Herbicides are chemicals that kill plants, and selective herbicides are those that can kill certain plant species (weeds) while not injuring other plant species (crops). Since the introduction and commercialization of synthetic organic herbicides in the late 1940s, farmers have been increasingly reliant on herbicides for weed control. This is particularly true for agriculture in many developed countries, where there has also been a shift toward increased mechanization with larger machines and reduced tillage (less cultivation of the soil between plantings). Although herbicides have been widely used for only the past 60 years, societal changes have occurred in developed countries that would make it very difficult to return to previous weed control practices. These societal changes include a shift to fewer but larger farms with the simultaneous depopulation of rural areas, shortages of farm labor, and an increasing average wage paid to farm workers, which makes handweeding uneconomic in many instances.