Variations in seasonal rainfall, temperature, and other climatic factors are generally the principal causes of the fluctuations, although the isolation of their effects from those of the other factors involved is difficult and has been accomplished for only a few crops in some developed countries. Fluctuations in production lead to changes in prices, employment, incomes and demand which in turn influence the course of production, generally accentuating the fluctuations caused by the weather. Changes in price and other agricultural policies can cause production to vary sharply from the trend. Civil disturbance, hostilities, natural disasters unrelated to the weather, and pests and diseases (sometimes linked with weather) are other powerful contributors.