New scientific discoveries gave birth to new machinery, thus creating the Industrial Revolution, making wages the main source of income, and shifting residential patterns to the city. By 1900, 40 percent of the U.S. population lived in urban areas. These revolutions also produced new medical advancements, vaccines, and antibiotics, which, in turn, lowered the mortality rates of both infants and adults and lengthened life expectancy, thus creating exponential population growth and new stages in life development (childhood, adolescence, old age, retirement, grandparenting).