European universities will need to shrink by 2020 or recruit large numbers of overseas students to adjust to demographic changes, researchers have said.

A paper in the journal International Higher Education says that the number of students in Europe was likely to fall because of a drop in the population of 18 to 24-year-olds.

The authors - Manja Klemencic, of Harvard University, and Jochen Fried, the director of education initiatives at think-tank Salzburg Seminar - say that universities will have to adjust their academic programmes and become less socially selective to attract a more diverse student population.

Their paper says: "The potential implications for higher education are enormous: unless foreign students are being imported in large numbers, higher education institutions in Europe will inevitably be reduced in numbers and size."

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The retirement of academics could result in "a stagnation of knowledge societies" unless a steady stream of young researchers is recruited, the authors say.

US universities will not face a decline in demand for higher education because the country's population is expected to grow largely thanks to relatively high birth rates among African and Hispanic Americans. It is also likely to continue to attract overseas students, they say.

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Asian and African states are likely to see an increased demand for university places because of growing populations in African states and economic improvements in Asia, as well as government attempts to improve access to higher education.