That may be true in US PPE courses, just like a basic US econ major is insufficient for grad school (you need to take math econ instead). However at Oxford most people drop one subject (usually Philosophy) after the first year, and economics in the UK is far more mathematical than a standard US economics course, and so much more attuned to what grad schools need. A first in PPE there is certainly enough preparation, although the typical UK path is to do a one year masters followed by a three year PhD: a friend went on to Stanford for a PhD in econ and was very successful. The problem is that as @Mumfromca points out, it’s extremely competitive, especially on the politics side, these are the best and most ambitious students in the country and some will end up becoming famous journalists and politicians.
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