Here's an interesting fact: for the last five years in UK universities, foreign postgraduate students have outnumbered British ones. International student numbers have grown by 90% in the past decade while the number of homegrown students has fallen by 12% in the past three years. And this despite the best efforts of the government and the Border Agency to dissuade students from coming to the UK.

The disproportionate growth in foreign postgraduates is good news for UK universities (because overseas students pay hefty fees), but bad for the society that supports those institutions. And it looks as though the situation will get worse. Many of those who work in higher education are worried that enrolment of home postgraduates will decline even further in 2016 when students graduating with debts of £27,000 from their undergraduate degree courses ponder whether they can afford to opt for further study. The situation is further exacerbated by the Research Councils' policy of cutting grant support for UK-domiciled postgraduate students.

As usual, there's an inequality angle to this too: there are some indications that poorer UK students are becoming less likely to apply for further study because of a lack of financial support; privately educated students are 25% more likely to apply for postgraduate courses than those from state schools.

But this isn't just about money. Culture plays a role in it too. For years, educators (and some government ministers) have been agonising over the decline in the number of secondary-school pupils studying the stem (science, technology, engineering and mathematics) subjects at A-levels. Why this should be so is a mystery. The most common explanation is that these subjects are perceived to be difficult (which they are) and therefore a bit risky for students wanting to ensure a place at a university of their choice.

But here again there's an inequality angle. The Russell Group (which comprises leading UK research universities) says: "State school pupils are significantly less likely to take separate sciences and other stem subjects, despite the fact that studying these subjects increases a student's future options. They are also far less likely to be taught stem from teachers with a degree in the subject.

For example, 80% of physics teachers in independent schools had a degree in physics, compared to only 30% of those in state schools. Just under half of all science A grades at A-level are from independent schools." So the school pipeline that feeds the undergraduate courses that eventually feed postgraduate studies is producing far fewer stem students than the country needs and a disproportionate number of them come from private schools.

Why does the decline in UK postgraduates matter? Basically because, in a knowledge-based economy, having a primary degree is a necessary but not sufficient condition for employment in some of the fastest-growing sectors. You need a master's degree and, in some cases, a PhD. So a decline in the numbers of UK students with these qualifications points towards a growing competitive disadvantage for UK plc. Especially in a political climate that seems hellbent on preventing equivalently qualified foreigners from coming to the UK to work.

The statistics for overseas postgraduate students are staggering. According to the Higher Education Statistics Agency, the data for 2011-12 (the latest I could find) show that 69% of all full-time taught postgraduate (mostly master's degrees) and 48% of full-time PhD students in UK universities were from overseas.

The vast majority of those students will return to their homelands, armed with the skills and knowledge that they have acquired in excellent UK universities and that their societies will put to good competitive use.

The strategic implications of these trends are keeping some people awake at night. One of them is Philip Greenish, the chief executive of the Royal Academy of Engineering (the body that is for the engineering profession what the Royal Society is for scientists). In a recent letter to the Financial Times he laid out the scale of the challenge.

"We need about 1.25m science, engineering and technology professionals and technicians by 2020," he wrote, "including a high proportion of engineers, to support the UK's economic recovery. However, the real problem lies predominately in the productive manufacturing and innovative technology businesses across the country that are struggling to recruit the stem talent they need. Indeed it is often the smaller companies that are experiencing the lack of stem skills more keenly, rather than the high-profile global institutions."

Why are the stem subjects important? Basically because the next surges in innovation are going to come from them. Contrary to what many people think, innovation generally doesn't come from lone geniuses brooding in laboratories, but from combining technologies that already exist or have recently evolved. A good example is the Google self-driving car, which is made possible not by some startling breakthrough in artificial intelligence but by putting together massive processing power with machine-learning software, detailed mapping and new kinds of sensor technology. The societies that will be best at harnessing this kind of combinatorial innovation will be those most richly endowed with people who understand the component sciences and technologies involved, that is, people with stem backgrounds.

Britain ought to be such a society. It's a country stuffed with talent that has traditionally punched way above its weight in the IQ stakes, especially in the stem subjects.

A single British university (Cambridge), for example, has had 90 Nobel laureates, which is more than France's tally. It would be absurd to allow a society so rich in human capital to become a laggard in the new world economy. But that's what will happen if current trends in British higher education continue.

Instead of obsessing about things such as HS2, a really far-sighted government would treat the decline in stem subjects in secondary and higher education as one of the greatest threats facing the country and act accordingly.

It would boost the status and pay of teachers in these subjects in state schools, provide massive incentives to students to study them at university (halving the tuition fees, for example) and pour resources into postgraduate studentships in strategic disciplines. It would, in other words, realise that the best way to predict the future is to invent it.