crises Marx's models of economic reproduction in capitalism have often been interpreted as stating the conditions for economic equilibrium, or balanced economic growth. After all, there are certain "necessary proportions" between different branches of production, which have to adjust their output levels to each other. If those proportions do not reach a minimum acceptable level, then products remain unsold or producers cannot obtain the inputs they require, in which case production begins to slow down or break down. So there are necessary proportions between production, distribution, and consumption that must be maintained if society is to survive and grow. In this sense, Marx distinguishes between the production of means of production, consumer goods, and luxury goods, and he considers the commercial interactions between the sectors producing them. If the growth of different sectors of production occurs very unevenly, bottlenecks can occur, so that a supply or demand cannot be met. In the worst case, an interruption in the normal reproduction process triggers a sequence of disturbances, a chain reaction, which spreads from some branches of production to the whole economy, meaning that products are left unsold and that producers receive insufficient income to pay their bills. The result is rising unemployment, idle productive capacity, and a drop in output and productive investment. This in turn means lower economic growth. Disproportionality theories. This idea was the basis for numerous Marxist crisis theories devised in the first three decades of the 20th century, most famously by Rudolf Hilferding, Rosa Luxemburg, Nikolai Bukharin, Otto Bauer, and Henryk Grossmann. The argument is that balanced economic growth is only a temporary phenomenon, because a private enterprise economy is incapable of sustaining the necessary proportionalities required. It cannot do so because there is no overall, conscious coordination of production activities in an economic system in which producers compete to cut costs, increase sales, and increase profits. There has been much dispute about which disproportionalities exactly are of decisive importance and why those specific disproportionalities would occur and recur, but all these theorists agree that some disproportionalities must necessarily occur, making recurrent crises inevitable. Uneven development theories. Other theorists such as Ernest Mandel and Roman Rosdolsky argued that capitalist economic development is always an "imbalanced" rather than "balanced" development in space and time. At most, an approximate balance of supply and demand is achieved in particular areas. In that case, an economic equilibrium never exists in reality;