The theory also assumes that technology is freely traded and available to developing countries that are attempting to catch-up. Capital that is expensive or unavailable to these economies can also prevent catch-up growth from occurring, especially given that capital is scarce in these countries. This often traps countries in a low-efficiency cycle whereby the most efficient technology is too expensive to be acquired. The differences in productivity techniques is what separates the leading developed nations from the following developed nations, but by a margin narrow enough to give the following nations an opportunity to catch-up. This process of catch-up continues as long as the following nations have something to learn from the leading nations, and will only cease when the knowledge discrepancy between the leading and following nations becomes very small and eventually exhausted.