Final answer:
Life expectancy significantly increased over the 20th century due to clean water systems, healthier public behaviors, and medical advancements like vaccines and antibiotics.
Step-by-step explanation:
Twentieth Century Increases in Life Expectancy and Lifespan
The question addresses the observed increases in both life expectancy and lifespan over the 20th century. During this period, a remarkable shift occurred, with life expectancy in the United States rising from about 47 years in 1900 to 79 years by 2015. This increase can be attributed to several factors.
First, the development of clean water systems and proper waste disposal significantly reduced the transmission of diseases.
Second, there was a shift in public behavior towards healthier practices, such as the boiling of bottles for food storage, hand washing, and food safety measures. Additionally, the reduction in tobacco use and precautions against sexually transmitted diseases have contributed to this rise.
Third, advancements in medicine, including the development of vaccines for various diseases and the discovery of penicillin, vastly improved public health. These factors combined have led to a dramatic improvement in life expectancy, representing one of the most profound changes in human standard of living.