As you argue, immigration is central to America’s ability to innovate (“Give Me Your Scientists…,” March 7, 2009). Immigrants now make up nearly a half of scientists and engineers with doctoral degrees in the country. They contribute a disproportionate share of all patents given to scientists in America. What is more, a scientific diaspora gives countries of origin a certain advantage in terms of access to the latest research, thus mitigating some of the problems associated with brain drain. It is therefore strange that contemporary theories of growth, all of which emphasize the importance of innovation, fail to assign a special rôle to immigration. As you also point out, innovation benefits from clusters, such as the Silicon Valley or Boston, Massachusetts, where half a million students and teachers from all over the world can be found at any one time. The only addition I would make to your ideas about how to improve theories of growth is to focus on the innovators themselves and ask them, too, whether they would rather be in an American innovation cluster or in their village someplace in India or China. Chances are that the only force conspiring against America’s ability to innovate are its unenlightened immigration officials.