In Biology, 55% of 'A' Level candidates are girls, in Chemistry it's 50%, but in Physics, it's only 20%.
Now my initial response was:
"So what?" If we were to look at the figures for Nursing, English or Sociology, we would find that boys were in the minority.
However that wouldn't make much of a post, so let's have a look at the three possible reasons.
1) Girls on average are not as good at Physics as boys.
2) Girls are not very interested in Physics.
3) Something else discourages girls from studying Physics.
Let's look at each in turn:
1) Doesn't seem very likely as girls do better at the Physics GCSE than boys. (As in pretty much all subjects.)
Now you could argue that the GCSE is female biased with its modular assessment and coursework. Maybe the male brain is more analytic and logical, which makes it more suited to studying Physics at a higher level rather than a woolly subject like Sociology for example. Maybe girls are also put off because of the Maths content.
Hard to say without doing a lot more research. It's also a taboo topic in teaching.
2) On average this is certainly true. My completely unscientific questioning of seven women (one of whom was a science teacher!) concluded that with the exception of Prof. Brian Cox, no aspect of Physics held any interest for them whatsoever. I then asked seven males and two admitted to finding the subject interesting. Maybe the things that most boys naturally gravitate towards- cars, guns mechanics, taking things apart, explosives, computers and football lead them in some way towards having more of an interest in the physical world.
3) Most Physics teachers are male, as are the vast majority of Physics students at University. (The same is true of Maths, Computing and Engineering). Most physics related jobs would involve working in a male dominated environment. I suppose that would certainly put off many females.
A BBC article stated that girls are much more likely to study Physics if they go to an all-girls school. However they failed to note that this might simply be because such schools are almost always private and tend to be very selective academically. Physics is one of the hardest subjects and so it's not surprising that more of the brighter girls study it.
Finally you could ask- do we actually need more women Physicists?