Short-term trends, even on the scale of decades or centuries, cannot prove or disprove the existence of mechanisms promoting a Malthusian catastrophe over longer periods. However, due to the prosperity of a major fraction of the human population at the beginning of the 21st century, and the debatability of the predictions for ecological collapse made by Paul R. Ehrlich in the 1960s and 1970s, some people, such as economist Julian L. Simon, question its inevitability.[10]