When I first studied economics ( a long, long time ago) the textbook explanation of why income differed between countries was based on capital. In the simplest version for example, that of Harrod and Domar), rich countries had a bigger stock of capital than poor countries, and the problem was one of accumulating sufficient capital to catch up. In more sophisticated versions, rich countries had more modern capital stocks, and therefore benefited from embodied technological progress.
Even when I was a student, this kind of thinking was already being superseded by notions such as human capital theory[1]. Still, I’ve never seen a really convincing refutation. It strikes me that computers and the Internet provide one, at least as far as differences among developed countries are concerned.