The use of crop breeding, better management of soil nutrients, and improved weed control have greatly increased yields per unit area. At the same time, the use of mechanization has decreased labor input. The developing world generally produces lower yields, having less of the latest science, capital, and technology base. More people in the world are involved in agriculture as their primary economic activity than in any other, yet it only accounts for four percent of the world's GDP. The rapid rise of mechanization in the 20th century, especially in the form of the tractor, reduced the necessity of humans performing the demanding tasks of sowing, harvesting, and threshing. With mechanization, these tasks could be performed with a speed and on a scale barely imaginable before. These advances have resulted in a substantial increase in the yield of agricultural techniques that have also translated into a decline in the percentage of populations in developed countries that are required to work in agriculture to feed the rest of the population.