And pretending that some blonde woman in a photograph is a scientist is probably not the place to begin.