You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Imperialism

Context:
A controversial aspect of imperialism is the defense and justification of empire-building based on seemingly rational grounds. J. A. Hobson identifies this justification on general grounds as: "It is desirable that the earth should be peopled, governed, and developed, as far as possible, by the races which can do this work best, i.e. by the races of highest 'social efficiency'". Many others argued that imperialism is justified for several different reasons. Friedrich Ratzel believed that in order for a state to survive, imperialism was needed. Halford Mackinder felt that Great Britain needed to be one of the greatest imperialists and therefore justified imperialism. The purportedly scientific nature of "Social Darwinism" and a theory of races formed a supposedly rational justification for imperialism. The rhetoric of colonizers being racially superior appears to have achieved its purpose, for example throughout Latin America "whiteness" is still prized today and various forms of blanqueamiento (whitening) are common.

Question:
what is the most controversial aspect of imperialism?

empire-building